Why does a child in a wagon seem to fall backwarvnj/nia 1d-or 96yg n 3mw;(cqd when you give the wagon a sharp pull forward w3m/ngyno n1v;-j(r d q6aic9?

A box rests on the (frictioh24r6 izj2 rjdnless) bed of a truck. The truck driver starts the truck and accelerates forward. The box immediately starts 24ihr jj6 zrd2to 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 e8lo)r yjn;wrki 0r-j076uhdan object is zero, are no forces acting on it? Explain.

Only one force acts on an obj4jizxue o+e: qs0x)ocu5* 2qzect. Can the object have zero acceleration? Can it have zero velocity? Exz: xuuq+qsc j *5)20eeo oxi4zplain.

When a golf ball is dropped to the pavement, it sz4e pg9vvu3:;8 8 cwirxiri1bounces back up. (a) Is a force needed to make it bounce back up? (b) If so, what exerts thv1pz4re;v3sii u8 :igxc9 w8re force?

If you walk along a log floating on a lia*yszv-(x- l v5lyr0c)3yjp ake, why does the log move in the opposite *0-j l(y-5)yv vz3aprxcisyl direction?

Why might your foot hurt if you kick a heavy desk7wvk3: n/ dun)b252c /q(q oaxfxvfll or a wall?

  When you are running and want to sto)i kaq*sye2wovau *7q dg;-l9 p quickly, you must 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 at top speed to come to reqw7a ;-vs 9qeoyiul da*g)k *2st.    m/s$^2$

A stone hangs by a fine thread from the ceiling, ando5zn+44h)( ygl/ha6jzlrjy v 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 the person gives a slow and stead4j+(65 zn zgory/v 4) jyhhllay pull? Explain your answers.

The force of gravity on a 2-kg rock is twice as great as that on a 1-kg rock. Whhl(jksfqk84 */qql 7f0 la+xx y then doesn’t the heavier rock fall fasteh x4qqf7k/ l*+x8jklsl(q af0 r?

Would a spring scale carried to thev,m y:6emtdm b:f*v0 i Moon give accurate results if the scale had been calibrated (a) in pounds, or (b) in kilograms?fy::vm,v m t0mie*6db

You pull a box with a constant force across a frictionless table usis ky u c;4l,59xbvweq2ng an attached rope held horizontally. If you now pull the rope with the same force at an angle to the horizontaq9;x5w,b2k cu l4yevsl (with the box remaining flat on 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 gravitx pslo0k- vm9q/o8f+c e*vhq-y there is a net force mg exerted on it by the Earth. Yet by Newton’s third law the object exerts an eqlxq vk9o--he*fc sm+8 /ov0p qual and opposite force on the Earth. Why doesn’t the Earth move?

Compare the effort (or force) needed to lift a 10-kg obj2vq3+xix .ar14yzr)8 2de yk+jcd z dzect when you are on the Moon with the force needed to lift it on Earth. Compare the fz de ryac 8)vi3.4dxyzr12zkq +j+2 xdorce needed to throw a 2-kg object horizontally with a given speed on the Moon and on Earth.

According to Newton’s third law, each team in a tug of war (Fig. 4–37)(f k)lqtm p;vb ,az9h3 pulls with equal force on the other team. Whk,)p abm f;(93hlzv qtat, then, determines which team will win?

A person exerts an upward force of 4yj7km9o 4,s-o3e54 zt eveug d0 N to hold a bag of groceries. Describe the “reaction” force (Newton’s third law) by stating (a) its magnitude, (b) z9dvu 3mogojees5et, 44-7kyits direction, (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 the ground exerb7v3j,hswke 4p edfa1)8fkv)i(gb 8pt on you? Why doesn’t this force make you rise up into the air?3 p1ed)wjb8vfk7,4 hfeag 8skpbv( i)

Whiplash sometimes results from an automobile accident when the vicik) ln zxkvf* (s1:fr8tim’s car is struck violently from the rear. Explain why the head of the victim seems to be th:k)rikf n1 szv8lf(*xrown backward in this situation. Is it really?

A heavy crate rests on the bed of a flatbed truck. When the truck accelerates+pvhddf1bd 7kc fh 1 *x4)/bmd, thckd+d b 7hdxhf d*)4 vb1pf1m/e crate remains where it is on the truck, so it, too, accelerates. What force causes the crate to accelerate?

A block is given a push so that ,y pv25 wx5m2:ydfaon it slides up a ramp. After the block reaches its highest point, it slides back down but the magnid wonampfx ,2y2 5:vy5tude of its acceleration is less on the descent than on the ascent. Why?

What would your bathroom scale read if you weighed yourself on an inclined plany7o9sz as of::po9+ jio)xns4e? Assup)y szj: 4xo+an oiso997s: fome the mechanism functions properly, even at an angle.

  What force is needed to acclg rez*zh g3j- 6s+muxy0 2-axelerate a child on a sled at 1.25m/s$^2$?   

  A net force of 265 N accelerates a bike and rider5. xmaa:kj,2 ngjo: bu 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 97tnk.f vgu7eif it 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 nb7j tmvo/ *3z;2af8vlyih u: 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 pwfj-2u8 tf- fgp rew++ (kj.ean 1100-kg car in 8.0 s if the car is traveling at 95km/h?(u krfjj.f- t+ -ewg8f+e 2ppw -  

  What average force is needed to accelerate a 7.00-gram pelle j 9ap3spy:64qrllm nohe3,w3t from rest to 125m/s over a distance of oa,qp e46: jpw 3lly3s9nmr3h0.800 m along the barrel of a rifle?   

A fisherman yanks a fish vvq) oaw)pp( o+z bfr)5ertically out of the water with an acceleration of 2.p +)qowz)fa)5vporb (5 $\mathrm{~m} / \mathrm{s}^{2}$ 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 quat-:3+dfq qei1z +t emc6ax ;.qp*kbxsrter-mile track (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 designed so that the maximum accele0xu+ e) n i*yc+yo5jk z)gv(jeration is gk0u)5)xey*ocvjnjyze+ i (+0.0680 g . What are 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 sckmh(1q4 xjbo2di 2j a3ale in a motionless elevator. When the elevator begins to move, the scale briefly readd 2qj4ox3hi2am1(kbj s only 0.75 of the person’s regular weight. Calculate the acceleration of the elevator, and find the direction of acceleration.     ----待选择----downupward 

  The cable supporting a 2123,b gj ms.ro0jwu 0a6x5-kg elevator has a maximum strength of 21,750 N. What maximum upwam0 b.rg a6xj 3jw,o0surd acceleration can it give the elevator without breaking?    ( two decimal places)

  (a) What is the acceleration of twk b1q8 sm-crg4o falling sky divers (mass 132 kg including parachute) when the 18mqk-rgb 4sc upward force of air resistance is equal to one-fourth of their weight?   
(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 house 3.9-m high. When he /t3yp6irq 8 lkstrikes the ground below, he bends his knees so that his torso decelerates over an approximate distance of 0.70 m. If the mass of his torso (excluding legs)rl tpi68qy/3 k 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. A rope tied to the box run ;cczr-f : )gork75neus vertically upwa)r r5fko n7cu-c;ez:grd over a pulley and a weight is hung from the other end (Fig. 4-40). Determine 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 bas;*f t0 :7f0b2b qagas-ho jltmketball player
(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 moment it is hit by the bfhmq 7wy++g1(m+j lf uat, and again (b) after it has left the bat and is flying toward the outfilm7++f+h wujy1mf( gqeld.

A 650-N force acts in a northwesterly direction. A seconl2 aufv; +5pl9gccpd 9d 650-N force must be exerted in what direction so that the resultant of the two forces points westward? Illustrate your answer with a vector dialcp2g+laf c;v u9pd 59gram.

  Arlene is to walk across a "high wi hkn9g7n ;i; hei/b7sore" strung horizontally between two buildings 10.0 m apart. The sag in the rope when she is at the midpoint is 97 ehoins7khbg;/i;n10.0$^{\circ}$ 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 -a ,x 5+vv9yzsp+lpmf is hanging by a massless cord from another 3.2$-\mathrm{kg}$ 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 14.0s 10;1hvrnp ocxea6j: $-\mathrm{kg}$ 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, Antarck ;dhem4 iahw;)g2 8yytica, as shown in Fig. 4-46. The sum ohw yei; ;d4y2 hak)8mgf 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 pulling two cars of the same mass behindkwnx.8gtsr09 f+v vty*v/ o .f (hlzo2 it, Fig. 4-47 . Determine the ratio of the tension in the coupling between the locomotive and the firstn2 98tzvsyx/hg(fkf *o.v+ltv.rw 0o 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 u .4mn3rebizo ff;0xd*6a g edc7y0a*s pward using the bucket-pulley apparatus shown in Fig. 4-48. (a) How hard must she pull downward to raise herself slowly at constant spaa0ee rm.7 ox* *06ffbci4gy d;d3zn seed?   
(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 bff: m*i/1plb)qw cvt )egan, 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 (mzrwkfjb9 k 1mh42vex:5qfakj .h r1-/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 96 bokh jqzot ;rklg*b:1j /0ghorizontal surface are in contact with each other, as sh1gkh *oljqzgk/9o0t6rb b ; :jown 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 are each initially 1.80 m above the 2xy:x8 (qqy/z)dagusb5s w4tground, and the massless frictionless pulley is 4.8 m above the8:4xdqs/5()u2 satgwzb yy q x ground. What maximum height does the lighter object reach 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 amgot*wr 5 leu-jy5.1y heavy cord of mass 1.0 kg . Calculate the acceleration of each box and the tension at each end of the cord, using the fre.wy-rojltmu g5*1ey5 e-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 fric) l: e2babp.kqtion between a 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 required to start a 5.0klc(pni-vy a 3w n1:qv2oc:+i$-\mathrm{kg}$ 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 train accelb 3v t6(4iyuk+kjx ;0sx8o2z cdcwi:n erating at 0.20g. What minimum coefficient of static frictu 0(kxnd328iv; c4w:6x +yojs ikcbztion must exist between your feet and the floor if you are not to slide?   

  What is the maximum acceleration a car can undetd(*e 5l01 i lhjz7(vkexyk )16z pqdgrgo if the coefficient of static friction between the tires and the ground is 0.e1g6xp(d1t(ji z l z*5e)h07dklvyq k80?   

  The coefficient of static friction between hard ruz3/qza zcq60z5knj1 cbber and normal street pavement is about 0.8. On how steep a hill (maximum angle) can you leave a carz cz 1n q6z5c0zjqka/3 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 -4.80pynbye :8v71 s, l5osg $\mathrm{~m} / \mathrm{s}^{2}$ 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 8zv0iaaq,*fo5* qufs u gz; d+on 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 coefficiecu y9a)8):s3armf0fmddde1he 2zw z.nt of static friction.dyz fac9:8he)mmawu) 02 r1e z.3fd ds

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 kilamze-jsedg-z .q4*d i zv*.. netic 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 (Exampl7f-v 1 cfecgd-p8uwe)e 4-20) how large a mass would box A have to have to prevent any motion fro w-c8 -ffe)evgucd7 1pm occurring? Assume $\mu_{\mathrm{s}}=0.30$ .   

  A box is given a push so thatd ,2dz ;np+4yudbprf- it slides across the floor. How far will it go, given that the coefficien- r,d;pnyfdz42bpu+d t of kinetic friction is 0.20 and the push imparts an initial speed of 4.0 $\mathrm{~m} / \mathrm{s}$ ?   

  Two crates, of mass 75 kg and 110 kg , are in con/,jndv,. xoe +iqdek3t:ri7 vtact and at rest on a horizontal surface (Fig. 4-54). d7qrxdt iei,ev3 +: vjn,/ko.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 coefficiev7)c zgsl+ie4m,f5 ksnt of static friction between the crate and the bed of the truck is 0.75. What is the maximum rate at which the driverci, lsk)7s ge+ 5vmf4z can decelerate and still avoid having the crate slide against the cab of the truck?   

On an icy day, you worry abouht4h6o + ppv of,5d0yl 4r+ghvt parking your car in your driveway, which has an inclinerpto+p+v ,5h4ylg f640hohdv of 12$^{\circ}$ . 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 ,pbqf t,d/h0a36v8dzn5v zq4mso u k/slide 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-dkncofezc *(uc d1 +7rlc1/(v0g t:qu 55 lies on a plane tilted at an 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 jpw6dkf*rb5kc/tbo x7 9 jw 0;on*v*lspeed 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 ,n1zqc9ifz,h vs.irhw;+mg8 of 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 1kdz6,ow pska(7 grp2a rough wall by someone pushing on it with a force k6wp72drkg a1 zs(, podirected 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 ruuq n7c,9u rm/oofs can become dangerous projectiles as they melt. Consider a chunk of snow at th7u ruuc,n/mq9e ridge of a roof 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 stopping distance for an automobile traveling +r a6( hp:fktozx;:wun5(i hxat speed v is equal t:w rh5;fuzhx px:nkta +i(6o( o $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 dashbo ,:wme) tvurz;rh pv(6ard of a car slides forward on the dash when the driver decelerates f emp)zr,v6tu :( ;hwvrrom 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 given an initial speed aw:*o+lc0ubg am1ocg ,wo+4j y- gm- z$ 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 :s(dr,ki/utpxwl15(57t3 kkbgh xz va5) x vbthe packing statiptsw(5huk vt z xi kbx5g,rd( 7xak)l31/5v :bon 11.0 m away. The belt is initially stationary and must finish with zero speed. The most rapid transit is accomplished if the belt 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 fn)zk e4ue bj4t t4jk0r2ploa+y 2s1(of kinetic friction between $m_{1}$ 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 the bicycle9t,mkgv sr 9 .drq*,rc) 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 pulse ap /-fmob:cz g.zproximately 20 g of blood is acc-fb.:mz oz g/celerated 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 chayw2cwz7 s0 x tmc90rtdr* y8e-nce of surviving an automobile crash if the deceleration is no more than 30 " g 's." Calculate the force on 0yy* 8d0rc- ewr m2twtcxs7z9 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 is1azk(7twe eg6 a , and if an object is going to "hold its plz67agwe1etk (ace" 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 anxgb j7 w4ws57x accident involving two cars, measure 72-m-long skid mw5gw xsb 47x7jarks of one of the cars, which nearly came to a stop before colliding. The coefficient of kinetic friction between 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-t xlsdut.qz -4rbw.8 54 means that for each 4 m traveled along the road, the elevation change is 1 m). How fast iz8d .4slw-.uxt5t b rqs it going when it reaches the bottom after traveling 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 j:(ge/w,) 2l eu itmv)xakm:fof Pisa and falls 55 m before reaching the ground with a speed of What was the average force of air resistance? le )mxwe :u tm:g( v2if/k,)ja  

  A cyclist is coasting at a steady speed of 12m/s but enters ando+rs )xg )b4 muddy stretch where the effective coefficient of friction is 0.60. Will the cyclist emerge from the muddy stretch without having to pedal if the mud lasts for 11 m? If so, what will be txs rgd) o)+4nbhe speed upon emerging?   

  A city planner is working on the redesign of a hilly portion of a city. An ih ;+pgg4vcne .mportant consideration is how 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 accelg n ;e4hp.cvg+erate 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 thiql/ukcb(v. c:igorh+. i7nt)n piece of string while the jetlinv7 k g(o inh.tur:)icb+cql./ er she is in takes off from JFK Airport (Fig. 4-58). She notices that the string makes an angle 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 several ramps connecting diff-+h+ bh:s ahv-t zr6nkerent parts of the st:sr 6-vhk+b n-h t+hzaore.
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 6 os05.4su;upr jwn btmh 61azlift the piano (mass M ) using the pulley apparatus shown 4o 01 tz;bm s6a6 nhuw.s5uprjin

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 afal t5q( yh-i)t 3.5 $\mathrm{~m} / \mathrm{s}^{2}$ 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 enginee f*0 07yf1jnmrq; nyumr considers the folmyn;u01jy fm 70 q*rfnlowing 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-car Italian train cvp,e 7*r+ischas 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 of similar length. Karen7v, lihan 7 a)is13nsv'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 by 30 cm only. How many times his weight3aa7vsn1n,i li h v7)s will the rope pull on him? Which climber is more likely to be hurt?

A fisherman in a boat ivae-)z t1e-cv s using 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 w1xfiwrsg+m18 u * z4wt*+sn vbuilding is allowed to reach 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 together are ascending an ice field inclind* 9ko ntpe75x -.zc-yr(*pp)ivspjxed at xkc pi*p 7*5rtzd)o p vjn 9p-.(yx-se21.0$^{\circ}$ 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.