Essential University Physics, 3e (Wolfson) 3e (Wolfson) Chapter 4 Force and Motion
4 . 1 Conc e p t ua lQue s t i o ns 1) You You swing a bat and hit a heavy box with a force of 1500 N. The force the box exerts on the bat is A) exactly 1500 N only if the box does not move. ) exactly 1500 N whether or not the box moves. !) greater than 1500 N if the box moves. ") less than 1500 N if the box moves. #) greater than 1500 N if the bat bounces bac$. Answer% &ar% 1
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) +oint P +oint P in in the figure indicates the 'osition of an obect traveling at constant s'eed cloc$wise around the circle. /hich arrow best re'resent the direction the obect would travel if the net external force on it were suddenly reduced to ero
A)
)
!) ")
#)
Answer% " &ar% 1
2) An obect is moving to the right, and ex'eriencing a net force that is directed to the right. The magnitude of the force is decreasing with time. The s'eed of the obect is A) increasing. ) decreasing. !) constant in time. Answer% A &ar% 1
3) -n order to get an obect moving, you must 'ush harder on it than it 'ushes bac$ on you. A) True ) 4alse Answer% &ar% 1 !o'yright ( 01* +earson #ducation, -nc.
5) -n order to lift a buc$et of concrete, you must 'ull u' harder on the buc$et than it 'ulls down on you. A) True ) 4alse Answer% &ar% 1
*) A *15 N student standing on a scale in an elevator notices that the scale reads *35 N. 4rom this information, the student $nows that the elevator must be moving A) downward. ) u'ward. !) You cannot tell if it is moving u'ward or downward. Answer% ! &ar% 1
) A car is being towed at constant velocity on a horiontal road using a horiontal chain. The tension in the chain must be e6ual to the weight of the car in order to maintain constant velocity. A) True ) 4alse Answer% &ar% 1
7) You are standing in a moving bus, facing forward, and you suddenly fall forward as the bus comes to an immediate sto'. The force acting on you that causes you to fall forward is A) the force of gravity. ) the normal force due to your contact with the floor of the bus. !) the force due to static friction between you and the floor of the bus. ") the force due to $inetic friction between you and the floor of the bus. #) No forces were acting on you to cause you to fall. Answer% # &ar% 1
8) !onsider what ha''ens when you um' u' in the air. /hich of the following is the most accurate statement A) -t is the u'ward force exerted by the ground that 'ushes you u', but this force cannot exceed your weight. ) You are able to s'ring u' because the earth exe rts a force u'ward on you that is greater than the downward force you exert on the earth. !) 9ince the ground is stationary, it cannot exert the u'ward force necessary to 'ro'el you into the air. -nstead, it is the internal forces of your muscles acting on your body itself that 'ro'els your body into the air. ") /hen you 'ush down on the earth with a force greater than your weight, the earth will 'ush bac$ with the same magnitude force and thus 'ro'el you into the air. #) /hen you um' u' the earth exerts a force F 1 on you and you exert a force F on the earth. You go u' because F 1 : F . Answer% " &ar% 1 2 !o'yright ( 01* +earson #ducation, -nc.
10) A 0;ton truc$ collides with a 1500;lb car and causes a lot of damage to the car. "uring the collision A) the force on the truc$ due to the collision is slightly greater than the force on the car. ) the force of on the truc$ due to the collision is exactly e6ual to the force on the car. !) the force on the car due to the collision is much greater than the force on the truc$. ") the car and the truc$ have the same magnitude acceleration. Answer% &ar% 1
11) Two obects, each of weight W , hang vertically by s'ring scales as shown in the figure. The 'ulleys and the strings attached to the obects have negligible weight, and there is no a''reciable friction in the 'ulleys. The reading in each scale is
A) W. ) more than W , but not 6uite twice as much. !) less than W . ") W . #) more than W . Answer% A &ar% 1
1) A fish weighing 1* N is weighed using two s'ring scales, each of negligible weight, as shown in the figure. /hat will be the readings of the scales
A) The bottom scale will read 1* N, and the to' scale will read ero. ) #ach scale will read 1* N. !) The to' scale will read 1* N, and the bottom scale will read ero. ") The scales will have different readings, but the sum of the two readings will be 1* N. #) #ach scale will read 7 N. Answer% &ar% 1 3 !o'yright ( 01* +earson #ducation, -nc.
12) /hich one of the following free;body diagrams best re'resents the free;body diagram, with correct relative force magnitudes, of a 'erson in a n elevator that is traveling u'ward with an unchanging velocity
f is the force of the floor on the 'erson and
the 'erson. A)
)
!)
")
#)
Answer% &ar% 1
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g is the force of gravity on
13) /hich one of the following free;body diagrams best re'resents the free;body diagram, with correct relative force magnitudes, of a 'erson in a n elevator that is traveling u'ward but is gradually slowing down at a rate of 8 m
)
!)
")
#)
Answer% &ar% 1
15) A box slides down a frictionless 'lane inclined at an angle θ above the horiontal. The gravitational force on the box is directed A) 'arallel to the 'lane in the same direction as the movement of the box. ) 'arallel to the 'lane in the o''osite direction as the movement of the box. !) 'er'endicular to the 'lane. ") vertically. #) at an angle θ below the inclined 'lane. Answer% " &ar% 1
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1*) An obect is moving forward with a constant velocity. /hich statement about this obect =>9T be true A) The net force on the obect is ero. ) The net force on the obect is in the forward direction. !) No forces are acting on the obect. ") The acceleration of the obect is in the forward direction. Answer% A &ar% 1
1) 9u''ose you are 'laying hoc$ey on a new;age ice surface for which there is no friction between the ice and the hoc$ey 'uc$. You wind u' and hit the 'uc$ as hard as you can. After the 'uc$ loses contact with your stic$, the 'uc$ will A) start to slow down. ) not slow down or s'eed u'. !) s'eed u' a little, and then slow down. ") s'eed u' a little, and then move at a constant s'eed. Answer% &ar% 1
17) A ball is tossed vertically u'ward. /hen it reaches its highest 'oint ?before falling bac$ downward) A) the velocity is ero, the acceleration is directed downward, and the force of gravity acting on the ball is directed downward. ) the velocity is ero, the acceleration is ero, and the force of gravity acting on the ball is ero. !) the velocity is ero, the acceleration is ero, and the force of gravity acting on the ball is directed downward. ") the velocity and acceleration reverse direction, but the force of gravity on the ball remains downward. #) the velocity, acceleration, and the force of gravity on the b all all reverse direction. Answer% A &ar% 1
18) A dog is standing in the bed of a 'ic$u' truc$. The bed is coated with ice, causing the force of friction between the dog and the truc$ to be ero. The truc$ is initially at rest, and then accelerates to the right, moving along a flat road. As seen from a stationary observer ?watching the truc$ move to the right), the dog A) does not move left or right, but the bac$ of the truc$ moves towards the dog. ) moves to the right, but not as 6uic$ly as the truc$ is moving to the right, causing it to slide towards the bac$ of the truc$. !) moves to the right at the same rate as the truc$, so it doesn@t slide. ") moves to the left, as the truc$ moves to the right, causing the dog to slide towards the bac$ of the truc$. Answer% A &ar% 1
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0) You are seated in a bus and notice that a hand stra' that is hanging from the ceiling hangs away from the vertical in the bac$ward direction. 4rom this observation, you can conclude that A) the velocity of the bus is forward. ) the velocity of the bus is bac$ward. !) You cannot conclude anything about the direction of the velocity of the bus. Answer% ! &ar% 1
1) !onsider a 'lot of the dis'lacement ? x) as a function of the a''lied force ? F ) for an ideal elastic s'ring. The slo'e of the curve would be A) the s'ring constant. ) the reci'rocal of the s'ring constant. !) the acceleration due to gravity. ") the reci'rocal of the acceleration of gravity. #) the mass of the obect attached to the s'ring. Answer% &ar% 1
) /hich of the gra'hs in the figure illustrates oo$e@s Baw
A) Cra'h a ) Cra'h b !) Cra'h c ") gra'h d Answer% &ar% 1
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2) /hich of the gra'hs in the figure re'resents a s'ring that gets less stiff the more it is stretched
A) Cra'h a ) Cra'h b !) Cra'h c ") Cra'h d Answer% " &ar% 1
4 . 2 Pr o bl e ms 1) A bloc$ lies on a horiontal frictionless surface. A horiontal force of 100 N is a''lied to the bloc$ giving rise to an acceleration of 2.0 m
) -f a 5.0 $g box is 'ulled simultaneously by a 10.0 N force and a 5.0 N force, then its acceleration must be A) 2.0 m
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2) The figure shows an acceleration;versus;force gra'h for three obects 'ulled by rubber bands. The mass of obect is 2* $g. /hat are the masses of obects 1 and 2
A) 13 $g and 80 $g ) $g and 17 $g !) 80 $g and 17 $g ") 13 $g and $g Answer% A &ar% 50D
3) A .0;$g obect is acted on by two forces. Ene of the forces is 10.0 N acting toward the east. /hich of the following forces is the other force if the acceleration of the obect is 1.0 m
5) The -nternational 9'ace 9tation has a mass of 1.7 F 105 $g. A 0.0;$g astronaut inside the station 'ushes off one wall of the station so she accelerates at 1.50 m
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*) En a horiontal frictionless floor, a wor$er of weight 0.800 $N 'ushes ho riontally with a force of 0.00 $N on a box weighing 1.70 $N. As a result of this 'ush, which statement could be true A) The box will not move because the 'ush is less than its weight. ) The wor$er and box will both have an acceleration of 1.07 m
) A 50.0;N box is sliding on a rough horiontal floor, and the only horiontal force acting on it is friction. You observe that at one instant the box is sliding to the right at 1.5 m
7) A bloc$ is on a frictionless horiontal table, on earth. This bloc$ accelerates at 1.8 m< a
when
horiontal force is a''lied to it. The bloc$ and table are then set u' on the moon where the acceleration due to gravity is 1.* m
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8) A bloc$ is on a frictionless horiontal table, on earth. This bloc$ accelerates at 2.* m< a
when
horiontal force is a''lied to it. The bloc$ and table are then set u' on the moon where
the acceleration due to gravity is 1.* m< . A horiontal force of 35 N is a''lied to the bloc$ when it is on the moon. /hat acceleration does this force im'art to the bloc$ A) 1.7 m< ) 1.* m< !) .0 m< ") . m< #) .2 m< Answer% A &ar% 50D
10) An 1100;$g car traveling at .0 m
11) En its own, a certain tow;truc$ has a maximum acceleration o f 2.0 m
1) -n a ballistics test, a 1.50;g bullet is fired through a 7.0;$g bloc$ traveling horiontally toward the bullet. -n this test, the bullet ta$es 11.3 ms to 'ass through the bloc$ as it reverses the bloc$@s velocity from 1.5 m
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12) A 10,000;$g roc$et blasts off from earth with a uniform u'ward acceleration of .00 m
13) 15) um'ers on cars are not of much use in a collision. To see why, calculate the average force a bum'er would have to exert if it brought a 100;$g car ?a so;called com'act model) to a rest in 15 cm when the car had an initial s'eed of .0 m
15) A box of mass 50 $g is at rest on a horiontal frictionless surface. A constant horiontal force F then acts on the box and accelerates it to the right. -t is observed that it ta$es the box *.8 seconds to travel 7 meters. /hat is the magnitude of the force Answer% 58 N &ar% 50D
1*) A locomotive is 'ulling 18 freight cars, each of which is loaded with the same amount of weight. The mass of each freight car ?with its load) is 2,000 $g. -f the train is accelerating at on a level trac$, what is the tension in the cou'ling between the second and third cars ?The car nearest the locomotive is counted as the first car, and friction is negligible.) Answer% 130,000 N &ar% 50D
1) A 1000;$g car is driving toward the north along a straight horiontal road at a s'eed of 0.0 m
N south
!) 1.00 F
N south
") 1.00 N south #) 100 N south Answer% ! &ar% 5
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17) A construction wor$er 'ulls a box of tools on a smooth horiontal floor with a force of 100 N in a direction of 2.0G above the horiontal. The mass of the box and the tools is 30.0 $g. ?a) "raw a free;body diagram for the box. ?b) !alculate the acceleration of the box. ?c) ow hard does the floor 'ush u' on the box Answer% ?a) The box is acted on by the force of gravity which 'oints downward toward the center of the earth. The normal force is directed toward the box 'er'endicular to the surface of the floor. The 'ulling force is directed away from the box at an angle 2.0 degrees above the horiontal. ?b) .00 m
18) A *0.0;$g 'erson rides in an elevator while standing on a scale. The scale reads 300 N. The acceleration of the elevator is closest to A) 2.12 m
0) A *0.0;$g 'erson rides in elevator while standing on a scale. The elevator is traveling downward but slowing down at a rate of .00 m
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1) Three boxes in contact rest side;by;side on a smooth, horiontal floor. Their masses are 5.0; $g, 2.0;$g, and .0;$g, with the 2.0;$g box in the center. A force of 50 N 'ushes on the 5.0;$g box, which 'ushes against the other two boxes. ?a) "raw the free;body diagrams for each of the boxes. ?b) /hat magnitude force does the 2.0;$g box exert on the 5.0;$g box ?c) /hat magnitude force does the 2.0;$g box exert on the .0;$g box Answer% ?a) The following forces act on the 5.0;$g box% the force due to gravity, normal force, contact force between 5.0;$g mass and 2.0;$g mass, the force of 50 N 'ushing on the box. The following forces act on the 2.0;$g box% the force due to gravity, normal force, contact force between the 5.0;$g box and the 2.0;$g box, the contact force between the 2.0;$g b ox and the .0; $g box. The following forces act on the .0;$g box% the force due to gravity, normal force, contact force between the 2.0;$g box and the .0;$g box. ?b) 5 N ?c) 10 N &ar% 1
) A *.00;$g bloc$ is in contact with a 3.00;$g bloc$ on a horiontal frictionless surface as shown in the figure. The *.00;$g bloc$ is being 'ushed by a horiontal 0.0;N force as shown. /hat is the magnitude of the force that the *.00;$g bloc$ exerts on the 3.00;$g bloc$
A) *.00 N ) 0.0 N !) 7.00 N ") 3.00 N #) 10.0 N Answer% ! &ar% 1
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2) A series of weights connected by very light cords are given an u'ward acceleration of 3.00 m
A) *80 N. ) 380 N. !) 80 N. ") 00 N. #) 50 N. Answer% A &ar% 1
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3) A series of weights connected by very light cords are given an u'ward acceleration of 3.00 m
A) 70.0 N. ) 18* N. !) * N. ") 372 N. #) *1 N. Answer% ! &ar% 1
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5) The figure shows a gra'h of the acceleration of a 15;g obect as a function of the net force acting on it. /hat is the acceleration at 'oints A and B
Answer% A% 1* m
*) The figure shows a gra'h of the acceleration of an obect as a function of the net force acting on it. The mass of this obect, in grams, is closest to
A) 120. ) 11. !) 78. ") 7000. Answer% A &ar% 1
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) Two forces act on a obect. Ene force has magnitude *5 N directed 58G cloc$wise from the 'ositive x;axis, and the other has a magnitude 25 N at 2G cloc$wise from the 'ositive y;axis. /hat is the magnitude of this obect@s acceleration A) 1.1 m
7) The gra'h in the figure shows the x com'onent of the acceleration of a .3;$g obect as a function of time ?in ms).
?a) At what time?s) does the x com'onent of the net force on the obect reach its maximum magnitude, and what is that maximum magnitude ?b) /hat is the x com'onent of the net force on the obect at time t H 0.0 ms and at t H 3.0 ms Answer% ?a) At 2.0 ms, 37 N ?b) 1 N, ;3 N &ar% 1
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8) The gra'h in the figure shows the net force acting on a 2.0;$g obect as a function of time.
?a) /hat is the acceleration of this obect at time t H .0 s ?b) "raw, to scale, a gra'h of the acceleration of this obect as a function of time over the range t H 0.00 s to t H .0 s. Answer% ?a) .0 m
20) A s'ring stretches by that would stretch the s'ring by A) 188 N ) 81.0 N !) 135 N ") 8 N Answer% A
when a
obect is attached. /hat is the weight of a fish
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21) An obect attached to an ideal massless s'ring is 'ulled across a frictionless surface. -f the s'ring constant is 35 N
2) -n the figure, two identical ideal massless s'rings have unstretched lengths of 0.5 m and s'ring constants of 00 N
A) 7 N. ) 5 N. !) 1 N. ") 17 N. #) 13 N. Answer% A &ar% 1
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