Key Concepts
How do forces affect the motion of an object?
What are the four main types of friction?
How do gravity and air resistance affect a falling object?
In what direction does Earth's gravity act?
Why does a projectile follow a curved path?
Vocabulary
force
newton
net force
friction
static friction
sliding friction
rolling friction
fluid friction
air resistance
gravity
terminal velocity
projectile motion
Reading Strategy
Relating Text and Visuals Copy the table below. As you read, look carefully at Figures 2,3, and 5. Complete the table by describing the forces and motion shown in each figure.
Figure |
Is Net Force 0? |
Effect on Motion |
---|---|---|
2A |
a. |
b. |
2B |
c. |
d. |
3 |
e. |
f. |
5A |
g. |
h. |
5B |
g. |
h. |
Figure 1 The wind pushes against the man and his umbrella. The push from the wind is a force.
A powerful storm is approaching. The weather forecast calls for gale-force winds. Many people in the city decide to leave work earlyin order to get home before things get worse. As shown in Figure 1, a man pushes ahead into a strong wind and shields himself from the driving rain with an umbrella. The strong wind makes it very difficult for him to hold onto his umbrella. To keep the umbrella from being pulled from his hands, he tightly squeezes the umbrella handle. Elsewhere, a store owner attempts to bring in a folding sign that hasn't blown awaybecause it is chained to a pole.
Wind is but one example of the many forces you experience every day. The study of forces is a very important part of physics. As you read this section you'll learn what forces are and how they make things move.
The man out in the storm is battling the forces of wind. A force is a push or a pull that acts on an object. A force can cause a resting object to move, or it can accelerate a moving object by changing the object's speed or direction. The force of the wind pushing against the man slows his speed. A strong gust could even change the direction in which he was moving.