College Physics: A Strategic Approach, 3e (Knight)
Chapter 5 Applying Newton’s Laws
5.1 Conceptual Questions
1) In the figure, a 10-lb weight is suspended from two spring scales, each of which has negligible
weight. Which one of the following statements about the readings in the scales is true?
A) Each scale will read 5 lb.
B) The top scale will read zero, the lower scale will read 10 lb.
C) The lower scale will read zero, the top scale will read 10 lb.
D) Each scale will show a reading between one and 10 lb, such that the sum of the two is 10 lb.
However, exact readings cannot be determined without more information.
E) None of these is true.
2) Two blocks, A and B, are being pulled to the right along a horizontal surface by a horizontal
100-N pull, as shown in the figure. Both of them are moving together at a constant velocity of
2.0 m/s to the right, and both weigh the same.
Which of the figures below shows a correct free-body diagram of the horizontal forces acting on
the upper block, A?
A)
B)
C)
D)
E)
3) Two blocks, A and B, are being pulled to the right along a horizontal surface by a horizontal
100-N pull, as shown in the figure. Both of them are moving together at a constant velocity of
2.0 m/s to the right, and both weigh the same.
Which of the figures below shows a correct free-body diagram of the horizontal forces acting on
the lower block, B?
A)
B)
C)
D)
E) None of these diagrams is correct.
4) As shown in the figure, a woman is straining to lift a large crate, but without success because
it is too heavy. We denote the forces on the crate as follows: P is the magnitude of the upward
force being exerted on the crate by the person, C is the magnitude of the vertical contact force on
the crate by the floor, and W is the weight of the crate. How are the magnitudes of these forces
related while the person is trying unsuccessfully to lift the crate?
A) P + C = W
B) P + C < W
C) P + C > W
D) P = C
5) A push of magnitude P acts on a box of weight W as shown in the figure. The push is directed
at an angle θ below the horizontal, and the box remains a rest. The box rests on a horizontal
surface that has some friction with the box. The normal force on the box due to the floor is equal
to
A) W.
B) W + P.
C) W + P cos θ.
D) W + P sin θ.
E) W – P sin θ.
6) A push of magnitude P acts on a box of weight W as shown in the figure. The push is directed
at an angle θ below the horizontal, and the box remains a rest. The box rests on a horizontal
surface that has some friction with the box. The friction force on the box due to the floor is equal
to
A) P sin θ
B) P cos θ
C) 0
D) P cos θ + W
E) P + W
7) In the figure, what does the spring scale read? The pulleys are ideal and the strings and scale
are also massless.
A) exactly 2.0 N.
B) exactly 1.0 N
C) 0.50 N
D) 0.00 N
E) more than 19.6 N
8) Two boxes P and Q on a perfectly smooth horizontal surface are connected by a light
horizontal cord. The mass of P is greater than that of Q. A horizontal force is applied to box Q
as shown in the figure, accelerating the bodies to the right. The magnitude of the force exerted by
the connecting cord on body P is
A) zero.
B) less than F but not zero.
C) equal to F.
D) greater than F.
9) Three boxes are pulled along a horizontal frictionless floor by a constant horizontal pull P.
The boxes are connected by very light horizontal strings having tensions T1 and T2 as shown in
the figure. Which of the following statements about the tensions is correct? (There could be more
than one correct choice.)
A) T1 = P
B) T2 = P
C) T1 + T2 = P
D) T2 > T1
E) T1 > T2
10) Two boxes, A and B, are connected by a horizontal string S on a horizontal floor. A very
light wire pulls horizontally on box B, as shown in the figure, with a force of 100 N. The reaction
force to this pull is
A) the pull that box B exerts on string S.
B) the pull that string S exerts on box B.
C) the pull that string S exerts on box A.
D) the pull of box B on the wire.
E) the force that box A exerts on box B.
11) Two boxes are connected to each other by a string as shown in the figure. The 10-N box
slides without friction on the horizontal table surface. The pulley is ideal and the string has
negligible mass. What is true about the tension T in the string?
A) T = 10 N
B) T = 20 N
C) T = 30 N
D) T < 30 N
E) T > 30 N
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12) Two objects of unequal masses, M and m (M > m), are connected by a very light cord passing
over an ideal pulley of negligible mass. When released, the system accelerates, and friction is
negligible.
Which one of the following free-body diagrams most realistically represents the forces acting on
the two objects in the moving system?
A)
B)
C)
D)
13) A 200-N sled of slides down a frictionless hillside that rises at 37° above the horizontal.
What is the magnitude of the force that the surface of the hill exerts on the sled?
A) 200 N
B) 120 N
C) 160 N
D) 150 N
E) 0 N
14) A 200-N sled of slides down a frictionless hillside that rises at 37° above the horizontal.
What is the magnitude of the force that the hill exerts on the sled parallel to the surface of the
hill?
A) 200 N
B) 120 N
C) 160 N
D) 150 N
E) 0 N
15) Two objects have masses m and 5m, respectively. They both are placed side by side on a
frictionless inclined ramp and allowed to slide down from rest without any air resistance. Which
one of the following statements about these objects is correct?
A) It takes the lighter object 5 times longer to reach the bottom of the ramp than the heavier.
B) It takes the lighter object 10 times longer to reach the bottom of the ramp than the heavier.
C) It takes the heavier object 5 times longer to reach the bottom of the ramp than the lighter.
D) It takes the heavier object 10 times longer to reach the bottom of the ramp than the lighter.
E) The two objects reach the bottom of the ramp at exactly the same time.
16) A block of mass m sits at rest on a rough inclined ramp that makes an angle θ with the
horizontal. What must be true about force of static friction f on the block?
A) f > mg
B) f > mg cos θ
C) f > mg sin θ
D) f = mg cos θ
E) f = mg sin θ
17) A block of mass m sits at rest on a rough inclined ramp that makes an angle θ with the
horizontal. What must be true about normal force F on the block due to the ramp?
A) F > mg
B) F > mg cos θ
C) F > mg sin θ
D) F = mg cos θ
E) F = mg sin θ
1) A 50.0-kg crate is being pulled along a horizontal smooth surface. The pulling force is 10.0 N
and is directed 20.0° above the horizontal. What is the magnitude of the acceleration of the crate?
A) 0.0684 m/s2
B) 0.188 m/s2
C) 0.200 m/s2
D) 0.376 m/s2
E) 0.0728 m/s2
2) A 40-kg box is being pushed along a horizontal smooth surface. The pushing force is 15 N
directed at an angle of 15° below the horizontal. What is the magnitude of the acceleration of the
crate?
A) 0.16 m/s2
B) 0.36 m/s2
C) 0.47 m/s2
D) 0.39 m/s2
E) 0.68 m/s2
3) A 1000-kg barge is being towed by means of two horizontal cables. One cable is pulling with
a force of 80.0 N in a direction 30.0° west of north. In what direction should the second cable
pull so that the barge will accelerate northward, if the force exerted by the cable is 120 N?
Assume that the water exerts no appreciable frictional drag on the barge.
A) 19.5° east of north
B) 21.1° east of north
C) 39.0° east of north
D) 47.5° east of north
E) 54.7° east of north
4) Two forces act on a object. One force has magnitude 65 N directed 59° clockwise from
the positive x-axis, and the other has a magnitude 35 N at 32° clockwise from the positive y-axis.
What is the magnitude of this object’s acceleration?
A) 1.1 m/s2
B) 1.3 m/s2
C) 1.5 m/s2
D) 1.7 m/s2
5) A tightrope walker walks across a 30-m long wire tied between two poles. The center of the
wire is displaced vertically downward by 1.0 m when he is halfway across. If the tension in both
halves of the wire at this point is what is the mass of the tightrope walker? Neglect the
mass of the wire.
A) 85 kg
B) 43 kg
C) 74 kg
D) 91 kg
6) A 30.0-kg load is being held in place using massless wires in the ideal pulley arrangement
shown in the figure. What is the magnitude of the force F?
7) An object is being acted upon by three forces and as a result moves with a constant velocity.
One force is 60.0 N along the +x-axis, and the second is 75.0 N along the +y-axis. What is the
magnitude of the third force?
A) 135 N
B) 15.0 N
C) 48.0 N
D) 67.5 N
E) 96.0 N
8) An object is being acted upon by three forces and moves with a constant velocity. One force is
60 N along the +x-axis, the second is 75 N along a direction making a counterclockwise angle of
150° with the +x-axis.
(a) What is the magnitude of the third force?
(b) What is the direction of the third force, measured clockwise from the +x-axis?
9) A piano mover raises a 1000-N piano at a constant speed using a very light rope in a
frictionless pulley system, as shown in the figure. With what force is the mover pulling down on
the rope?
A) 2000 N
B) 1000 N
C) 500 N
D) 250 N
E) Depends on the speed of the piano.
10) A 6.00-kg ornament is held at rest by two light wires that form 30° angles with the vertical,
as shown in the figure. An external force of magnitude F acts vertically downward on the
ornament. The tension exerted by each of the two wires is denoted by T. A free-body diagram,
showing the four forces that act on the box, is shown in the figure. If the magnitude of force F is
410 N, what is the magnitude of the tension T?
A) 271 N
B) 235 N
C) 188 N
D) 376 N
E) 470 N
11) The figure shows a block of mass M hanging at rest. The light wire fastened to the wall is
horizontal and has a tension of 38 N. The wire fastened to the ceiling is also very light, has a
tension of and makes an angle θ with the ceiling. Find the angle θ.
A) 50°
B) 40°
C) 33°
D) 65°
E) 45°
12) A very light wire is used to hang a series of 8.0-kg bricks. This wire will break if the tension
in it exceeds 450 N. The bricks are hung one below the other from a hook in the ceiling using
this wire, as shown in the figure.
(a) How many whole bricks can be hung without breaking the wire?
(b) If you add one more brick to the number found in part (a), which string will break?
13) A 400-kg box is lifted vertically upward with constant velocity by means of two cables
pulling at 40.0° on either side of the vertical direction. What is the tension in each cable?
A) 231 N
B) 400 N
C) 800 N
D) 2560 N
E) 3920 N
14) A 10.0-kg picture is held in place by two wires, the first one hanging at 50.0° to the left of
the vertical and the second one at 45.0° to the right of the vertical. What is the tension in the first
wire?
A) 69.6 N
B) 50.8 N
C) 98.1 N
D) 69.4 N
E) 23.8 N
15) Three objects are connected by weightless flexible strings as shown in the figure. The pulley
has no appreciable mass or friction, and the string connected to the block on the horizontal bench
pulls on it parallel to the bench surface. The coefficients of friction between the bench and the
block on it are µs = 0.66 and µk = 0.325. You observe that this system remains at rest.
(a) Find the mass of the hanging object A.
(b) What is the magnitude of the friction force on the block on the bench?
16) A load is being lifted vertically upwards by means of two cables attached to it. The first
cable exerts a force of 600 N at an angle of 35.0° to the left of the vertical. The second cable
exerts a force of 1300 N. At what angle to the right of the vertical is the second cable pulling?
A) 15.4°
B) 16.2°
C) 30.8°
D) 67.8°
E) 75.8°