AP Physics 1: Forces 3: Newton's Third Law of Motion Part 1

Summary

This video explains Newton's third law of motion, highlighting the principle that every action has an equal and opposite reaction. Through demonstrations, the video explores how forces act on objects, including friction, the deformation of materials like walls, and how these forces lead to motion. It covers everyday examples like walking, jumping, and swimming, showing how pushing or pulling against surfaces results in reactions that move us or objects. The video also emphasizes how action-reaction pairs work without canceling each other when acting on different objects, with engaging visuals and experiments to illustrate key concepts.

Takeaways

• 😀 Newton's Third Law states that for every action force, there is an equal and opposite reaction force.
• 😀 When you push on an object, the object pushes back with an equal amount of force in the opposite direction.
• 😀 The amount of force you apply to an object does not depend on its movement, but the resistance (like friction) determines whether it moves.
• 😀 Friction plays a critical role in determining whether an object moves when force is applied to it.
• 😀 In a tug-of-war, the winning team pulls harder, but friction plays a key role—team A wins because team B has less friction.
• 😀 The wall responds to force through molecular deformation—stiffer materials deform less, but they push back harder when deformed.
• 😀 Forces don't cancel each other out unless they act on the same object. For example, when pushing on a book, your force acts on the book, and the book's force acts on you, so they don't cancel.
• 😀 You cannot push or lift yourself because the action and reaction forces act on the same object, canceling each other out.
• 😀 You can walk or jump because when you push on the ground, the ground pushes back with a force that creates a net movement in the direction you want.
• 😀 Swimming works by pushing on the water; the water pushes back, propelling you forward. The harder you push, the faster you go.
• 😀 A toy car demonstrates Newton’s Third Law—when the car pushes backward against the ground, the ground pushes the car forward, causing it to accelerate.

Q & A

What does Newton's third law state?

Newton's third law states that for every action force, there is an equal and opposite reaction force.

What happens when you push on an object, like a wall, with a force of 20 newtons?

When you push on the wall with 20 newtons, the wall pushes back on you with an equal force of 20 newtons in the opposite direction.

Why does your hand hurt when you hit a table?

Your hand hurts because the table exerts an equal and opposite force back on your hand, and the harder you hit the table, the greater the force it applies, causing more pain.

If you push a book with 10 newtons, how much force does the book exert back on you?

The book exerts exactly 10 newtons of force back on you, in the opposite direction, according to Newton's third law.

Why does the book move while you don’t, when you push it?

The book moves because the force you apply on it is enough to overcome its friction with the surface, whereas your own friction with the ground prevents you from moving.

What determines whether you or the table moves when you push on it?

Friction determines which object moves. If the force you apply is not enough to overcome the friction between the table and the floor, the table won’t move. Similarly, if the friction between you and the floor is too strong, you won't move either.

In a tug-of-war, why does the team with less friction win?

In a tug-of-war, both teams exert equal forces on each other, but the team with less friction can move more easily, allowing them to win.

How does deformation affect the reaction force of an object, like a wall?

Deformation affects the reaction force because harder surfaces have stiffer springs (molecules), which resist deformation more strongly. The more you push on a surface, the more it deforms, and the greater the reaction force becomes.

Why do action and reaction forces not cancel each other out when pushing on different objects?

Action and reaction forces do not cancel out if they act on different objects. They only cancel if they act on the same object.

How does jumping involve Newton's third law of motion?

When you jump, you push down on the ground with a force greater than your weight, and in response, the ground pushes you upward with a force greater than your weight, allowing you to lift off the ground.