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University Physics, 13e (Young/Freedman)

Chapter 4 Newtons Laws of Motion

4.1 Conceptual Questions

1) 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.

B) exactly 1500 N whether or not the box moves.

C) greater than 1500 N if the box moves.

D) less than 1500 N if the box moves.

E) greater than 1500 N if the bat bounces back.

Answer: B

Var: 1

2) Point P in the figure indicates the position of an object traveling at constant speed clockwise around the circle. Which arrow best represent the direction the object would travel if the net external force on it were suddenly reduced to zero?

A)

B)

C)

D)

E)

Answer: D

Var: 1

3) An object is moving to the right, and experiencing a net force that is directed to the right. The magnitude of the force is decreasing with time. The speed of the object is

A) increasing.

B) decreasing.

C) constant in time.

Answer: A

Var: 1

4) A stalled car is being pushed up a hill at constant velocity by three people. The net force on the car is

A) zero.

B) up the hill and equal to the weight of the car.

C) down the hill and equal to the weight of the car.

D) up the hill and greater than the weight of the car.

E) down the hill and greater than the weight of the car.

Answer: A

Var: 1

5) In order to get an object moving, you must push harder on it than it pushes back on you.

A) True

B) False

Answer: B

Var: 1

6) In order to lift a bucket of concrete, you must pull up harder on the bucket than it pulls down on you.

A) True

B) False

Answer: B

Var: 1

7) A car is being towed at constant velocity on a horizontal road using a horizontal chain. The tension in the chain must be equal to the weight of the car in order to maintain constant velocity.

A) True

B) False

Answer: B

Var: 1

8) A box of mass m is pulled with a constant acceleration a along a horizontal frictionless floor by a wire that makes an angle of 15 above the horizontal. If T is the tension in this wire, then

A) T = ma.

B) T > ma.

C) T < ma.
Answer: B
Var: 1
9) You are standing in a moving bus, facing forward, and you suddenly fall forward as the bus comes to an immediate stop. The force acting on you that causes you to fall forward is
A) the force of gravity.
B) the normal force due to your contact with the floor of the bus.
C) the force due to static friction between you and the floor of the bus.
D) the force due to kinetic friction between you and the floor of the bus.
E) No forces were acting on you to cause you to fall.
Answer: E
Var: 1
10) Consider what happens when you jump up in the air. Which of the following is the most accurate statement?
A) It is the upward force exerted by the ground that pushes you up, but this force cannot exceed your weight.
B) You are able to spring up because the earth exerts a force upward on you that is greater than the downward force you exert on the earth.
C) Since the ground is stationary, it cannot exert the upward force necessary to propel you into the air. Instead, it is the internal forces of your muscles acting on your body itself that propels your body into the air.
D) When you push down on the earth with a force greater than your weight, the earth will push back with the same magnitude force and thus propel you into the air.
E) When you jump up the earth exerts a force F1 on you and you exert a force F2 on the earth. You go up because F1 > F2.

Answer: D

Var: 1

11) A 20-ton truck collides with a 1500-lb car and causes a lot of damage to the car. During the collision

A) the force on the truck due to the collision is slightly greater than the force on the car.

B) the force of on the truck due to the collision is exactly equal to the force on the car.

C) the force on the car due to the collision is much greater than the force on the truck.

D) the car and the truck have the same magnitude acceleration.

Answer: B

Var: 1

12) Two objects, each of weight W, hang vertically by spring scales as shown in the figure. The pulleys and the strings attached to the objects have negligible weight, and there is no appreciable friction in the pulleys. The reading in each scale is

A) W.

B) more than W, but not quite twice as much.

C) less than W.

D) 2W.

E) more than 2W.

Answer: A

Var: 1

13) A fish weighing 16 N is weighed using two spring scales, each of negligible weight, as shown in the figure. What will be the readings of the scales?

A) The bottom scale will read 16 N, and the top scale will read zero.

B) Each scale will read 16 N.

C) The top scale will read 16 N, and the bottom scale will read zero.

D) The scales will have different readings, but the sum of the two readings will be 16 N.

E) Each scale will read 8 N.

Answer: B

Var: 1

14) An object is moving forward with a constant velocity. Which statement about this object MUST be true?

A) The net force on the object is zero.

B) The net force on the object is in the forward direction.

C) No forces are acting on the object.

D) The acceleration of the object is in the forward direction.

Answer: A

Var: 1

15) Suppose you are playing hockey on a new-age ice surface for which there is no friction between the ice and the hockey puck. You wind up and hit the puck as hard as you can. After the puck loses contact with your stick, the puck will

A) start to slow down.

B) not slow down or speed up.

C) speed up a little, and then slow down.

D) speed up a little, and then move at a constant speed.

Answer: B

Var: 1

16) A ball is tossed vertically upward. When it reaches its highest point (before falling back downward)

A) the velocity is zero, the acceleration is directed downward, and the force of gravity acting on the ball is directed downward.

B) the velocity is zero, the acceleration is zero, and the force of gravity acting on the ball is zero.

C) the velocity is zero, the acceleration is zero, and the force of gravity acting on the ball is directed downward.

D) the velocity and acceleration reverse direction, but the force of gravity on the ball remains downward.

E) the velocity, acceleration, and the force of gravity on the ball all reverse direction.

Answer: A

Var: 1

17) A dog is standing in the bed of a pickup truck. The bed is coated with ice, causing the force of friction between the dog and the truck to be zero. The truck is initially at rest, and then accelerates to the right, moving along a flat road. As seen from a stationary observer (watching the truck move to the right), the dog

A) does not move left or right, but the back of the truck moves towards the dog.

B) moves to the right, but not as quickly as the truck is moving to the right, causing it to slide towards the back of the truck.

C) moves to the right at the same rate as the truck, so it doesnt slide.

D) moves to the left, as the truck moves to the right, causing the dog to slide towards the back of the truck.

Answer: A

Var: 1

18) You are seated in a bus and notice that a hand strap that is hanging from the ceiling hangs away from the vertical in the backward direction. From this observation, you can conclude that

A) the velocity of the bus is forward.

B) the velocity of the bus is backward.

C) You cannot conclude anything about the direction of the velocity of the bus.

Answer: C

Var: 1

4.2 Problems

1) A block lies on a horizontal frictionless surface. A horizontal force of 100 N is applied to the block giving rise to an acceleration of 3.0 m/s2.

(a) Determine the mass of the block.

(b) Calculate the distance the block will travel if the force is applied for 10 s.

(c) Calculate the speed of the block after the force has been applied for 10 s.

Answer: (a) 33 kg (b) 150 m (c) 30 m/s

Var: 1

2) If a 5.0 kg box is pulled simultaneously by a 10.0 N force and a 5.0 N force, then its acceleration must be

A) 3.0 m/s2.

B) 2.2 m/s2.

C) 1.0 m/s2.

D) We cannot tell from the information given.

Answer: D

Var: 1

3) The figure shows an acceleration-versus-force graph for three objects pulled by rubber bands. The mass of object 2 is 36 kg. What are the masses of objects 1 and 3?

A) 14 kg and 90 kg

B) 72 kg and 18 kg

C) 90 kg and 18 kg

D) 14 kg and 72 kg

Answer: A

Var: 50+

4) A 7.0-kg object is acted on by two forces. One of the forces is 10.0 N acting toward the east. Which of the following forces is the other force if the acceleration of the object is 1.0 m/s2 toward the east?

A) 6.0 N east

B) 3.0 N west

C) 12 N east

D) 9.0 N west

E) 7.0 N west

Answer: B

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5) The following four forces act on a 4.00 kg object:

1 = 300 N east

2 = 700 N north

3 = 500 N west

4 = 600 N south

What is the acceleration of the object?

A) 224 N in a direction 63.4 north of west

B) 300 N in a direction 63.4 north of west

C) 300 N in a direction 26.6 north of west

D) 224 N in a direction 26.6 north of west

E) 2100 N in a direction 26.6 north of west

Answer: D

Var: 1

6) The International Space Station has a mass of 1.8 105 kg. A 70.0-kg astronaut inside the station pushes off one wall of the station so she accelerates at 1.50 m/s2. What is the magnitude of the acceleration of the space station as the astronaut is pushing off the wall? Give your answer relative to an observer who is space walking and therefore does not accelerate with the space station due to the push.

A) 5.8 10-4 m/s2

B) 1.50 m/s2

C) 4.7 10-4 m/s2

D) zero

E) 3.9 10-3 m/s2

Answer: A

Var: 1

7) On a horizontal frictionless floor, a worker of weight 0.900 kN pushes horizontally with a force of 0.200 kN on a box weighing 1.80 kN. As a result of this push, which statement could be true?

A) The box will not move because the push is less than its weight.

B) The worker and box will both have an acceleration of 1.08 m/s2, but in opposite directions.

C) The worker and box will both have an acceleration of 2.17 m/s2, but in opposite directions.

D) The worker will accelerate at 1.08 m/s2 and the box will accelerate at 2.17 m/s2, but in opposite directions.

E) The worker will accelerate at 2.17 m/s2 and the box will accelerate at 1.08 m/s2, but in opposite directions.

Answer: E

Var: 1

8) A 50.0-N box is sliding on a rough horizontal floor, and the only horizontal force acting on it is friction. You observe that at one instant the box is sliding to the right at 1.75 m/s and that it stops in 2.25 s with uniform acceleration. What magnitude force does friction exert on this box?

A) 3.97 N

B) 8.93 N

C) 38.9 N

D) 50.0 N

E) 490 N

Answer: A

Var: 1

9) A block is on a frictionless horizontal table, on earth. This block accelerates at 1.9 m/s2 when a 90 N horizontal force is applied to it. The block and table are then set up on the moon where the acceleration due to gravity is 1.62 m/s2. What is the weight of the block on the moon?

A) 77 N

B) 67 N

C) 58 N

D) 48 N

E) 39 N

Answer: A

Var: 50+

10) A block is on a frictionless horizontal table, on earth. This block accelerates at 3.6 m/s2 when a 90 N horizontal force is applied to it. The block and table are then set up on the moon where the acceleration due to gravity is 1.62 m/ s2. A horizontal force of 45 N is applied to the block when it is on the moon. What acceleration does this force impart to the block?

A) 1.8 m/s2

B) 1.6 m/s2

C) 2.0 m/s2

D) 2.2 m/s2

E) 2.3 m/s2

Answer: A

Var: 50+

11) An 1100-kg car traveling at 27.0 m/s starts to slow down and comes to a complete stop in 578 m. What is the magnitude of the average braking force acting on the car?

A) 690 N

B) 550 N

C) 410 N

D) 340 N

Answer: A

Var: 1

12) On its own, a certain tow-truck has a maximum acceleration of 3.0 m/s2. What would be the maximum acceleration when this truck was towing a bus of twice its own mass?

A) 2.5 m/s2

B) 2.0 m/s2

C) 1.5 m/s2

D) 1.0 m/s2

Answer: D

Var: 1

13) In a ballistics test, a 1.50-g bullet is fired through a 28.0-kg block traveling horizontally toward the bullet. In this test, the bullet takes 11.4 ms to pass through the block as it reverses the blocks velocity from 1.75 m/s to the right to 1.20 m/s to the left with constant acceleration. Find the magnitude of the force that the bullet exerts on the block during this ballistics test.

Answer: 7.25 103 N

Var: 1

14) A 10,000-kg rocket blasts off from earth with a uniform upward acceleration of 2.00 m/s2 and feels no air resistance. The upward thrust force its engines must provide during this acceleration is closest to

A) 20,000 N.

B) 980,000 N.

C) 118,000 N.

D) 78,000 N.

Answer: C

Var: 1

15) Bumpers on cars are not of much use in a collision. To see why, calculate the average force a bumper would have to exert if it brought a 1200-kg car (a so-called compact model) to a rest in 15 cm when the car had an initial speed of 2.0 m/s (about 4.5 mph). (Bumpers are built with springs that compress to provide a stopping force without, hopefully, denting the metal.)

A) 1.8 104 N

B) 1.6 104 N

C) 5.4 104 N

D) 6.5 105 N

E) 3.2 104 N

Answer: B

Var: 1

16) A box of mass 50 kg is at rest on a horizontal frictionless surface. A constant horizontal force F then acts on the box and accelerates it to the right. It is observed that it takes the box 6.9 seconds to travel 28 meters. What is the magnitude of the force?

Answer: 59 N

Var: 50+

17) A locomotive is pulling 19 freight cars, each of which is loaded with the same amount of weight. The mass of each freight car (with its load) is 37,000 kg. If the train is accelerating at 0.22 m/s2 on a level track, what is the tension in the coupling between the second and third cars? (The car nearest the locomotive is counted as the first car, and friction is negligible.)

Answer: 140,000 N

Var: 50+

18) A 1000-kg car is driving toward the north along a straight horizontal road at a speed of 20.0 m/s. The driver applies the brakes and the car comes to a rest uniformly in a distance of 200 m. What are the magnitude and direction of the net force applied to the car to bring it to rest?

A) 1.00 N north

B) 10.0 103 N south

C) 1.00 103 N south

D) 1.00 N south

E) 100 N south

Answer: C

Var: 5

19) A construction worker pulls a box of tools on a smooth horizontal floor with a force of 100 N in a direction of 37.0 above the horizontal. The mass of the box and the tools is 40.0 kg.

(a) Draw a free-body diagram for the box.

(b) Calculate the acceleration of the box.

(c) How hard does the floor push up on the box?

Answer:

(a) The box is acted on by the force of gravity which points downward toward the center of the earth. The normal force is directed toward the box perpendicular to the surface of the floor. The pulling force is directed away from the box at an angle 37.0 degrees above the horizontal.

(b) 2.00 m/s2

(c) 332 N

Var: 1

20) A series of weights connected by very light cords are given an upward acceleration of 4.00 m/s2 by a pull P, as shown in the figure. A, B, and C are the tensions in the connecting cords. The pull P is closest to

A) 690 N.

B) 490 N.

C) 290 N.

D) 200 N.

E) 50 N.

Answer: A

Var: 1

21) A series of weights connected by very light cords are given an upward acceleration of 4.00 m/s2 by a pull P, as shown in the figure. A, B, and C are the tensions in the connecting cords. The SMALLEST of the three tensions, A, B, and C, is closest to

A) 80.0 N.

B) 196 N.

C) 276 N.

D) 483 N.

E) 621 N.

Answer: C

Var: 1

22) The figure shows a graph of the acceleration of a 125-g object as a function of the net force acting on it. What is the acceleration at points A and B?

Answer: A: 16 m/s2, B: 4.0 m/s2

Var: 1

23) The figure shows a graph of the acceleration of an object as a function of the net force acting on it. The mass of this object, in grams, is closest to

A) 130.

B) 11.

C) 89.

D) 8000.

Answer: A

Var: 1

24) Two forces act on a 55-kg 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 objects acceleration?

A) 1.1 m/s2

B) 1.3 m/s2

C) 1.5 m/s2

D) 1.7 m/s2

Answer: A

Var: 46

25) The graph in the figure shows the x component of the acceleration of a 2.4-kg object as a function of time (in ms).

(a) At what time(s) does the x component of the net force on the object reach its maximum magnitude, and what is that maximum magnitude?

(b) What is the x component of the net force on the object at time t = 0.0 ms and at t = 4.0 ms?

Answer: (a) At 3.0 ms, 48 N (b) 12 N, -24 N

Var: 1

26) The graph in the figure shows the net force acting on a 3.0-kg object as a function of time.

(a) What is the acceleration of this object at time t = 2.0 s?

(b) Draw, to scale, a graph of the acceleration of this object as a function of time over the range t = 0.00 s to t = 7.0 s.

Answer:

(a) 2.0 m/s2

(b) The acceleration-time graph looks the same as the force-time graph except on the vertical axis the numbers (starting at 2.0) are replaced by 0.67, 1.3, 2.0, 2.7, 3.3, and 4.0.

Var: 1

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