Exploring Forces · Lesson 1 of 7
Force as Push, Pull, and Interaction
“Build the idea of force from familiar pushes and pulls, then connect force to interaction and changes in motion or shape.”
• Describe force as a push or pull produced through interaction between objects. • Identify the main effects a force can have on motion and shape. • Distinguish between the object applying a force and the object experiencing it. • Use newton (N) as the SI unit of force. • Explain why an object at rest can still have forces acting on it.
Forces Are Everywhere
Cycling against the wind, turning a handlebar, pressing a brake, pushing a box, or pulling a drawer all involve forces. In each case, one object interacts with another and produces a push or a pull. The effect may be obvious, such as making something move, or more subtle, such as slowing it down or changing its direction.
A useful way to begin is to try moving a large cardboard box in different ways. You may push it, pull it, lift it, or carry it. Although these actions look different, they all involve a push or pull acting on the box.
A force is a push or pull on an object resulting from the object's interaction with another object.
What Can a Force Do?
A force is recognized by the effect it produces. It may start the motion of an object at rest, slow or speed up a moving object, change its direction, or change its shape. Sometimes one force produces more than one effect at the same time.
| Situation | Type of action | Effect of force |
|---|---|---|
| A friend holds a moving bicycle from behind | Pull | The bicycle slows down or stops |
| A bat hits a moving ball | Push | The ball's direction changes |
| An inflated balloon is pressed | Push | The balloon changes shape |
| A football is kicked | Push | The ball can start moving and change speed |
| A drawer is opened | Pull | The drawer moves from rest |
Problem
A moving football is kicked from the side and changes direction without stopping. What effect has the force produced?
- 1.The ball was already moving, so the force did not need to start its motion.
- 2.The kick acted from the side and changed the direction of motion.
- 3.Therefore, the main observed effect of the force is a change in direction.
Force Comes from Interaction
A force does not appear by itself. When your hand pushes a table, your hand and the table are interacting. At least two objects are involved: one object exerts a force and the other experiences that force. When the interaction ends, that particular force also ends.
The interaction works both ways. If you push hard on a table, you can also feel the table pushing back on your hand. This is useful evidence that both interacting objects experience forces during the interaction.
The SI unit of force is the newton. Its symbol is N.
An object being at rest does not necessarily mean that no force acts on it. Forces may be present but balance one another, so the object's state of motion does not change.
Quiz
Which statement best describes a force?
Which is NOT a possible effect of force?
A fielder stops a moving ball. What effect of force is most directly shown?
Why must at least two objects be involved for a force to arise?
What is the SI unit of force?
Practice Problems
- Give one example each of a push and a pull from daily life.
- Explain how a force can change the motion of an object without changing its shape.
- A bicycle is moving and the rider applies the brakes. Describe the effect of the force.
- Why is pushing a table considered an interaction between two objects?
- An object is at rest on a table. Explain why this does not prove that no force acts on it.
Key Takeaways
• A force is a push or pull produced through interaction between objects. • A force can start or stop motion, change speed, change direction, or change shape. • At least two interacting objects are involved whenever a force acts. • The SI unit of force is the newton (N). • An object can remain at rest even while forces act on it if those forces balance.
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Next · Lesson 2
Contact Forces: Muscular Force and Friction