Newton's third law of motion | Forces and Newton's laws of motion | Physics | Khan Academy

Summary

This video explains Newton's third law of motion, which states that every action has an equal and opposite reaction. Through various examples, such as pushing a block, walking on sand, and rocket propulsion, the script demonstrates how forces come in pairs, with each object exerting an equal force on the other. The video also touches on real-life applications like astronaut movement in space, where objects are used to push against to alter motion. Overall, it emphasizes how this fundamental law governs interactions and movement in both everyday and complex scenarios.

Takeaways

• 😀 Newton's Third Law of Motion states that for every action, there is an equal and opposite reaction.
• 😀 Newton wrote in Latin because it was considered a more serious language at the time, even though he was English.
• 😀 The action force and reaction force are always equal in magnitude but opposite in direction.
• 😀 An example of Newton's Third Law is when you push on a block, the block pushes back on you with equal force.
• 😀 When you press your hand against a desk, you feel the desk's force pushing back against you, demonstrating the law in action.
• 😀 Stepping on sand at the beach is another example, where your weight exerts a force on the sand, and the sand exerts an equal and opposite force on you.
• 😀 The equal and opposite forces in Newton's Third Law are why you don't fall through the ground when standing on it.
• 😀 Rockets work based on Newton's Third Law, as they expel gases backward, which creates an equal and opposite force that propels the rocket forward.
• 😀 Even in the vacuum of space, rockets can accelerate by expelling gas, using Newton's Third Law to propel them without needing anything to push off of.
• 😀 If an astronaut drifts away from their spacecraft in space, they can throw an object in the opposite direction to accelerate back toward it, demonstrating Newton's Third Law in action.

Q & A

What does Newton's third law of motion state?

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

Why did Newton write his laws in Latin?

Newton wrote his laws in Latin because, at that time, Latin was considered a more serious and formal language for scholarly works.

What happens when you push a block on ice?

When you push a block on ice, you exert a force on the block, which moves it. Simultaneously, the block exerts an equal and opposite force back on your hand, which you can feel as compression.

How does Newton's third law apply when you push against a desk?

When you push against a desk, your hand exerts a force on the desk, and the desk exerts an equal and opposite force back on your hand. This is why you feel pressure on your hand as you push.

What is the evidence that the sand is exerting a force on you when you step on it?

The evidence is that you don't accelerate downwards when stepping on the sand; instead, the sand exerts an upward force that balances the downward gravitational force, preventing you from sinking.

How do rockets work based on Newton's third law?

Rockets work by expelling gases at high velocities out of the back. The force exerted on the gases pushes them in one direction, and the equal and opposite force pushes the rocket in the opposite direction, propelling it forward.

What should an astronaut do if they find themselves drifting in space?

An astronaut should find the heaviest object they can and throw it in the opposite direction of their drift. The action of throwing the object exerts a force on it, and the equal and opposite reaction pushes the astronaut back toward the spacecraft.

What role does friction play when pushing the block on ice?

Friction must be overcome for the block to move. On ice, friction is minimal, allowing the block to slide more easily when pushed.

Why do you feel pressure when pushing on a desk but not when pushing on air?

When pushing on a desk, the desk exerts an equal and opposite force back on your hand, creating pressure. Air, however, has a much lower density and offers less resistance, so you don't feel the same reaction force.

What would happen if there was no equal and opposite reaction when pushing against an object?

If there were no equal and opposite reaction, you would not feel any force acting back on you, and you would be unable to push or move objects as you do in real life.