In what ways is the word “work” as used in everyday language ts;m/d76b snbz 0dpf;t -yedn .he same as that defined in physics? In what ways is it different? Give examples pdznds 7-tn f de /sy60b;;mb.of both.

Can a centripetal force ever do work on an object l 3pcs(,)xrzq8iy,af? Explain.

Can the normal force on an object ever do work? Explain.2c pmq)v r)x/x

A woman swimming upstream is nq6l:slvkxh-sp6:zp / ot moving with respect to the shore. Is she doing any work? If she stops swimming asq6hp: : zllv/skp 6x-nd merely floats, is work done on her?

Is the work done by kinetic friction forces m j) j;qd0o42elbpq3w always negative? [Hint: Consider what happens to the dishesl2mejj 30)qb4;qw p do when you pull a tablecloth out from under them.]

Why is it tiring to push hard against a solid wall even though you -c(q) fw lyt h2u6r)ujare doing no workf)-6 uqh r 2uwly)(tjc?

You have two springs that are identical exce p.9 y)s bpldb:ato1f7pt that spring l is stiffer than spring 2fdlb1p7.osby9 ta) p: $(k_1>k_2).$ On which spring
is more work done $(a)$ if they are stretched using the same force, $(b)$ if they are stretched the same distance?

A hand exerts a constant horizontal force on a block that is free to slide o9; inhsd r gr294 .q-yhitmm.qn a frictionless surface (Fig.6-30). The block starts from rest at þoint A, and by the time it has traveled a y gq 4 9-;.9h2i mqnsrhrmid.tdistance $d$ to point B it is traveling withe bint $o$ point B it is traveling with speed $\nu_\mathrm{B}$.When the block has traveled another distance $d$ to point C, will its speed be greater than, less than, or equal to 2$\nu_\mathrm{B}$? Explain your reasoning

By approximately how much does your gra; r w7(m p:h/efju8zjrvitational potential energy change when you jumz m7frh;ep:8/w ujrj (p as high as you can?

In Fig. 6–31, water ballfud x, qu 8+.(absxc6loons are tossed from the roof of a building, all with the same speed but with different launch angles. Which one has the highest speed on impact? Ignore air rq+,s ucx.xa6 l(dfu b8esistance.

A pendulum is launched from a point that is a height h above its lowestlsp)ap hp-9v zvw b5b++ xl+e, point in two different ways (Fig. 6–32). During both launches, the pendulum is given an initial speed of 3.0m/s On the first launch, the initial velocity of the pendulum is directed upward alon,xh9+ l-+ +zlewv5b)a pvp bspg the trajectory, and on the second launch it is directed downward along the trajectory. Which launch will cause it to swing the largest angle from the equilibrium position? Explain.

A coil spring of mass m rests upright on a table. If you compress th9wcnpp )tx1in1s8p:gei nf 4: e spring by pressing down with your hand and then release it, ca)8 ipp 1c n1:: 49witgxsfpnnen the spring leave the table? Explain, using the law of conservation of energy.

A bowling ball is hung from the ceiling by a steel wire ( +z37 sxkb;wj.l fu:nwyh*k 9gl2in8jFig. 6–33). The instructor pulls the ball back and stands against the wall with the ball against his nose. To avoid injury the instructor +:; ng3kw lw.izsujbylhf 2kx 9*7n8jis supposed to release the ball without pushing it. Why?

What happens to the gravitational potential energy when water at 1868per,ig boex+sx uz o t: q2q0b1evthe top of a waterfall fallsx2u81 btzoi e,gs b: o1xqq8p0+6erev to the pool below?

Describe the energy :vxehp;e5tmcf ;/wn0transformations when a child hops around on a pogo stick.

Describe the energy transformations that take place when a r2rtg)5/pl f2ls tw k2op6c0jskier starts skiing down a hill, but after a time is brought to rest by s6k 02tr p)2/2to5 l jwcglpsfrtriking a snowdrift.

A child on a sled (total mass m) starts from rest at the top of a hill of h3y0snu+z0gclz ip3 ,qtj 13p:jrs m x8eight h and slides down. Does the velocity at x1,qi jpp nzz uy8+3sr03:cl0g jmts3the bottom depend on the angle of the hill if (a) it is icy and there is no friction, and (b) there is friction (deep snow)?

Seasoned hikers prefery:1n3 l2bvosg to step over a fallen log in their path rather than stepping on top and jumping down on the other side. Explaig3v:soy 12lbnn.

Two identical arrows, one with twice the speed o0 (e+u:irwmnzsf+ n5zf the other, are fired into a bale of hay. Assuming the hay exerts a constant frictional force on the arrows, the faster arrow wr:e 5m (ifn 0w+zs+nuzill penetrate how much farther than the slower arrow? Explain.

Analyze the motion of a simple swinging pendulum in terms of energy, (a) ignor r ozt;l7:2dqsing friction, and (b) taking frictio: 2d7oqrlz; tsn into account. Explain why a grandfather clock has to be wound up.

When a “superball” is dropped, can it rebound to a height greater than its o(*hatff.d5gj peq8o6ue(k, triginal hei( g fta e jpkq.5h(otefd*6u,8ght? Explain.

Suppose you lift a suitcase from the floor to :h:tewt49x 0 zouuek7 a table. The work you do on the suitcase depends on which of the following: (a) whether you lift it straight up or along a more complicated path, (b) the time it takes, (c) the hei h 0u7:w:t4kz9uoxte eght of the table, and (d) the weight of the suitcase?

Repeat Question 22 for the power needed rather th+(,ijk3bsp x/x tw,dyan the work.

Why is it easier to climb a m 692-z0j3.fwbe r2pc bswrhlj ountain via a zigzag trail than to climb straight upse3l z0frb wr6c2ph.b2-w jj9 ?

Recall from Chapter 4, Example 4–14, that you can use a pull) y84 wh;4iyjbl5nn(y qwj7beey and ropes to decrease the force needed to raise a heavy load (s4j y5yqb 7ilyewnjbw8) ( 4;nhee Fig. 6–34). But for every meter the load is raised, how much rope must be pulled up? Account for this, using energy concepts.

  How much work is done by the gravitational force when a 265-kg pile driv rvby3gmx 8pc 4q m)jwn.e(/.ler falls 2.8 cqe/ l3n8gbj)vx. 4(mymwp. r0 m?    J

  A 65.0-kg firefighter climbs a flight of stairs 20.0 m high. How much7c3fo5;xe qis work is requiree;i 35cs7 fqoxd?    J

  A 1300-N crate rests on the floor. How much work is requi* li*p;hvi oot) j cut*.0xmq/red to move it at constant spt c;)*q.jt xolv hoim*/i p0u*eed (a) 4.0 m along the floor against a friction force of 230 N,    J
(b) 4.0 m vertically?    J

  (I) How much work did the movers do (horizontally) pushing a 160-kg crate hv/iia:rh 2 t*10.3 m across a rough floorh:ahi2i /tv *r without acceleration, if the effective coefficient of friction was 0.50?    J

  A box of mass 5.0 kg is acc -g+rc1xju4 a.k.m zcy4 a-iicelerated by a force across a floor at a rate of for 7.0 s. Find the net w4 + .g-ux4a.cc aii1mjrzykc -ork done on the box.   J

  Eight books, each 4.3 cm thick with mass 1.7 kg, lie flat on a table. Hon ;ntwpm ;fglr*/ -/dfw much work is required to stackd t;g/n pmw f/*;fr-ln them one on top of another?    J

A lever such as that shown in Fig. 6-35 can be usec)vpy eqmea,j2yc7g c1t b51+n cn; t(d to liff objects we might not otherwise be able to liff. Show that the ratio of output forc q( 7;yc tj 5+ga1,cntycbcpne)ve2m1e, $F_{\mathrm{O}}$, to input force, $F_{\mathrm{I}}$, is related to the lengths $\iota_\mathrm{I}$ and $\iota_\mathrm{o}$ from the pivot point by $F_{\mathrm{O}}/F_{\mathrm{I}}=l_{\mathrm{I}}/l_{\mathrm{O}}$ (ignoring friction and the mass of the lever), given that the work output equals work input.

  A 330-kg piano slides 3.6 m down ihn:wmtrdu8s 30 2q, ma 28° incline and is kept from accelerating by a man who is pushing back on it parallel to the incline (Fig. 6-36). The effective coefficient ofq0trmnsd8h ,3wm2iu : kinetic friction is 0.40. Calculate:
(a) the force exerted by the man,    N
$(b)$ the work done by the man on the piano,    J
$(c)$ the work done by the friction force,   J
$(d)$ the work done by the force of gravity,   J
$(e)$ the net work done on the piano   J

  (a) Find the force required to give a helico p k8mm.7of9v d*r,gocpter of mass M an acceleration of 0.10 g *dop7 vo.kc m8r fm,g9upward.    Mg
(b) Find the work done by this force as the helicopter moves a distance h upward.    Mgh

  $\text{What is the minimum work needed to push a 950-kg car 810 m up along a 9.0$^{\circ} $ incline? }$$(a)\text{ Ignore friction. }$    J
$(b)\\\text{Assume the effective coefficient of friction retarding the car is }0.25.$    J

  In Fig. 6-6a, assume the distance axi3r 6bf:r9nt f)1ufh n;a ,1)zwg(yfgz wt6qvms is linear and that $d_{\mathrm{A}}=10.0$ m and $d_{\mathrm{B}}=35.0$m. Estimate the work
done by force $F$ in moving a 2.80-kg object from $d_\mathrm{A}$ to $d_\mathrm{B}$   J

  The force on an object, acting along the x axis, varies as shown in Fildy+u3+sb3mk ,xd ds,g. 6–37. Determls 3bks,3 mddu,+y+ dxine the work done by this force to move the object (a) from x=0 m to x = 10 m    J
(b) from x = 0 m to x = 15 m    J

  A spring has $k=88$ N/m. Use a graph to determine the work needed to stretch it from $x=3.8$ cm to
$x=5.8$cm, where $x$ is the displacement from its unstretched length    J

  The net force exerted on a particle acts in to 3wtaltf) try9 5)0(v tb.mukxxf -j(m /bg7the $+x$ direction. Its magnitude increases linearly from zero at
$x=0$, to 24.0 N at $x=3.0$ m. It remains constant at 24.0 N from $x=3.0$ m and then decreases
linearly to zero at $x=13.0$m. Determine the work done to move the particle from $x=0$ to $x=13.0$ m
graphically by determining the area under the $F_x$ vs. $x$ graph    J

  At room temperature, an oxygen molecule, with mass tdqy4k.usq,-.a 5e eeof $5.31\times10^{-26}$typically has a KE of about$6.12\times10^{-21}$ J .How fast is the molecule moving?    m/s

  (a) If the KE of an arrow is doubled, by , ce:xdol d60pwhat factor has its speed increased? $ \sqrt{a}$ a =    (b) If its speed is doubled, by what factor does its KE increase?   

  How much work is requ-5b zseud ,j,gired to stop an electron $\left(m=9.11\times10^{-31}\mathrm{~kg}\right)$ which is moving with a speed of$1. 90\times 10^{6}\mathbf{m/ s}$ ? $\boxed{a\times10^{b} \mathrm{J}}$ a =    b =   

  How much work must be done to stop a 12zbmlp *zlb;u9zv9 e,950-kgcar traveling at 105km/h?$\boxed{a\times10^b\text{J}}$ a =    b =   

  An 88-g arrow is fired from a bow whose string exerts an avera,45f )scpyzcw5d zd6 yge force of 110 N on the arrow over a distance of 78 cm. What is the sps4c,zy5dy d6 5 )fzwcpeed of the arrow as it leaves the bow?    m/s

  A baseball (m=140kg) traveling 32m/s move3 bvsjab-b1v2 5 ikxa:s a fielder’s glove backward 25 cm when the ball is caught. What was the average force exerted by the ball on the-5i1k: bab32 vxbsjv a glove?    N

If the speed of a car is i)ag i6 3j43lwupq ira4ncreased by 50%, by what factor will its minimum braking distan rai4jl3) up6ag w4q3ice be increased, assuming all else is the same? Ignore the driver’s reaction time.

  At an accident scene on a level road, investigators measure a car’s skid markosixld- r,(lk a,o t2 9mco,xgm81 z.p to be 88 m long. The accident occurred on a rainy day, and the coefficient of kinetic friction was estimated to be 0.42. Use these data to determine the speed of the car when the driver slammed on (and locked) the brakes. (Why does the car’s mass(,c8,s9l ik.2o xmx ,oa-tmop1ldg zr not matter?)    m/s

  A softball having a massvrj hg6xrg+oqa61e. * of 0.25 kg is pitched at By the time it reaches the plate, it may have slowed by 10%. Neglecting gravity, estimate the average force of jqra1*g egv6x +.hr6o air resistance during a pitch, if the distance between the plate and the pitcher is about 15 m.    N

  How high will a 1.85- .pid16n/t 8v ehrtyx2kg rock go if thrown straight up by someone who does 80.0 J of rtpin62v thd8./1yxework on it? Neglect air resistance.    m

  A 285-kg load is lifted 22.0 m vertically with an acg yd*dw p8;ly4zd+m rw/0cdd/celeration by a single cable. Dedz;y0cy/8dm/rdl dwg+ p4 d *wtermine
(a) the tension in the cable,    N
(b) the net work done on the load,    J
(c) the work done by the cable on the load,    J
(d) the work done by gravity on the load,    J
(e) the final speed of the load assuming it started from rest.    m/s

  A spring has a spring stiffness constant, k,o7 ;u hpndgj bkbc*s45:-,nl- ivv4h h of 440N/m How much must this spring be stretched to store 25 J of potential energyh:lg u;, hvboi5vhsnc4d4- p* nb7j-k?    m

  A 7.0-kg monkey swings from one branch to anin12r5 /u3ym1okkk3 jw3pz tfhg 7er. other 1.2 m higher. What is the change in p 32 kngrw3tyeu7o3j1f.rk1/z mik 5hpotential energy?    J

  By how much does the gravitational potential energy of a 64-kg pole vauum y7 gx08h8glv7 ,mymlter change if his center of mass rises about 4.0 m during tym 7lmg 88uvx,ym h07ghe jump?    J

  A 1200-kg car rolling on a horizontal surface ha drel0c txtx(b55n(mr5 t 8k+f6zdg 1rs speed v = 65 km/h when it strikes a horizontal coiled sprin( ktcrefx5+nz61bdr 0t85lxt5r d (gmg and is brought to rest in a distance of 2.2 m. What is the spring stiffness constant of the spring?    N/m

  A 1.60-m tall person lifts a 2.10-kg book c3/uk2kqezs1kp6w pgov:e)o h ,a x--from the ground so it is 2.20 m above the ground. What is the potential energy of the ,o -1u) opx kwh-a2s ek6c:3kqz/egvpbook relative to
(a) the ground,    J
(b) the top of the person’s head?    J
(c) How is the work done by the person related to the answers in parts (a) and (b)?    J

  A 55-kg hiker starts at an elevation of 1600 m and climbtog:bj/sen1(qf5d 8 mn98 qkk ga9-oes to the top of a 3300-m pem b-nsj(9ont9:81go a /k e5ek dqfg8qak.
(a) What is the hiker’s change in potential energy?    J
(b) What is the minimum work required of the hiker?    J
(c) Can the actual work done be more than this? Explain why. ----待选择----yesno 

  A spring with k = 53N/m hangs vertically next to a ruler. The end op9hs; y6 ow-iylv94mn f the spring is next to the 15-cm mark on ths ;l hvyywm99o64-nip e ruler. If a 2.5-kg mass is now attached to the end of the spring, where will the end of the spring line up with the ruler marks?    cm

  Jane, looking for Tarzan, is running at top speed (5.3m/s) and grabs a vi.n 2uegwq1eev 01 z9+yhfvv)fne hanging vertically from a tall tree in the jungle. Heqhv0vfyu29 gvzef w1.)+en1ow high can she swing upward?    m
Does the length of the vine affect your answer? ----待选择----yesno 

  A novice skier, starting from r4x aba4gyg0a7est, slides down a frictionless 35.0$^{\circ}$ incline whose vertical height is 185 m. How fast is she going when she reaches the bottom?    m/s

  A sled is initially given a shove up a f3szemaz6g q ;:rictionless 28.0$^{\circ}$ incline. It reaches a maximum vertical height 1.35 m higher than where it started. What was its initial speed?    m/s

  In the high jump, Fran’s kinetic energy is transformed into gravitational - l8bcmj+nc8jpotential energy without the aid of a pole. With what minimum speed must Fran leave the ground in order to lift her center of mass 2.10 m and cross thejbcmlc+88n-j bar with a speed of 0.7m/s.    m/s

  A 65-kg trampoline artist jumps vertically upward from the toa2d+bg 5, tdffgv m(u(p of a platform wit(5 da,udvbf mfg+(tg2h a speed of 5.0 m/s. ($\alpha)$ How fast is he going as he lands on the trampoline, 3.0 m below (Fig. 6-38)?$v_2$ $\approx$    m/s
(b) If the trampoline behaves like a spring with spring stiffness constant 6.2×10$^{4}$ N/m , how far does he depress it? $y_3$ =    m

  $\text{A projectile is fired at an upward angle of 45.0o from the top of a 265-m cliff with a speed of 185m/s. What will}\\\text{be its speed when it strikes the ground below?(Use conservation of energy.)}$    m/s

  A vertical spring (ignore its mass), whose spring so4tg1mybmj -up8:n /bin ( 4sjtiffness constant is 950N/m is attached to a table and is compressed down 0.1bs-it1u(j m ynmb j8on g/:44p50 m. (a) What upward speed can it give to a 0.30-kg ball when released?    m/s
(b) How high above its original position (spring compressed) will the ball fly?    m

  A block of mass m slides without friction along the jc i6by 6r. yzx0w/(6avdp.d vlooped track shown in Fig. 6–39. If the block is to remain on the 0r6 d6avyzyd.bx. (vpcj/ i6wtrack, even at the top of the circle (whose radius is r), from what minimum height h must it be released?    r

A block of mass m is attached to the end of a spring (spring stiffness c)4jjuuvw fd.hdamd *: ;2r8 c8cts)g bonstant k), Fig. 6–40. The block is given an in dr)jsu;.)b2v8t ch4:ddgu w amf*j8citial displacement $x_0$ after which it oscillates back and forth. Write a formula for the total mechanical energy (ignore friction and the mass of the spring) in terms of $x_0$ position x, and speed

  A 62-kg bungee jumperxo* 8kk qfg2)v2pcpzq2 /log0 jumps from a bridge. She is tied to a bungee cord whose unstretched length is 12 0vcx8og k)q /pkg2 fq2oz*pl 2m, and falls a total of 31 m. (a) Calculate the spring stiffness constant k of the bungee cord, assuming Hooke’s law applies. $\approx$    N/m
(b) Calculate the maximum acceleration she experiences. $\approx$   m/$s^2$

  The roller-coaster car shoj ;gj4r3 awdo.gs0 e(wwn in Fig. 6–41 is dragged up to point 1 where it is released from rest. Assuming no friction,jsgr3;. gjd0eo wa4 w( calculate the speed at points
2    m/s
3    m/s
4    m/s

  A 0.40-kg ball is thrown with a speed of 12 m/s at andv 3homn:e1bt1 tga.* angle of 33 $^{\circ} $. (a) What is its speed at its highest point    m/s
(b) how high does it go? (Use conservation of energy, and ignore air resistance.)   m

An engineer is designin3 .c)q. shrb xt0dtwpfng:d46g a spring to be placed at the bottom of an elevator shaft. If the elevator cable should break when the elevator is at a height h.hb6 s3).: wd rdgxqntftc40 p above the top of the spring, calculate the value that the spring stiffness constant k should have so that passengers undergo an acceleration of no more than 5.0 g when brought to rest. Let M be the total mass of the elevator and passengers.

  A cyclist intends to cycle upui;c oeq z/ d4;jn4xs- a 7.8 $^{\circ} $ hill whose vertical height is 150 m. Assuming the mass of bicycle plus cyclist is 75 kg,
(a) calculate how much work must be done against gravity.    J
(b) If each complete revolution of the pedals moves the bike 5.1 m along its path, calculate the average force that must be exerted on the pedals tangent to their circular path. Neglect work done by friction and other losses. The pedals turn in a circle of diameter 36 cm.    N

  Two railroad cars, each of mass 7650 kg and traveling 95.,kqr0 8:ax5d xm,k*nrdqpv(qbh j8b km/h in opposite directions, collide head-on and come to rest. How much thermal energy is prod jdr m:*b.8qakrpq,v0x8 h5xd(kn b,quced in this collision? $a\times10^b$ J a =    b =   

  A 21.7-kg child descends a slide 3.5 m high and reaches the lpr3+:j po5kb bottom with a speed of 2.2m/s How much thermal energy due to frk:lop+5 j prb3iction was generated in this process?    J

  A ski starts from rest and slides down a 22 a 9e: zndddk ..b,(u3 mnx1pbb$^{\circ} $ incline 75 m long.
(a) If the coefficient of friction is 0.090, what is the ski’s speed at the base of the incline? $\approx$    m/s
(b) If the snow is level at the foot of the incline and has the same coefficient of friction, how far will the ski travel along the level? Use energy methods.$\approx$    m

  A 145-g baseball is dropped from a tree 13.0x xpe91d+ v4yn:r15sqjnl7iz m above the ground.
(a) With what speed would it hit the ground if air resistance could be ignored?    m/s
(b) If it actually hits the ground with a speed of 8m/s what is the average force of air resistance exerted on it?    N

  You drop a ball from a height of 2.0 m, and it bounce4f( imbtex)(zhef 4y0(hh ,v fs back to a height of 1.5 m. t,0hyi ez (v he(hm44b( fxff)
(a) What fraction of its initial energy is lost during the bounce?    %
(b) What is the ball’s speed just as it leaves the ground after the bounce?    m/s
(c) Where did the energy go?

  A 110-kg crate, starting k n(u)hdy8v;gfrom rest, is pulled across a floor with a constant horizontal force of 350 N. For the first 15 m the floor is frictionless, and for the next 15 m the coefficient of friction is 0.30. Wh(y;ugn) 8h dkvat is the final speed of the crate?    m/s

  Suppose the roller coaster in Fig. 6–41 passes point 1 with a speed of do 5 th6fp7s kys*+0fsd.me *cIf the average force of friction is equal to one-fifth of its weight, with what speed will it r+y mhfs60 pdof5ek7 **dc.tsseach point 2? The distance traveled is 45.0 m. $\approx$    m/s

  A skier traveling 12m/s / 8vsi8etuyv8reaches the foot of a steady upward 18.0 $^{\circ} $ incline and glides 12.2 m up along this slope before coming to rest. What was the average coefficient of friction?   

  A 0.620-kg wood block is firmly attached to a very lly-t2 pjzr45mff:l 2d ight horizontal spring (k=180N/m) as shown in Fig. 6–40. It is noted that the block–spring system, when compressed 5.0 cm and released, stretches out 2.3 cm beyond the equilibrium position before stopping and turning back. What is the 5z-j2: yf dfrm 4pt2llcoefficient of kinetic friction between the block and the table?   

  A 280-g wood block is firmly attached to a very l,0pihh2 xx4* (paft)t b862sew ;xxccrdq qi -ight horizontal spring, Fig. 6–40. Th*p0qx-qh e xc2)aiwd,bt8pxx stf6;ic2hr4 (e block can slide along a table where the coefficient of friction is 0.30. A force of 22 N compresses the spring 18 cm. If the spring is released from this position, how far beyond its equilibrium position will it stretch on its first swing?    m

  Early test flights for wq330)cg;ah-h+xgi ffsbeg7the space shuttle used a “glider” (mass of 980 kg including pilot) that was launched horizontally at 500km/h from a height of 3500 m. The glider eventually landed at a speed of 200km/h (a) What would its landing speed have been in the absence of air resistance;-g07sbfhecq g g+i wxf)ah33 ?$\approx$    km/h
(b) What was the average force of air resistance exerted on it if it came in at a constant glide of 10 $^{\circ} $ to the Earth?$\approx$    N

  How long will it take a 1)f/7s,f jzcu5lmak)k ha8d/k c,p;zy 750-W motor to lift a 315-kg piano to a sixth-story window 16.0 mjk zcha8)a yz /;)5,fp7u kkmfscd/l, above?    s

  If a car generates 18 hp when travelngo4vw3pgoehi 33i*1 ing at a steady 88km/h what must be the average force exerted on the car due to friction and aigpv h in3oe14*o3 ig3wr resistance?    N

  A 1400-kg sports car accelerates from rest to 95km/h iiua jzg/xnqra*qq.u jb73i*;s. (2 kmn 7.4 s. What is the average power delivered by the ej(mg . /u3n j xriqiaa2kqsu*qz*7b .;ngine?$\approx$    hp

  (a) Show that one British horsepo 1kxjn-d2f-( xw -njl7qxke)awer$ \mathrm{ft}\cdot\mathrm{lb/s}$ is equal to 746 W.
(b) What is the horsepower rating of a 75-W light bulb?    hp

  Electric energy units are often expressed in the form of “kilowatw/zoiu s ,t uj-54e)qut-hours.”
(a) Show that one kilowatt-hour (kWh) is equal to $3.6\times10^6$J
(b) If a typical family of four uses electric energy at an average rate of 520 W, how many kWh would their electric bill be for one month $\approx$   kW*h
(c) how many joules would this be?$a\times10^b$ J a =    b =   
(d) At a cost of 0.12 dollars per kWh, what would their monthly bill be in dollars? Does the monthly bill depend on the rate at which they use the electric energy?    dollars

  A driver notices that her 1150-kg car slows down from to 65km/h in abn;g d4 j/5oohwy 9uq *rj45fcnout 6.0 s on the level when it is in neutral. Approximately what power (watts and hp)j *gfjo/9y5c r;qhndn u4wo54 is needed to keep the car traveling at a constant 75km/h.    hp

  How much work can a 3y(n)tau-0+pk u( jgru.0-hp motor do in 1.0 h? $a\times10^b$ J a =    b =   

  A shot-putter accelerates a 7.3-kg shot from rest to 14m/s If this motion ta z v:pgbdb gkfk 3wle7rg2gol)/;/k/9 kes 1.5 s, what average power wasl)/kgl o;v2/kgk7rwp9/bg 3ged:zfb developed?$\approx$    W

  A pump is to lift 18.0 kg of water per minute through a heiv)) tust5 m9v,sd ap2ight of 3.60 m. What output rating (watt m)u)5v,a tvs pdt2is9s) should the pump motor have?    W

  During a workout, the football players at Staw aipmkkw 5)r5vw :/k1te U. ran up the stadium stairs in 66 s. The stairs are kk:1)aip 5mk/rwvw 5w140 m long and inclined at an angle of 32 $^{\circ} $. If a typical player has a mass of 95 kg, estimate the average power output on the way up. Ignore friction and air resistance.    W

  How fast must a cyclist cl3 iz/j+osb x7cimb a 6.0 $^{\circ} $ hill to maintain a power output of 0.25 hp? Neglect work done by friction, and assume the mass of cyclist plus bicycle is 68 kg.   m/s

  A 1200-kg car has a maximum power output of 120 hp. How steep a hill can it clim x:16b6f fomurnhl:)unuz,g)b at a constant speed of 75k nu)fo1 ur:xnz, :mluh bgf)66m/h if the frictional forces add up to 650 N?    $^{\circ} $

  What minimum horsepower must a motor have to be able tgdb u-p )q,x*eo drag a 310-kg box along a level floor at a speed of 1.2,de p )qb-gux*m/s if the coefficient of friction is 0.45?    hp

  A bicyclist coasts down a 7.0 r7s um l,r ;9b*z/e ;9ojs)mw htyte-f$^{\circ} $ hill at a steady speed of 5m/s Assuming a total mass of 75 kg (bicycle plus rider), what must be the cyclist’s power output to climb the same hill at the same speed?    W

  Designers of today’s cars have built “5mi/h(8km/h) bumpers” teqz1z b (y4j1qhat are designed to compress and rebound elastically without any physical damage at speeds below 8k1 zjq(eb1q z4ym/h If the material of the bumpers permanently deforms after a compression of 1.5 cm, but remains like an elastic spring up to that point, what must the effective spring stiffness constant of the bumper be, assuming the car has a mass of 1300 kg and is tested by ramming into a solid wall? $a\times10^b$ N/m a =    b =   

  In a certain library the first shelf is 10.0 cm off the gra- i(mtjxvo ;x/r)wc :ound, and the remaining four shelves are each spaced 30.0 cm above the previous one. If the average book has a mass of 1.5 kg with a hat (; -ojc):mwrxv/x ieight of 21 cm, and an average shelf holds 25 books, how much work is required to fill all the shelves, assuming the books are all laying flat on the floor to start? $\approx$    J

  A film of Jesse Owens’s famous lox .8k* vypjto-ng jump (Fig. 6–42) in the 1936 Olympics shows that his center of mass rose 1.1 m from launch point to the top of the arc. What minimum speed did he need at launch if he was traveling ay-kot* v8pjx.t 6.5m/s at the top of the arc?   m/s

  The block of mass m sliding without friction along the looped trag 3b ljpb2d,-d ms,wf4ck shown in Fig. 6–39 is to remabd -bfw,d2s3gmlpj4, in on the track at all times, even at the very top of the loop of radius r.
(a) In terms of the given quantities, determine the minimum release height h (as in Problem 40). Next, if the actual release height is 2h, calculate    r
(b) the normal force exerted by the track at the bottom of the loop,    mg
(c) the normal force exerted by the track at the top of the loop,   mg
(d) the normal force exerted by the track after the block exits the loop onto the flat section.   mg

  An airplane pilot fell 370 mgac;1bz ;) r kbq 4z/si:l0lyt after jumping from an aircraft without his parachute opening. He landed in a snowbank, creating a crater 1.1 m deep, but survived with only minor injuries. Assuming the pilot’s mass was 78 kg and his terminal velocity was 35m/s es t:l1)4ca;sb rz y;lqgkz bi/0timate
(a) the work done by the snow in bringing him to rest; $\approx$    J
(b) the average force exerted on him by the snow to stop him;$\approx$    N
(c) the work done on him by air resistance as he fell.$\approx$    J

A ball is attached to igz ;x)t1,uoqfr rdml5x - -*ra horizontal cord of length L whose other end is fixed (Fim;- ql i1* odxr,rgzf -5tru)xg. 6–43).
(a) If the ball is released, what will be its speed at the lowest point of its path?
(b) A peg is located a distance h directly below the point of attachment of the cord. If h=0.8L what will be the speed of the ball when it reaches the top of its circular path about the peg?

  A 65-kg hiker climbs to the topw7c8wysx6ib /pri8+ f of a 3700-m-high mountain. The climb is made in 5.0 h startisbpyw8fr c7i+/8x w6 ing at an elevation of 2300 m. Calculate
(a) the work done by the hiker against gravity,   
(b) the average power output in watts and in horsepower,   W    hp
(c) assuming the body is 15% efficient, what rate of energy input was required.    hp

  An elevator cable breaks when a 920-kg elevator is 28 m above a hzzw h5wu:47 dw.lln 0guge spring ($k=2.2\times10^{5} \mathrm{N/m}$) at the bottom of the shaft. Calculate
(a) the work done by gravity on the elevator before it hits the spring,$\approx$    j
(b) the speed of the elevator just before striking the spring,$\approx$    m/s
(c) the amount the spring compresses (note that work is done by both the spring and gravity in this part).    m

  Squaw Valley ski area in Cab(wmhz7 .ic* vs6(ks mlifornia claims that its lifts can move 47,000 people per hour. If the average lift carries people about 200 m (vertically) higher, estimate the (bwm(s 7izvm*s.h ck6power needed.$a\times10^b$ W a =    b =   

  Water flows over a dam at the rate of 650kg/s and falls verti.ku mi:tr.3v5zrd:pv cally 81 m before striking th.zk5dvi :umr.: vrt3pe turbine blades. Calculate
(a) the speed of the water just before striking the turbine blades (neglect air resistance), $\approx$    m/s
(b) the rate at which mechanical energy is transferred to the turbine blades, assuming 58% efficiency.    W

  Show that on a roller coaster with a circular vertical loop (Figd8o40 ki c6 kt(dwwzj,pw,v9e wl32t t. 6–44), the difference in your apparent weight at the top of the circular loop and the bottom of the circular loop is 6 g’s — that is, six times your weight. Ignore friction. Show also that as long as your speed is above the minimum needed, this answer doesn’t depend on the size of the loop or how z dekw, tpt wld43 (89ck0 woivj2,w6tfast you go through it.    mg

  (a) If the human body could convert a candy bar ro. 5wkq;lne8 +(z illdirectly into work, how high could an 82-kg man climb a ladder if he were fueled by one bar (=1000kJ.en8 +z(rqlliwkl; o5)    m
(b) If the man then jumped off the ladder, what will be his speed when he reaches the bottom?    m/s

  A projectile is fired at an upward angle of ownarw8q1x8 r mti64vz8 /a dw9i-y1c45.0$^{\circ} $ from the top of a 165-m cliff with a speed of 175m/s What will be its speed when it strikes the ground below? (Use conservation of energy and neglect air resistance.)    m/s

  If you stand on a bathroom scale, the spring inside the scale comprr. x*2;uqa rou 1lqki0esses 0.60 mm, and it tells you your weight is 710 N. Now if you jump on the scale from a height of 1.0 mi1kqra0 q.xu2*lr uo;, what does the scale read at its peak?    N

  A 65-kg student runs at 5m/s grabs a rope rl7 vwxj2,tid,v;0da, and swings out over a lake (Fig. 6–45). He releases ,2,w;jddv07il x rvta the rope when his velocity is zero.
(a) What is the angle when he releases the rope?    $^{\circ} $
(b) What is the tension in the rope just before he releases it?    N
(c) What is the maximum tension in the rope?    N

  In the rope climb, a 72-kg athletexpzc6m3d n hg1 3u.z5z climbs a vertical distance of 5.0 m in 9.0 s. Whanhz z6 mu. x3gzpd31c5t minimum power output was used to accomplish this feat?    W

  Some electric-power companies us-f1 6c l +9cjlqw hdqh80j,wvve water to store energy. Water is pumped by reversible turbine pumps from a low to a high reservoir. To store the energy produced in 1.0 hour by a 120-MW 0j1+qh c wc-,f68dvqwllv9hj($120\times10^{6}$W) electric-power plant, how many cubic meters of water will have to be pumped from the lower to the upper reservoir? Assume the upper reservoir is 520 m above the lower and we can neglect the small change in depths within each. Water has a mass of 1000 kg for every 1$m^3$.    $m^3$

  A spring with spring stiffnessj4-mql ktm25 u constant k is cut in half. What is the spring stiffness constant fou54j 2qmm l-tkr each of the two resulting springs?   k

  A 6.0-kg block is pushed 8.0 m up a rouq *3 kd0zbz,bngh 37 $^{\circ} $ inclined plane by a horizontal force of 75 N. If the initial speed of the block is 2.2m/s up the plane and a constant kinetic friction force of 25 N opposes the motion, calculate
(a) the initial kinetic energy of the block; $\approx$    J
(b) the work done by the 75-N force; $\approx$    J
(c) the work done by the friction force;    J
(d) the work done by gravity;$\approx$    J
(e) the work done by the normal force;    J
(f) the final kinetic energy of the block.$\approx$    J

  If a 1500-kg car can accelerate from 35km/h to 55km/h in 3.2 s76nca4w6e:*fgk5b)6 kuqb i;l pjd bu, how long will it take to accelerate from 55km/h to 75km/h Assume the power stays the same, and neglec66wf l6jkbep5b:au *;bcun7i)gk4 qdt frictional losses.    s

  In a common test for cardiac function (the “stress test”), the patient walks on *z mx u5j8 p6enn:/kjman inclined treadmill (Fig. 6–46). Ezme 8/mn5p :n ujjx6*kstimate the power required from a 75-kg patient when the treadmill is sloping at an angle of 15 $^{\circ} $ and the velocity is 3.3km/h (How does this power compare to the power rating of a light bulb?)$\approx$    W

  (a) If a volcano spews a 500-kg rock vertically upward a dist nsez1ii5nvyya-j/7 :ance of 500 m, what was its velocity when it left the vol17:n- 5sva /znyiyji ecano? $\approx$    m/s
(b) If the volcano spews the equivalent of 1000 rocks of this size every minute, what is its power output? $\approx$ $a\times10^b$ W a =    b =   

  Water falls onto a water wheel from a height of 2.0 m at a;q n0 *s veut:6hxvhe- rate of 95kg/s (a) If this water wheel is*vh0e vsxe:t6uhn-;q set up to provide electricity output, what is its maximum power output?    W
(b) What is the speed of the water as it hits the wheel?    m/s