### Multiple Choice

#### 4.1 Force

Which of the following is a physical quantity that can be described by dynamics but not by kinematics?

- Velocity
- Acceleration
- Force

Which of the following is used to represent an object in a free-body diagram?

- A point
- A line
- A vector

#### 4.2 Newton's First Law of Motion: Inertia

What kind of force is friction?

- External force
- Internal force
- Net force

What is another name for Newton’s first law?

- Law of infinite motion
- Law of inertia
- Law of friction

True or False—A rocket is launched into space and escapes Earth’s gravitational pull. It will continue to travel in a straight line until it is acted on by another force.

- True
- False

A 2,000-kg car is sitting at rest in a parking lot. A bike and rider with a total mass of 60 kg are traveling along a road at 10 km/h. Which system has more inertia? Why?

- The car has more inertia, as its mass is greater than the mass of the bike.
- The bike has more inertia, as its mass is greater than the mass of the car.
- The car has more inertia, as its mass is less than the mass of the bike.
- The bike has more inertia, as its mass is less than the mass of the car.

#### 4.3 Newton's Second Law of Motion

In the equation for Newton’s second law, what does F_{net} stand for?

- Internal force
- Net external force
- Frictional force

What is the SI unit of force?

- Kg
- dyn
- N

What is the net external force on an object in freefall on Earth if you were to neglect the effect of air?

- The net force is zero.
- The net force is upward with magnitude
*mg*. - The net force is downward with magnitude
*mg*. - The net force is downward with magnitude 9.8 N.

Two people push a 2,000-kg car to get it started. An acceleration of at least 5.0 m/s^{2} is required to start the car. Assuming both people apply the same magnitude force, how much force will each need to apply if friction between the car and the road is 300 N?

- 4850 N
- 5150 N
- 97000 N
- 10300 N

#### 4.4 Newton's Third Law of Motion

One object exerts a force of magnitude F_{1} on another object and experiences a force of magnitude F_{2} in return. What is true for F_{1} and F_{2}?

- F
_{1}> F_{2} - F
_{1}< F_{2} - F
_{1}= F_{2}

A weight is suspended with a rope and hangs freely. In what direction is the tension on the rope?

- parallel to the rope
- perpendicular to the rope

A person weighing 55 kg walks by applying 160 N of force on the ground, while pushing a 10-kg object. If the person accelerates at 2 m/s^{2}, what is the force of friction experienced by the system consisting of the person and the object?

- 30 N
- 50 N
- 270 N
- 290 N

A 65-kg swimmer pushes on the pool wall and accelerates at 6 m/s^{2}. The friction experienced by the swimmer is 100 N. What is the magnitude of the force that the swimmer applies on the wall?

- −490 N
- −290 N
- 290 N
- 490 N

### Short Answer

#### 4.1 Force

True or False—An external force is defined as a force generated outside the system of interest that acts on an object inside the system.

- True
- False

- A positive sign ( $+$ )
- A negative sign ( $-$ )
- Either a positive or negative sign ( $\pm $ )
- No sign is assigned

A body is pushed downward by a force of 5 units and upward by a force of 2 units. How would you draw a free-body diagram to represent this?

- Two force vectors acting at a point, both pointing up with lengths of 5 units and 2 units
- Two force vectors acting at a point, both pointing down with lengths of 5 units and 2 units
- Two force vectors acting at a point, one pointing up with a length of 5 units and the other pointing down with a length of 2 units
- Two force vectors acting at a point, one pointing down with a length of 5 units and the other pointing up with a length of 2 units

A body is pushed eastward by a force of four units and southward by a force of three units. How would you draw a free-body diagram to represent this?

- Two force vectors acting at a point, one pointing left with a length of 4 units and the other pointing down with a length of 3 units
- Two force vectors acting at a point, one pointing left with a length of 4 units and the other pointing up with a length of 3 units
- Two force vectors acting at a point, one pointing right with a length of 4 units and the other pointing down with a length of 3 units
- Two force vectors acting at a point, one pointing right with a length of 4 units and the other pointing up with a length of 3 units

#### 4.2 Newton's First Law of Motion: Inertia

A body with mass m is pushed along a horizontal surface by a force F and is opposed by a frictional force *f*. How would you draw a free-body diagram to represent this situation?

- A dot with an arrow pointing right, labeled F, and an arrow pointing left, labeled
*f*, that is of equal length or shorter than F - A dot with an arrow pointing right, labeled F, and an arrow pointing right, labeled
*f*, that is of equal length or shorter than F - A dot with an arrow pointing right, labeled F, and a smaller arrow pointing up, labeled
*f*, that is of equal length or longer than F - A dot with an arrow pointing right, labeled F, and a smaller arrow pointing down, labeled
*f*, that is of equal length or longer than F

- The mass of the first object is less than that of the second object.
- The mass of the first object is equal to the mass of the second object.
- The mass of the first object is greater than that of the second object.
- No inference can be made because mass and force are not related to each other.

Two similar boxes rest on a table. One is empty and the other is filled with pebbles. Without opening or lifting either, how can you tell which box is full? Why?

- By applying an internal force; whichever box accelerates faster is lighter and so must be empty
- By applying an internal force; whichever box accelerates faster is heavier and so the other box must be empty
- By applying an external force; whichever box accelerates faster is lighter and so must be empty
- By applying an external force; whichever box accelerates faster is heavier and so the other box must be empty

True or False—An external force is required to set a stationary object in motion in outer space away from all gravitational influences and atmospheric friction.

- True
- False

#### 4.3 Newton's Second Law of Motion

A steadily rolling ball is pushed in the direction from east to west, which causes the ball to move faster in the same direction. What is the direction of the acceleration?

- North to south
- South to north
- East to west
- West to east

A ball travels from north to south at 60 km/h. After being hit by a bat, it travels from west to east at 60 km/h. Is there a change in velocity?

- Yes, because velocity is a scalar.
- Yes, because velocity is a vector.
- No, because velocity is a scalar.
- No, because velocity is a vector

What is the weight of a 5-kg object on Earth and on the moon?

- On Earth the weight is 1.67 N, and on the moon the weight is 1.67 N.
- On Earth the weight is 5 N, and on the moon the weight is 5 N.
- On Earth the weight is 49 N, and on the moon the weight is 8.35 N.
- On Earth the weight is 8.35 N, and on the moon the weight is 49 N.

An object weighs 294 N on Earth. What is its weight on the moon?

- 50.1 N
- 30.0 N
- 249 N
- 1461 N

#### 4.4 Newton's Third Law of Motion

A large truck with mass 30 m crashes into a small sedan with mass *m*. If the truck exerts a force F on the sedan, what force will the sedan exert on the truck?

- $\frac{\text{F}}{30}$
- F
- 2F
- 30F

A fish pushes water backward with its fins. How does this propel the fish forward?

- The water exerts an internal force on the fish in the opposite direction, pushing the fish forward.
- The water exerts an external force on the fish in the opposite direction, pushing the fish forward.
- The water exerts an internal force on the fish in the same direction, pushing the fish forward.
- The water exerts an external force on the fish in the same direction, pushing the fish forward.

True or False—Tension is the result of opposite forces in a connector, such as a string, rope, chain or cable, that pulls each point of the connector apart in the direction parallel to the length of the connector. At the ends of the connector, the tension pulls toward the center of the connector.

- True
- False

True or False—Normal reaction is the force that opposes the force of gravity and acts in the direction of the force of gravity.

- True
- False

### Extended Response

#### 4.1 Force

True or False—When two unequal forces act on a body, the body will not move in the direction of the weaker force.

- True
- False

_{restore}?

- F
_{restore}is the force exerted by the hand on the spring, and it pulls to the right. - F
_{restore}is the force exerted by the spring on the hand, and it pulls to the left. - F
_{restore}is the force exerted by the hand on the spring, and it pulls to the left. - F
_{restore}is the force exerted by the spring on the hand, and it pulls to the right.

#### 4.2 Newton's First Law of Motion: Inertia

- No, the balls will not necessarily travel the same distance because the gravitational force acting on them is different.
- No, the balls will not necessarily travel the same distance because the angle at which they are thrown may differ.
- Yes, the balls will travel the same distance because the gravitational force acting on them is the same.
- Yes, the balls will travel the same distance because the angle at which they are thrown may differ.

A person pushes a box from left to right and then lets the box slide freely across the floor. The box slows down as it slides across the floor. When the box is sliding freely, what is the direction of the net external force?

- The net external force acts from left to right.
- The net external force acts from right to left.
- The net external force acts upward.
- The net external force acts downward.

#### 4.3 Newton's Second Law of Motion

A 55-kg lady stands on a bathroom scale inside an elevator. The scale reads 70 kg. What do you know about the motion of the elevator?

- The elevator must be accelerating upward.
- The elevator must be accelerating downward.
- The elevator must be moving upward with a constant velocity.
- The elevator must be moving downward with a constant velocity.

True or False—A skydiver initially accelerates in his jump. Later, he achieves a state of constant velocity called terminal velocity. Does this mean the skydiver becomes weightless?

- Yes
- No

#### 4.4 Newton's Third Law of Motion

How do rockets propel themselves in space?

- Rockets expel gas in the forward direction at high velocity, and the gas, which provides an internal force, pushes the rockets forward.
- Rockets expel gas in the forward direction at high velocity, and the gas, which provides an external force, pushes the rockets forward.
- Rockets expel gas in the backward direction at high velocity, and the gas, which is an internal force, pushes the rockets forward.
- Rockets expel gas in the backward direction at high velocity, and the gas, which provides an external force, pushes the rockets forward.

Are rockets more efficient in Earth’s atmosphere or in outer space? Why?

- Rockets are more efficient in Earth’s atmosphere than in outer space because the air in Earth’s atmosphere helps to provide thrust for the rocket, and Earth has more air friction than outer space.
- Rockets are more efficient in Earth’s atmosphere than in outer space because the air in Earth’s atmosphere helps to provide thrust to the rocket, and Earth has less air friction than the outer space.
- Rockets are more efficient in outer space than in Earth’s atmosphere because the air in Earth’s atmosphere does not provide thrust but does create more air friction than in outer space.
- Rockets are more efficient in outer space than in Earth’s atmosphere because the air in Earth’s atmosphere does not provide thrust but does create less air friction than in outer space.