Why does a child in a wagon seemu:-qmo,mlvd . buw 2.t to fall backward when you give the wagon a sharp pultm2:- ,.q .buldwmvou l forward?

A box rests on the (frictionless) bed of a truck. The tq 4z9/ c r ) ,(yht;zeun8jyb1as4n ys06snotnruck driver starts the truck and accelerates forward. The box immeditbsza1coy nyy9/n4tqn s08n,z 6srje ()h u4;ately starts to slide toward the rear of the truck bed. Discuss the motion of the box, in terms of Newton’s laws, as seen (a) by Mary standing on the ground beside the truck and (b) by Chris who is riding on the truck

If the acceleration of an object is zero, are no forces acting on in7xe1 fytinh 87:-57ilwx xfbt? Explain.

Only one force acts ozpig7a(6-r rexki j- .n an object. Can the object have zero acceleration? Can it have6apei- jxkg.i7-r (rz zero velocity? Explain.

When a golf ball is dropped to the pave e0ta 5k 4+ubn/):x;pde xfvxmendl3 9ment, it bounces back up. (a) Is a force needed to mx:9 xetpvaln0 x m)u4kdef/nd+ 3b5e; ake it bounce back up? (b) If so, what exerts the force?

If you walk along a log floating on a lake, why does the log move in theet;l24ux nuvtp3pol * p(fd-* opposite direction? d( -uu pe2v3;fp xl*n4p*tlto

Why might your foot hurt if you k4 mv5 aetz6ch,ick a heavy desk or a wall?

  When you are running and want to stop quickly, you mu t-bzh*+ ;baqvst decelerate quickly. (a) What is the origin of the force that causes you to stop? (b) Estimate (using your own experience) the maximum rate of deceleration of a person running a bbhzt+ a-qv*;t top speed to come to rest.    m/s$^2$

A stone hangs by a fine rbmdqr n .bq.8yq:v32thread from the ceiling, and a section of the same thread dangles from the bottom of the stone (Fig. 4–36). If a person gives a sharp pull on the dangling thread, where is the thread likely to break: below the stone or above it? What if th3.qm 2rrb byqnd.: 8qve person gives a slow and steady pull? Explain your answers.

The force of gravity qt xb+mpra.)8 on a 2-kg rock is twice as great as that on a 1-kg rock. Why then doesn’t the heavier rock fatarq )p m+x.b8ll faster?

Would a spring scale carried to the Moon give accur :m73n l((2 qs3j;elbhtyxzkbee)x; vate results if the scale had :);e3q ln v(27 bhsemejbx(xzl yk;t3 been calibrated (a) in pounds, or (b) in kilograms?

You pull a box with a constant force across a frictionless table using an atta ssdn.x6a z +buq*l;2oched rope held horizontally. If you now pull the rope with the same force at an angle to the horizontal (with the box remaining flat onz* 2b+os ns;6.dul xaq the table), does the acceleration of the box (a) remain the same, (b) increase, or (c) decrease? Explain.

When an object falls freely under the influence of gravity there is a net forgd((dlu1isgkv c:k 6 3ce mg exerted on it by the Earth. Yet by Newton’s third law the object 6l((uc kvi:1g3gds kd exerts an equal and opposite force on the Earth. Why doesn’t the Earth move?

Compare the effort (or force) needed to lift a 10-kg object when r(xpcv +q91sq1x9jv (dh7z mwyou are on the Moon with the force needed to lift it on Earth. Compare the force needed to throw a 2-kg object horizontally with a given speed on the Moon and on Ea91+9 (dqx q7 xs(1jcwhpvrvzm rth.

According to Newton’s third law, each team in a tug of war (ns+7 wp,*ip+: yk wsxdFig. 4–37) pulls with equal force on the other team. What, then, determine, wskids7+y:w npx+ p*s which team will win?

A person exerts an upward force of 40 N to hold a bag of groceries. Describe 1 0 zn4o:py)bmo; ibrbthe “reaction” force (Newton’s third law) by stating (a) its magnitude, (b) its bbr) znp :miy;oo 4b10direction, (c) on what object it is exerted, and (d) by what object it is exerted.

When you stand still on the ground, how large a force does the3:;*fw gote7w+ ri xmn ground exert on you? Why donx*+3w ; e:mrwfog it7esn’t this force make you rise up into the air?

Whiplash sometimes results from an automobileq4e*+m 3mde : bw gsbmy:zt4/s accident when the victim’s car is st4w3 edg e bymm:sq*mt4s+zb/:ruck violently from the rear. Explain why the head of the victim seems to be thrown backward in this situation. Is it really?

A heavy crate rests on the bed of a flatbed truck. Wef /9(j:iy ueo/2bbmkv29 sdchen the truck accelerates, the crate remains where it is on the truck, so it, too, accelerates. What forcebko2/b9i( y:v/jd e9 cuesm2f causes the crate to accelerate?

A block is given a push so that it slides up a ramp. After the tyxc6z i590vi dot-ocf8pe81block reaches its highest point, it slides back down but the magnitude of its acceleration is less on the descf5y9dpo tx61vzc8cit 8o0- ieent than on the ascent. Why?

What would your bathroom scale read if you weighed yourself on an inclined ubh ea tr8(jr,q4-+wpplane? Assume the mechanism functions pr-,hjp48w ub( rer+q atoperly, even at an angle.

  What force is needed to ac oyp 4.p,hy/w; gn/avlcelerate a child on a sled at 1.25m/s$^2$?   

  A net force of 265 N accelerates a bike and rgkb4u vzu:3j+dh v b0m:3ac-ec:8 kdmider at 2.30$ \mathrm{~m} / \mathrm{s}^{2}$ . What is the mass of the bike and rider together?   

  How much tension must a rope withstand if if -s;mpqy67 sbt is used to accelerate a 960$-\mathrm{kg} $ car horizontally along a frictionless surface at 1.20 $\mathrm{~m} / \mathrm{s}^{2}$ ?   

  What is the weight of i a7phip54 t*hl5*agm a $76-\mathrm{kg}$ astronaut
(a) on Earth,   
(b) on the Moon $\left(g=1.7 \mathrm{~m} / \mathrm{s}^{2}\right)$ ,   
(c) on Mars $\left(g=3.7 \mathrm{~m} / \mathrm{s}^{2}\right)$,   
(d) in outer space traveling with constant velocity?   

  A 20.0-$\mathrm{kg}$ box rests on a table. (a) What is the weight of the box and the normal force acting on it?   
(b) A 10.0-$\mathrm{kg}$ box is placed on top of the 20.0-$\mathrm{kg}$ box, as shown in Fig. 4-38. Determine the normal force that the table exerts on the 20.0-$\mathrm{kg}$ box    and the normal force that the 20.0-$\mathrm{kg}$ box exerts on the 10.0-$\mathrm{kg}$ box.   

  What average force is required to stop an 1100-kg car*ere dmv k w.6y*m00qx in 8.0 s if the car is travelingvw.*yk deq0mx* 0e mr6 at 95km/h? -  

  What average force is needed to acc7/egxgc 7i,(ez rogc.nhh23 nelerate a 7.00-gram pellet from rest to 125m/s over a distance of 0.800 m along tho7i(2n xgn.e3h r,g zcghce7 /e barrel of a rifle?   

A fisherman yanks a fish vertically out of the water with an accelery9th4w -g ui*w-wu6phxm 4wo+mn7 f n2ation of 2. 9n-oy 4 ft*wh2 uw-muxmwgnip7 w4h+65 $\mathrm{~m} / \mathrm{s}^{2}$ using very light fishing line that has a breaking strength of 22 N . The fisherman unfortunately loses the fish as the line snaps. What can you say about the mass of the fish?

  A 0.140$-\mathrm{kg}$ baseball traveling 35.0 $\mathrm{~m} / \mathrm{s}$ strikes the catcher's mitt, which, in bringing the ball to rest, recoils backward 11.0 cm . What was the average force applied by the ball on the glove? -  

  How much tension musta rope withstand if it is used to accelerate a 1200-kg car vertically upward at0.8m/s$^2 ?   

  A particular race car can cover a quarter-mile tnwm+xby:;md1 :6gwc zrack (402 $\mathrm{~m}$) in 6.40 s starting from a standstill. Assuming the acceleration is constant, how many " g 's" does the driver experience?    If the combined mass of the driver and race car is 485 kg , what horizontal force must the road exert on the tires?   

  A 12.0$-\mathrm{kg}$ bucket is lowered vertically by a rope in which there is 163 N of tension at a given instant.
What is the acceleration of the bucket? Is it up or down?     ----待选择----downupward 

  An elevator (mass 4850 kg ) is to be design yw1f 6s3kwct.p8da -kv/c1g9hrx6c sed so that the maximum acceleration is 0.0680 g . What are9 k 3w ckhcs16f.ar-stv/p gxd16cwy8 the maximum and minimum forces the motor should exert on the supporting cable? max =    min =   

A 75$-\mathrm{kg}$ petty thief wants to escape from a third-story jail window. Unfortunately, a makeshift rope made of sheets tied together can support a mass of only 58 kg . How might the thief use this "rope" to escape? Give a quantitative answer.

  A person stands on a bathroom scale itx os ,t9 nu* lc48sf6/gc6r7 ;snbhcon a motionless elevator. When the elevator begins to move, the scale briefly reads only 0.75 of the person’s regular weight. Calculate the acceleration of the elevator, and find the direct;4 bhcc68ltr ,o9soc6fnx 7 ts su*/ngion of acceleration.     ----待选择----downupward 

  The cable supporting a 2125-kg elevator has a maximum stren3 x7k3 vm zn6bdsd4i9hgth of 21,750 N. What maximum upward acceleration can it dmzn xkh3bs3 7vi4d96give the elevator without breaking?    ( two decimal places)

  (a) What is the acceleration of two f b +,3yxhm4gysqlh4(f:w k ,cnalling sky divers (mass 132 kg including parachute) when the upward force of air resistance is equal to one-fourth of their ghkm:qyb,w3nyhl +sx 4 4f(,cweight?   
(b) After popping open the parachute, the divers descend leisurely to the ground at constant speed. What now is the force of air resistance on the sky divers and their parachute?    See Fig. 4–39.

  A person jumps from the roof of a houyj qy;j-ks *s*se 3.9-m high. When he strikes the ground below, he bends his knees so that his torso decelerates over an approximate distanc s *jq-yyskj*;e of 0.70 m. If the mass of his torso (excluding legs) is 42 kg, find
(a) his velocity just before his feet strike the ground, and   
(b) the average force exerted on his torso by his legs during deceleration.   

  A box weighing 77.0 N rests on a table.ykk -z8svp) 2nzilhh3*w a* 0j A rope tied to the box runs vertically upward over a pulley and a weight is hung from the other end (Fig. 4-40). Determyvw k 2h0jz*is-h*8alk zpn 3)ine the force that the table exerts on the box if the weight hanging on the other side of the pulley weighs
(a) 30.0 N ,   
(b) 60.0 N ,   
(c) 90.0 N .   

Draw the free-body diagram for a basketball plagwu3s sfn+f; 1yer
(a) just before leaving the ground on a jump, and
(b) while in the air. See Fig. 4–41.

Sketch the free-body diagram of a baseball (a) at the a;m. 3 gm2v*hlpuqhv(moment it is hit by the bat, and again (b) after it has left the bat and is flying toward th v huvaph qm2mg.*3(;le outfield.

A 650-N force acts in a northwesterly direction. A second 650-N force must sskm1t;am5 dyf6d)sv .5 o,kebe exerted in what direction so65kev , 15dms.tyfss)a d;m ok that the resultant of the two forces points westward? Illustrate your answer with a vector diagram.

  Arlene is to walk across a "high wire" strung horizontallpwgepn2 (w 2v,zh1( g62fbi yli)k a9iy between two buildings 10.0 m apart. The sag g2gz(pl9,w62a wenyi() pf1k ivhb i2 in the rope when she is at the midpoint is 10.0$^{\circ}$ as shown in Fig. 4-42 . If her mass is 50.0 kg , what is the tension in the rope at this point?   

  The two forces $\overrightarrow{\mathbf{F}}_{1}$ and $\overrightarrow{\mathbf{F}}_{2}$ shown in Fig. 4-43 $\mathrm{a}$ and b (looking down) act on a 27.0$-\mathrm{kg}$ object on a frictionless tabletop. If $ F_{1}=10.2 \mathrm{~N}$ and $ F_{2}=16.0 \mathrm{~N}$ , find the net force on the object and its acceleration for (a) and (b). (a) a =    at    $^{\circ}$;(b) a =    at    $^{\circ}$

  One 3.2-kg paint bucket is hanging by a massless cord from another 3+.7zekvd xaz 9.2$-\mathrm{kg}$ paint bucket, also hanging by a massless cord, as shown in Fig. 4-44.
(a) If the buckets are at rest, what is the tension in each cord? F1 =    F2 =    (The top rope is F1 and the bottom rope is F2)
(b) If the two buckets are pulled upward with an acceleration of 1.60 $\mathrm{~m} / \mathrm{s}^{2}$ by the upper cord, calculate the tension in each cord. F1 =    F2 =   

  A person pushes a 14.q*d5 (nzxk7l in sa3tm n c 5zhbeu:+3).5xjqx0$-\mathrm{kg}$ lawn mower at constant speed with a force of F=88.0 $\mathrm{~N}$ directed along the handle, which is at an angle of 45.0$^{\circ} $ to the horizontal (Fig. 4-45).
(a) Draw the free-body diagram showing all forces acting on the mower. Calculate
(b) the horizontal friction force on the mower,   
(c) the normal force exerted vertically upward on the mower by the ground.   
(d) What force must the person exert on the lawn mower to accelerate it from rest to 1.5 $\mathrm{~m} / \mathrm{s} $ in 2.5 seconds, assuming the same friction force?   

  Two snowcats tow a housing unit to a new location at McMurdo Base, Antarctix;kknxh() r2g ca, as shown in Fig. 4-46)x;h kg kn2x(r. The sum of the forces $\overrightarrow{\mathbf{F}}_{\mathrm{A}}$ and $\overrightarrow{\mathbf{F}}_{\mathrm{B}} $ exerted on the unit by the horizontal cables is parallel to the line L , and $ F_{\mathrm{A}}=4500 \mathrm{~N} $. Determine $ F_{\mathrm{B}}$ and the magnitude of $ \overrightarrow{\mathbf{F}}_{\mathrm{A}}+\overrightarrow{\mathbf{F}}_{\mathrm{B}}$ .$F_B$ =    $\overrightarrow{\mathbf{F}}_{\mathrm{A}}+\overrightarrow{\mathbf{F}}_{\mathrm{B}}$ =   

A train locomotive is p*p9 .k,zvy/xx vbe9zh ulling two cars of the same mass behind it, Fig. 4-47 . Determine the ratio of t9 he.v ,9v *xxzypkz/bhe tension in the coupling between the locomotive and the first car $\left(F_{\mathrm{T} 1}\right)$ , to that between the first car and the second car $\left(F_{\mathrm{T} 2}\right)$ , for any nonzero acceleration of the train.

  A window washer pulls herself uhoi l wdximptc8s j;6aj *(dz-:( 3pev24ly k;pward using the bucket-pulley apparatus shown in Fig. 4-48. (a) How hard must she pull downward to raise herself slowly at constant spmjx2pyls:delo vit(ah 6c*- dzp; w 34j(8;ik eed?   
(b) If she increases this force by 15 % , what will her acceleration be? The mass of the person plus the bucket is 65 kg .   

  At the instant a race bebhy-uv i.p7i xs+6j 6ngan, a 65$-\mathrm{kg}$ sprinter exerted a force of 720 N on the starting block at a 22$^{\circ}$ angle with respect to the ground. (a) What was the horizontal acceleration of the sprinter?    (Keep one decimal place)
(b) If the force was exerted for 0.32 s , with what speed did the sprinter leave the starting block?    (Keep one decimal place)

Figure 4-49 shows a block (m8a -an ,xkp-dv/+lv(mvn a im)jxu0r:ass $ m_{1}$ ) on a smooth horizontal surface, connected by a thin cord that passes over a pulley to a second block $\left(m_{2}\right)$ , which hangs vertically. (a) Draw a free-body diagram for each block, showing the force of gravity on each, the force (tension) exerted by the cord, and any normal force. (b) Apply Newton's second law to find formulas for the acceleration of the system and for the tension in the cord. Ignore friction and the masses of the pulley and cord.

Three blocks on a frictionless horizon n vnuu/gclg 1(;lurj/9wx *f6tal surface are in contact with each other, 9n /uwvlc6ur/n lu1f;gg( xj*as shown in Fig. 4-51. A force $\overrightarrow{\mathbf{F}}$ is applied to block 1 (mass .$m_{1}$) .
(a) Draw a free-body diagram for each block. Determine
(b) the acceleration of the system (in terms of $m_{1}$, $m_{2}$ , and $ m_{3}$ ),
(c) the net force on each block,
(d) the force of contact that each block exerts on its neighbor.
(e) If $m_{1}=m_{2}=m_{3}=12.0 \mathrm{~kg}$ and $ F=96.0 \mathrm{~N}$ , give numerical answers to (b),(c) , and (d) . Do your answers make sense intuitively?

  The two masses shown in Fig. 4-52 aroaay0ba7: 7us e each initially 1.80 m above the ground, and the massless frictionless pulley is 4.8 m above the ground. What maximum height does the lighter object reachaa: b77ay0uos after the system is released? [Hint: First determine the acceleration of the lighter mass and then its velocity at the moment the heavier one hits the ground. This is its "launch" speed. Assume it doesn't hit the pulley.]   

Suppose two boxes on a frictionless table are connected by a hqs w15v38nel9zk p5qh eavy cord of mass 1.0 kg . Calculate the acceleration of each box and the tension at each end of the cord, usinvpz3e9 s1ln8 qk5wqh5 g the free-body diagrams shown in Fig. 4-53. Assume $F_{\mathrm{P}}=40.0 \mathrm{~N}$ , and ignore sagging of the cord. Compare your results to Example 4-12 and Fig. 4-22.
(4-53)
(4-22)
(4-12)

If the coefficient of kinetic friction between a kos -8b.j+ ,gv 97 kxhn*nejoc 35$-\mathrm{kg}$ crate and the floor is 0.30 , what horizontal force is required to move the crate at a steady speed across the floor? What horizontal force is required if $\mu_{\mathrm{k}}$ is zero?

  A force of 48.0 N is rrf16d5iz wki + 4pp hx9t.ft5t1lnogqu5d.+ qequired to start a 5.0$-\mathrm{kg}$ box moving across a horizontal concrete floor. (a) What is the coefficient of static friction between the box and the floor?   
(b) If the 48.0$-\mathrm{N}$ force continues, the box accelerates at 0.70$ \mathrm{~m} / \mathrm{s}^{2}$ . What is the coefficient of kinetic friction?   

  Suppose that you are standing on a tr/y18 ao- rvnh 9e7lclkain accelerating at 0.20g. What minimum coefficient of static friction must exist between yo 1 7nlhe8/vlk a9yorc-ur feet and the floor if you are not to slide?   

  What is the maximum acceleration a car canlpk/+y k1 y1sv undergo if the coefficient of static frs1l kpv/+y1 ykiction between the tires and the ground is 0.80?   

  The coefficient of static friction between hard rubber and normal street pmq;s :2h e*hws+znj-l avement is about 0.8. On how steep a hill (maximum angle) can you qmss+2*: n;wlz jh -ehleave a car parked?   $^{/circ}$

  A 15.0$-\mathrm{kg}$ box is released on a 32$^{\circ}$ incline and accelerates down the incline at 0.30 $\mathrm{~m} / \mathrm{s}^{2}$ . Find the friction force impeding its motion.    What is the coefficient of kinetic friction?   

  A car can decelerate at 8m/vmjsd h9 /oosw51i -4.80 $\mathrm{~m} / \mathrm{s}^{2}$ without skidding when coming to rest on a level road. What would its deceleration be if the road were inclined at 13$^{\circ} $ uphill? Assume the same static friction coefficient.   

(a) A box sits at rest onpldku+yl u -27uy1 pzg8;j4e g a rough 30$^{\circ}$ inclined plane. Draw the free-body diagram, showing all the forces acting on the box.
(b) How would the diagram change if the box were sliding down the plane?
(c) How would it change if the box were sliding up the plane after an initial shove?

  Drag-race tires in contact with an asphalt surface have a very high coeffic :b74ur6i h/-tk(gkv vzbr)my0c : duhient of staticv:7/d:kckm 4r-v0htgzu b(b yu)ri h6 friction.

Assuming a constant acceleration and no slipping of tires, estimate the coefficient of static friction needed for a drag racer to cover 1.0 km in 12 s , starting from rest.   

  The coefficient of kineticniki xei0xr0b.:96 h m friction for a $ 22-\mathrm{kg}$ bobsled on a track is 0.10 . What force is required to push it down a $6.0^{\circ}$ incline and achieve a speed of 60 $\mathrm{~km} / \mathrm{h}$ at the end of 75 m ?   

  For the system of Fig. 4-32 (Example xqo3z xq *94s 9ene/ mxmzm4q54-20) how large a mass would box A have to have/s oqx xz5xemnq m9eq4z4 39m* to prevent any motion from occurring? Assume $\mu_{\mathrm{s}}=0.30$ .   

  A box is given a push so that it slides across the floor. Ho/ pgnfz2 0cd,ew far will it go, given that the coefficient of kinetic friction is c2/gn,f d0zep 0.20 and the push imparts an initial speed of 4.0 $\mathrm{~m} / \mathrm{s}$ ?   

  Two crates, of mass 75 kg and 11d j3-ik4,r+p qqx/np f0 kg , are in contact and at rest on a horizontal surface (Fig. 4-54).jfni /,4prqq p-3 kd+x A 620$-\mathrm{N}$ force is exerted on the 75$-\mathrm{kg}$ crate. If the coefficient of kinetic friction is 0.15 , calculate
(a) the acceleration of the system,   
(b) the force that each crate exerts on the other.   
(c) Repeat with the crates reversed.   

  A flatbed truck is carrying a heavy crate. The coefficient c1 1cfar),rflof static friction between the crate and the brc alf11r,c)fed of the truck is 0.75. What is the maximum rate at which the driver can decelerate and still avoid having the crate slide against the cab of the truck?   

On an icy day, you worubb;helwwi /7+ zy8:p-xr;ps ry about parking your car in your driveway, which has an incline of 1p-zw r/:+yp x; 8bel;whus7i b2$^{\circ}$ . Your neighbor's driveway has an incline of 9.0$^{\circ}$ , and the driveway across the street is at 6.0$^{\circ}$ . The coefficient of static friction between tire rubber and ice is 0.15 . Which driveway(s) will be safe to park in?

  A child slides down a slr3jiwgt.1p (tide with a 28$^{\circ}$ incline, and at the bottom her speed is precisely half what it would have been if the slide had been frictionless. Calculate the coefficient of kinetic friction between the slide and the child.   

  The carton shown in Fig. 4-55 lies on a plane tilted at an+aamk e ;8gw5c:ssfi + angle $\theta=22.0^{\circ}$ to the horizontal, with $\mu_{\mathrm{k}}=0.12$ . (a) Determine the acceleration of the carton as it slides down the plane.   
(b) If the carton starts from rest 9.30 m up the plane from its base, what will be the carton's speed when it reaches the bottom of the incline?   

  A carton is given an initial sa z/-hto njl3.peed of 3.0 $\mathrm{~m} / \mathrm{s}$ up the 22.0$^{\circ}$ plane shown in Fig. 4-55.
(a) How far up the plane will it go?   
(b) How much time elapses before it returns to its starting point? Ignore friction.   

  A roller coaster reaches the top of the steepest hill with a speed ofxrw 6k/b.pn0( oa bx3q 6.0$ \mathrm{~km} / \mathrm{h}$ . It then descends the hill, which is at an average angle of 45$^{\circ}$ and is 45.0 m long. What will its speed be when it reaches the bottom? Assume $\mu_{\mathrm{k}}$=0.18    km/h

  An 18.0$-\mathrm{kg}$ box is released on a 37.0$^{\circ}$ incline and accelerates down the incline at 0.270 $\mathrm{~m} / \mathrm{s}^{2}$ . Find the friction force impeding its motion.    How large is the coefficient of kinetic friction?   

  A small box is held in place against a rough wall66 hij1ft02fysxp: yju.mk (h by someone pushing on it with a force directed1 t.u6hy6f js m:xjif(ky2h0p upward at 28$^{\circ}$ above the horizontal. The coefficients of static and kinetic friction between the box and wall are 0.40 and 0.30 , respectively. The box slides down unless the applied force has magnitude 13
N. What is the mass of the box?   

  Piles of snow on slippery roofsvkub j1+4 c ft lx:,ba2gt(yq4 can become dangerous projectiles as they melt. Consider a chunk of snow at the ridge of a rooy(u44vf 2ltb g1+ c ,:xktaqjbf with a pitch of $ 30^{\circ}$ .
(a) What is the minimum value of the coefficient of static friction that will keep the snow from sliding down?   
(b) As the snow begins to melt, the coefficient of static friction decreases and the snow eventually slips. Assuming that the distance from the chunk to the edge of the roof is 5.0 m and the coefficient of kinetic friction is 0.20 , calculate the speed of the snow chunk when it slides off the roof.   
(c) If the edge of the roof is 10.0 m above ground, what is the speed of the snow when it hits the ground?   

(a) Show that the minimum stopping0rxsy,/m) yp* ll- po0l /ekgqs+gq4w distance for an automobile traveling at speed v is equal0yw p4-+gsm/l*e l,r )lq pky x/sqg0o to $v^{2} / 2 \mu_{\mathrm{s}} g $, where $ \mu_{\mathrm{s}}$ is the coefficient of static friction between the tires and the road, and g is the acceleration of gravity.
(b) What is this distance for a 1200$-\mathrm{kg}$ car traveling 95 $\mathrm{~km} / \mathrm{h}$ if $\mu_{\mathrm{s}}=0.75$ ?

  A coffee cup on the dashboarut /djl1u+ xwh /zjq8:d of a car slides forward on the dash when the driver decelerates frouh8t/zdx :jq 1+l/wuj m 45 $\mathrm{~km} / \mathrm{h}$ to rest in 3.5 s or less, but not if he decelerates in a longer time. What is the coefficient of static friction between the cup and the dash?   

A small block of mass m is givi- )ds/sd+r:gpg8qo 1ytw)quen an initial speed $ v_{0} $ up a ramp inclined at angle $\theta $ to the horizontal. It travels a distance d up the ramp and comes to rest. Determine a formula for the coefficient of kinetic friction between block and ramp.

  The $75-\mathrm{kg}$ climber in Fig. 4-56 is supported in the "chimney" by the friction forces exerted on his shoes and back. The static coefficients of friction between his shoes and the wall, and between his back and the wall, are 0.80 and 0.60 , respectively. What is the minimum normal force he must exert? Assume the walls are vertical and that friction forces are both at a maximum. Ignore his grip on the rope.   

  Boxes are moved on a conveyor belt from where they are filled to the dmfw0) is 2q7i 1mpdl5packing station 11.0 m away. The belt is initially stationary and must finish with zero speed. The most rapid transit is accomplished if the m5 2pqidl)0fi mdws 71belt accelerates for half the distance, then decelerates for the final half of the trip. If the coefficient of static friction between a box and the belt is 0.60, what is the minimum transit time for each box?   

A block (mass $m_{1}$ ) lying on a frictionless inclined plane is connected to a mass $ m_{2}$ by a massless cord passing over a pulley, as shown in Fig. 4-57. (a) Determine a formula for the acceleration of the system of the two blocks in terms of $ m_{1}$,$ m_{2}$,$ \theta $ and g . (b) What conditions apply to masses $m_{1}$ and $m_{2}$ for the acceleration to be in one direction (say, $m_{1}$ down the plane), or in the opposite direction?

  Suppose the coefficient of kinetic friction between - ttpfn*s96g2a s80*scpzm fm $m_{1}$ and the plane in Fig. 4-57 is $ \mu_{\mathrm{k}}=0.15$ , and that $m_{1}=m_{2}=2.7 \mathrm{~kg}$ . As $m_{2}$ moves down, determine the magnitude of the acceleration of $ m_{1}$ and $m_{2}$ , given $\theta=25$ ?   
(b) What smallest value of $\mu_{\mathrm{k}}$ will keep this system from accelerating?   

  A bicyclist of mass 65 kg (including t/)ezee19uu3sb* t(ggzmh m: mhe bicycle) can coast down a 6.0$^{\circ} $ hill at a steady speed of 6.0 $\mathrm{~km} / \mathrm{h}$ because of air resistance. How much force must be applied to climb the hill at the same speed and same air resistance?   

  According to a simplified model of a mammalian heart, at each pul23qds5r-ygo0ii odgj 4cl7a6 se approximately 20 5-32ggdj67oaq0 lcrs 4ydi oi g of blood is accelerated from 0.25 $\mathrm{~m} / \mathrm{s}$ to 0.35 $\mathrm{~m} / \mathrm{s}$ during a period of 0.10 s . What is the magnitude of the force exerted by the heart muscle?   

  A person has a reasonable chance of ;+ u3ivb3 ha2eo8cgov surviving an automobile crash if the deceleration is no more than 30 " g 's." Calculate thh8ob v ou;33+vc i2aege force on a 70$-\mathrm{kg}$ person undergoing this acceleration. What distance is traveled if the person is brought to rest at this rate from 100 $\mathrm{~km} / \mathrm{h}$ ?   

If the horizontal acceleration produced by an earthquake is a , an qp,3m f ggsj1(2gexqpqt8r *4d if an object is going to "holfr( 3,j1g* p8psq4gg2qqexm td its place" on the ground, show that the coefficient of static friction with the ground must be at least $\mu_{\mathrm{s}}=a / g$ . (b) The famous Loma Prieta earthquake that stopped the 1989 World Series produced ground accelerations of up to 4.0 $\mathrm{~m} / \mathrm{s}^{2}$ in the San Francisco Bay Area. Would a chair have started to slide on a linoleum floor with coefficient of static friction 0.25 ?

  An 1150$-\mathrm{kg} $ car pulls a $450-\mathrm{kg} $ trailer. The car exerts a horizontal force of 3.8$ \times 10^{3} \mathrm{~N}$ against the ground in order to accelerate. What force does the car exert on the trailer? Assume an effective friction coefficient of 0.15 for the trailer.   

  Police investigators, examining the scene of an accident involving two cars, m9fqvk-c 1tt u*7;xwxieasure 72-m-long skid marks of one of the cars, which nearly came to a stop before colliding. The coefficient of kinetic friction w-cx*q;ut1fv x t9k7ibetween rubber and the pavement is about 0.80. Estimate the initial speed of that car assuming a level road.   

  A car starts rolling down a 1-in-4 hill (1-in-4 meanshyk2c;s m0m 2ntvo9 y3 that for each 4 m traveled along the road, the elevation change is 1 m). How fast is it going when it reaches the bottom after trav9;k ocnsthy2v3 2m 0myeling 55 m? (a) Ignore friction.   
(b) Assume an effective coefficient of friction equal to 0.10.   

  A 2.0-kg purse is dropped from the top of the Leaning Tower of sws - xn2sl2-af8 au z-6lvc3pPisa and falls 55 m beforeflsa8v-x-zpwn3 usl c -26sa2 reaching the ground with a speed of What was the average force of air resistance?   

  A cyclist is coasting at a sterb:ka c3 2ol.fk-*w 2rxhzro0ady speed of 12m/s but enters a muddy stretch where the effective coefficient of friction is 0.60. Will the cyclist emerge from t-h w2ror3kx:r2az k 0locbf* .he muddy stretch without having to pedal if the mud lasts for 11 m? If so, what will be the speed upon emerging?   

  A city planner is working on the redesign +dw 6d2lw*mq rof a hilly portion of a city. An important consideration is hd 2l6w*qrwmd+ ow steep the roads can be so that even low-powered cars can get up the hills without slowing down. A particular small car, with a mass of 1100 kg , can accelerate on a level road from rest to 21$ \mathrm{~m} / \mathrm{s}(75 \mathrm{~km} / \mathrm{h})$ in 14.0 s . Using these data, calculate the maximum steepness of a hill.    $^{\circ}$

  Francesca, who likes physics experiments, dangles her watch from a tzodmxuz r,7d.gf2u* v1xz ;m-hin piece of string while the jetliner she is in takes off from JFK Airport (Fig. 4-58). She notices that the string makes an anxrv*dg 2zzf d;uo mum,z.7x1 -gle of 25$^{\circ}$ with respect to the vertical as the aircraft accelerates for takeoff, which takes about 18 s . Estimate the takeoff speed of the aircraft.   

  A 28.0$-\mathrm{kg}$ block is connected to an empty 1.35$-\mathrm{kg}$ bucket by a cord running over a frictionless pulley
(Fig. 4-59). The coefficient of static friction between the table and the block is 0.450 and the coefficient of kinetic friction between the table and the block is 0.320 . Sand is gradually added to the bucket until the system just begins to move. (a) Calculate the mass of sand added to the bucket.   
(b) Calculate the acceleration of the system.   

  In the design of a supermarket, there are to be sf6sc(ka 9jf5,qfjm 7leveral ramps connecting different parts oql kffj(cja,6f9 5m7sf the store.
Customers will have to push grocery carts up the ramps and it is obviously desirable that this not be too difficult. The engineer has done a survey and found that almost no one complains if the force directed up the ramp is no more than 20 N . Ignoring friction, at what maximum angle $\theta$ should the ramps be built, assuming a full $30-\mathrm{kg}$ grocery cart?    $^{\circ} $

(a) What minimum force F is needed to lift the pianflvp n n-nt: 9pw fp03lo2/m40zvif 4bo (mass M ) using the pulley appa-:o/ l w2t 0fm3p vfnpfp9ibv04 4nlnzratus shown in

Fig. 4-60? (b) Determine the tension in each section of rope: $F_{\mathrm{T} 1}, F_{\mathrm{T} 2}, F_{\mathrm{T} 3} $, and $ F_{\mathrm{T} 4}$ ,

  A jet aircraft is accelerating at 3.5 /ft2x ,sjzdn xl:0 vo;$\mathrm{~m} / \mathrm{s}^{2}$ at an angle of 45$^{\circ} $ above the horizontal. What is the total force that the cockpit seat exerts on the 75$-\mathrm{kg} $ pilot?    N    $^{\circ} $

  In the design process for a child-restraint chair, an enginehp+*uq (o35 kg8tc acrz--tu per considers the following5(*aq8uoz+- 3 kutt-cr gh pcp set of conditions:
A 12$-\mathrm{kg}$ child is riding in the chair, which is securely fastened to the seat of an automobile (Fig. 4-
61). Assume the automobile is involved in a head-on collision with another vehicle. The initial speed $v_{0}$ of the car is 45$ \mathrm{~km} / \mathrm{h}$ , and this speed is reduced to zero during the collision time of 0.20 s .

Assume a constant car deceleration during the collision and estimate the net horizontal force F that the straps of the restraint chair must exert on the child in order to keep her fixed to the chair. Treat the child as a particle and state any additional assumptions made during your analysis.   

  A 7650$-\mathrm{kg}$ helicopter accelerates upward at 0.80 $\mathrm{~m} / \mathrm{s}^{2}$ while lifting a 1250$-\mathrm{kg}$ frame at a construction site, Fig. 4-62.
(a) What is the lift force exerted by the air on the helicopter rotors?   
(b) What is the tension in the cable (ignore its mass) that connects the frame to the helicopter?   
(c) What force does the cable exert on the helicopter?   

  A new high-speed 12-ck 836jjsw)ci2)y7zne ch*ust n (pbg cc61s9car Italian train has a mass of 660 metric tons (660,000 $\mathrm{~kg}$) . It can exert a maximum force of 400 kN horizontally against the tracks, whereas at maximum velocity ( 300 $\mathrm{~km} / \mathrm{h}$) , it exerts a force of about 150 kN . Calculate (a) its maximum acceleration   
(b) estimate the force of air resistance at top speed.   

  A 65$-\mathrm{kg}$ ice skater coasts with no effort for 75 m until she stops. If the coefficient of kinetic friction between her skates and the ice is $\mu_{\mathrm{k}}=0.10$ , how fast was she moving at the start of her coast?   

Two rock climbers, Bill and Karen, use safety ropes o/rdp1x0:ydld26adq wo (( r3s- zo kpef similar length. Karen's rope is more elastic, called a dynamic rope by climbers. Bill has a static rope, not recommended for safety purposes in pro climbing. Karen falls freely about 2.0 m and then the rope stops her over a distance of 1.0 m (Fig. 463). (a) Estimate, assuming that the force is constant, how large a force she will feel from the rope. (Express the result in multiples of her weight.) (b) In a similar fall, Bill's rope stretches 6(l3-wed0yx1qo:zar 2/ ddso kpp (r dby 30 cm only. How many times his weight will the rope pull on him? Which climber is more likely to be hurt?

A fisherman in a boat is ; -cv te jmw)9)zjnw 8lq.hw6qusing a " 10-1 $\mathrm{~b}$ test" fishing line. This means that the line can exert a force of 45 N without breaking (1 $\mathrm{lb}=4.45 \mathrm{~N}$) . (a) How heavy a fish can the fisherman land if he pulls the fish up vertically at constant speed? (b) If he accelerates the fish upward at 2.0 $\mathrm{~m} / \mathrm{s}^{2}$ , what maximum weight fish can he land? (c) Is it possible to land a 15-lb trout on 10-1b test line? Why or why not?

  An elevator in a tall building is allowed to re,wg2k 8*poj yncg-cj0ach a maximum speed of 3.5 $\mathrm{~m} / \mathrm{s}$ going down. What must the tension be in the cable to stop this elevator over a distance of 2.6 m if the elevator has a mass of 1300 kg including occupants?   

  Two boxes, $m_{1}$=1.0 $\mathrm{~kg}$ with a coefficient of kinetic friction of 0.10 , and $m_{2}=2.0 \mathrm{~kg}$ with a coefficient of 0.20 , are placed on a plane inclined at $ \theta=30$ ?
(a) What acceleration does each box experience? $a_1$ =    $a_2$ =   
(b) If a taut string is connected to the boxes (Fig. 4-64), with $ m_{2}$ initially farther down the slope, what is the acceleration of each box?
(c) If the initial configuration is reversed with $m_{1}$ starting lower with a taut string, what is the acceleration of each box?   

  A 75.0$-\mathrm{kg}$ person stands on a scale in an elevator. What does the scale read (in N and in kg ) when the elevator is
(a) at rest,   
(b) ascending at a constant speed of 3.0 $\mathrm{~m} / \mathrm{s}$ ,   
(c) falling at 3.0 $\mathrm{~m} / \mathrm{s}$ ,   
(d) accelerating upward at 3.0 $\mathrm{~m} / \mathrm{s}^{2}$ ,   
(e) accelerating downward at 3.0 $\mathrm{~m} / \mathrm{s}^{2}$ ?   

  Three mountain climbers who are roped togzfnji*8be cz1;isci*. .vo, tether are ascending an ice field inclined at 2.18et **iiic j fozvs;n .,cbz1.0$^{\circ}$ to the horizontal. The last climber slips, pulling the second climber off his feet. The first climber is able to hold them both. If each climber has a mass of 75 kg , calculate the tension in each of the two sections of rope between the three climbers. Ignore friction between the ice and the fallen climbers.