Force Problems

rouzbehani (mr39949) – Force – bump – (1004) 1

This print-out should have 28 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering.

001 3.6 points One Newton is

1. the force that gives a l-g body an acceler- ation of 1 cm/s2.

2. the force that gives a 1-kg body an accel- eration of 1 m/s2.

3. the force of gravity on a 1-kg body.

4. the force of gravity on a 1-g body.

5. the force that gives a 1-kg body an accel- eration of 9.8 m/s2.

002 3.6 points When forces are balanced, the motion of the object

1. is decreased.

2. is completely stopped.

3. is increased.

4. does not change.

003 3.6 points A 5.4 kg object is accelerated from rest to a speed of 34.6 m/s in 31 s. What average force was exerted on the ob-

ject during this period of acceleration? Answer in units of N

004 (part 1 of 2) 3.6 points A book weighing 26 N is placed on a table. How much support force does a table exert

on the book? Answer in units of N

005 (part 2 of 2) 3.6 points What is the net force on the book in this case? Answer in units of N

006 3.6 points Force equals mass times

1. velocity.

2. friction.

3. acceleration.

4. inertia.

007 3.6 points The horizontal surface on which the block of mass 5.9 kg slides is frictionless. The force of 34 N acts on the block in a horizontal direction and the force of 68 N acts on the block at an angle as shown below.

5.9 kg

68 N

60◦

34 N

What is the magnitude of the resulting ac- celeration of the block? The acceleration of gravity is 9.8 m/s2 . Answer in units of m/s2

008 3.6 points Which forces always cause a change in mo- tion?

1. fluid

2. balanced

3. unbalanced

4. frictional

009 3.6 points You are pushing a wooden crate across the floor at a constant speed. You decide to turn the crate on end, reducing by half the area in contact with the floor. In the new orientation,

rouzbehani (mr39949) – Force – bump – (1004) 2

to push the same crate across the same floor with the same constant speed, the force that you apply must be about:

1. equally great

2. 2 times as great

3. 1

4 as great

as the force required before you changed the crate’s orientation.

4. 1

2 as great

5. 4 times as great

010 3.6 points What is the tendency of matter to resist any change in motion?

1. friction

2. pressure

3. momentum

4. inertia

011 3.6 points The block of mass 1.8312 kg has an accelera- tion of 5 m/s2 as shown.

1.8312 kg

F 28◦

a

What is the magnitude of F? Assume the acceleration due to gravity is 9.8 m/s2 and the surface is frictionless. Answer in units of N

012 3.6 points No force is necessary to

1. cause a change in the motion of an ob-

ject.

2. start an object moving.

3. stop an object from moving.

4. keep an object moving the way it is al- ready moving.

013 3.6 points Two forces are the only forces acting on a 6.8 kg object which moves with an accelera- tion of 3.9 m/s2 in the positive y direction. One of the forces acts in the positive x direc- tion and has a magnitude of 14 N. What is the magnitude of the other force

f2? Answer in units of N

014 3.6 points Two forces are applied to a 50-kg box. One is 60.0 N to the south and the other is 80.0 N to the east. These are the only forces acting on the box. The magnitude of the acceleration of the

box is:

1. 0.50 m/s2

2. 2.0 m/s2

3. 10 m/s2

4. 5.2 m/s2

5. 1.5 m/s2

015 3.6 points A push or pull that gives energy to an ob- ject, causing that object to start moving, stop moving, or change its motion is

1. inertia.

2. a force.

3. friction.

4. pressure.

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016 3.6 points Blocks of mass 3M (on the left) and 2M (on the right) are in contact with one another on a frictionless horizontal surface. An external force 4F acting to the right is applied to the block of mass 3M, and an external force 2F acting to the left is applied to the block of mass 2M. Find the magnitude of the force that mass 2M exerts on the block of mass 3M.

1. 3F/5

2. 7F/5

3. 2F/3

4. 14F/5

5. 3F/2

6. 4F/5

7. 9F/2

8. 2F/5

9. 11F/4

10. 7F/2

017 3.6 points Consider a car at rest. We can conclude that the downward gravitational pull of Earth on the car and the upward contact force of Earth on it are equal and opposite because

1. Neither of these

2. the net force on the car is zero.

3. the two forces form an interaction pair.

018 3.6 points Two students sit in identical office chairs fac- ing each other. Bob has a mass of 95 kg, while Jim has a mass of 77 kg. Bob places his bare feet on Jim’s knees, as shown to the right. Bob then suddenly pushes outward with his

feet, causing both chairs to move.

Bob Jim

In this situation, while Bob’s feet are in contact with Jim’s knees,

1. Each student exerts a force on the other, but Bob exerts a larger force.

2. Bob exerts a force on Jim, but Jim doesn’t exert a force on Bob.

3. Neither student exerts a force on the other.

4. None of these answers is correct.

5. Each student exerts a force on the other, but Jim exerts a larger force.

6. Each student exerts the same amount of force on the other.

019 3.6 points A 6.2 kg block initially at rest is pulled to the right along a horizontal, frictionless surface by a constant, horizontal force of 18.6 N. Find the speed of the block after it has

moved 4.1 m. Answer in units of m/s

020 3.6 points Note: The left-hand mass is equal to the sum of the other two masses. A massless pulley is attached to the ceiling,

in a uniform gravitational field, and rotates with no friction about its pivot. Another massless pulley is attached to the end of the cord on the right and rotates with no friction about its pivot. The acceleration of gravity is 9.8 m/s2 .

rouzbehani (mr39949) – Force – bump – (1004) 4

33 kg 23 kg 10 kg

What is the magnitude of the acceleration of the 23 kg mass? Answer in units of m/s2

021 3.6 points

Which of the statements A. When you push an object, you transfer

energy to the object; B. When the forces on an object are balanced,

motion occurs; C. The force needed to overcome sliding fric-

tion is greater than the force needed to overcome rolling friction.

is/are true?

1. C only

2. A and C only

3. A only

4. A and B only

5. B only

6. None is true.

7. All are true.

8. B and C only

022 3.6 points For every action, the reaction is

1. equal and in the same direction.

2. unequal and opposite in direction.

3. unequal and in the same direction.

4. equal and opposite in direction.

023 (part 1 of 3) 3.6 points

The following 3 questions refer to the colli- sions between a car and a truck whose weight is much heavier than the car (M ≫ m). For each description of a collision below, choose the one answer from the possibilities that best describes the size (or magnitude) of the forces between the car and the truck.

m M

v v

They are both moving at the same speed when they collide.

1. None of the answers above correctly de- scribes the situation.

2. The car exerts a greater amount of force on the truck than the truck exerts on the car.

3. Neither exerts a force on the other; the car gets smashed simply because it is in the way of the truck.

4. The truck exerts a greater amount of force on the car than the car exerts on the truck.

5. Not enough information is given to pick one of the answers above.

6. The truck exerts the same amount of force on the car as the car exerts on the truck.

024 (part 2 of 3) 3.6 points The car is moving much faster than the heav- ier truck when they collide.

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1. The car exerts a greater amount of force on the truck than the truck exerts on the car.

2. Neither exerts a force on the other; the car gets smashed simply because it is in the way of the truck.

3. The truck exerts a greater amount of force on the car than the car exerts on the truck.

4. The truck exerts the same amount of force on the car as the car exerts on the truck.

5. Not enough information is given to pick one of the answers above.

6. None of the answers above correctly de- scribes the situation.

025 (part 3 of 3) 3.6 points The heavier truck is standing still when the car hits it.

1. Not enough information is given to pick one of the answers above.

2. Neither exerts a force on the other; the car gets smashed simply because it is in the way of the truck.

3. The car exerts a greater amount of force on the truck than the truck exerts on the car.

4. The truck exerts a greater amount of force on the car than the car exerts on the truck.

5. None of the answers above correctly de- scribes the situation.

6. The truck exerts the same amount of force on the car as the car exerts on the truck.

026 3.6 points Suppose a large number of different forces acts upon an object. Is it possible for the object to have an ac-

celeration of zero?

1. Yes; the forces could add vectorially to zero.

2. No; nonzero force means nonzero acceler- ation: F = ma.

3. None of these

4. The question does not make sense. Forces and accelerations are entirely different con- cepts; forces are measured in Newtons and accelerations in meters per second per sec- ond.

027 (part 1 of 2) 3.6 points An object in equilibrium has three forces ex- erted on it. A(n) 37 N force acts at 65.6◦, and a(n) 48 N force acts at 47.3◦. What is the direction of the third force?

(Consider all angles to be measured counter- clockwise from the positive x-axis.) Answer in units of ◦

028 (part 2 of 2) 3.6 points What is the magnitude of the third force? Answer in units of N