Question

Section 12.1 Forces
(pages 356–362)
This section describes what forces are and explains how forces affect the
motion of various objects.
Reading Strategy (page 356)
Relating Text and Visuals As you read about forces, look carefully
at Figures 2, 3, and 5 in your textbook. Then complete the table by
describing the forces and motion shown in each figure. For more
information on this Reading Strategy, see the Reading and Study
Skills in the Skills and Reference Handbook at the end of
your textbook.
Forces and Motion
Figure
Is Net Force 0?
Effect on Motion
2A
2B
3
5A
5B
What is a Force? (pages 356–357)
1. A force is defined as a(n)
or a(n)
that acts on an object.
2. Is the following sentence true or false? A force can act to cause an
object at rest to move or it can accelerate an object that is already
moving.
3. How can a force change the motion of an object that is already moving?
4. Circle the letter of the best answer. What force causes a 1-kg mass to
accelerate at a rate of 1 meter per second each second?
a. 1 kg/m•s2
b. 1 kg/s
c. 1 kg•m
d. 1 newton
Combining Forces (pages 357–358)
5. The overall force acting on an object after all the forces are combined
is the
.
6. How do balanced and unbalanced forces affect the motion of an object?

Chapter 12 103
Yes
None
Yes
None
Yes
None
Yes
None
No
Potted plant accelerates

Friction (pages 359–360)
7. Is the following sentence true or false? Friction is a force that helps
objects that are touching move past each other more easily.
8. Circle the letters that identify types of friction.
a. rolling
b. gravity
c. static
d. sliding
9. The friction force that acts on objects that are at rest is
.
10. Why is less force needed to keep an object moving than to start
the object in motion?
11. Complete the table below about friction forces.
Types of Friction Forces
Friction Force
Example
Static
Pushing a book along your desk
Rolling
12. Is the following sentence true or false? Fluid friction is a force that
opposes the motion of an object through a fluid such as water.
Gravity (page 361)
13. Gravity is a(n)
force that pulls objects together.
14. Is the following sentence true or false? Earth’s gravity acts
downward toward the center of Earth.
15. Describe how gravity and air resistance affect the motion of a
falling object.
16. Is the following sentence true or false? Terminal velocity is the
constant velocity of a falling object when the force of air resistance
equals the force of gravity.
Projectile Motion (page 362)
17. The curved path caused by the combination of an initial forward
velocity and the downward force of gravity is known as
motion.

Answer 1

Forces and Motion (figures)
- 2A: Is net force 0? Yes. Effect on motion: None (no change in motion).
- 2B: Is net force 0? Yes. Effect on motion: None.
- 3: Is net force 0? Yes. Effect on motion: None.
- 5A: Is net force 0? Yes. Effect on motion: None.
- 5B: Is net force 0? No. Effect on motion: Potted plant accelerates.

What is a Force?
1. A force is defined as a(n) push or a(n) pull that acts on an object.
2. True.
3. A force can change the speed of a moving object (speed it up or slow it down), change its direction, or both (i.e., cause acceleration).
4. d. 1 newton

Combining Forces
5. The overall force after all forces are combined is the net force.
6. Balanced forces produce a net force of zero and do not change an object’s motion (the object remains at rest or moves with constant velocity). Unbalanced forces produce a nonzero net force and cause the object to accelerate (change speed and/or direction).

Friction
7. False. Friction is a force that opposes motion (it makes sliding/motion more difficult).
8. a, c, d (rolling, static, sliding)
9. Static friction
10. Less force is needed to keep an object moving because static friction (which must be overcome to start motion) is generally larger than kinetic (sliding) friction. Once moving, you only need to overcome the smaller kinetic friction.
11. Types of friction forces — examples:
- Static friction: prevents a heavy box from starting to slide on the floor (e.g., pushing a box that doesn’t move).
- Sliding (kinetic) friction: pushing a book along your desk.
- Rolling friction: a bicycle wheel rolling on the pavement.
- Fluid friction (drag): a skydiver falling through air or a boat moving through water.
12. True.

Gravity
13. Gravity is a noncontact (attractive) force that pulls objects together.
14. True.
15. Gravity pulls a falling object downward, causing it to accelerate. Air resistance acts upward (opposite the motion), increasing with speed, and reduces the object’s acceleration; at high enough speed air resistance can balance gravity.
16. True.
17. Projectile motion.