Newton's third law | Physics | Khan Academy

Summary

This video explores Newton’s third law of motion, explaining how every force has an equal and opposite reaction. It delves into everyday examples, like kicking a ball and pulling a bar, to demonstrate how forces work between objects. The video clarifies common misconceptions about force and acceleration, emphasizing that equal forces on objects with different masses result in different accelerations. It also addresses the misleading idea of ‘action’ and ‘reaction’, showing that these forces occur simultaneously. The script concludes with real-world applications, like fish swimming and rocket propulsion, showcasing the law’s relevance in everyday phenomena.

Takeaways

• 😀 Newton's Third Law states that for every action, there is an equal and opposite reaction, with forces always acting on two different objects.
• 😀 When you kick a ball, the ball exerts an equal and opposite force on your foot, which is why it hurts when you kick it hard.
• 😀 Similarly, when pulling down on a bar, the bar pulls up on you with an equal and opposite force.
• 😀 Newton’s Third Law applies universally: the Earth pulls on the apple, and the apple pulls on the Earth with equal force.
• 😀 The acceleration of an object depends on both the force applied and its mass. Objects with less mass accelerate more easily.
• 😀 Despite equal forces, the Earth doesn’t noticeably accelerate towards the apple because of its enormous mass compared to the apple's small mass.
• 😀 Forces do not cancel out because they act on different objects. For example, the force of kicking a ball acts on the ball, while the opposite force acts on your foot.
• 😀 The terms 'action' and 'reaction' are misleading because forces in Newton's Third Law occur simultaneously, not in sequence.
• 😀 In physics problems, when you know the force on one object, you can calculate the acceleration of the other object using Newton’s Third Law.
• 😀 The principles behind Newton's Third Law also apply to everyday phenomena like a balloon rising when air is expelled, and even rocket propulsion.
• 😀 In gravity interactions, such as between Batman and the Earth, the gravitational force between Earth and Batman is equal and opposite, but the normal force from the ground on Batman is not part of the Third Law.

Q & A

What does Newton's Third Law state?

Newton's Third Law states that for every force that one object exerts on another, there is an equal and opposite force exerted back on the first object.

Why does the apple fall towards the Earth but the Earth doesn't move noticeably towards the apple?

While the Earth and the apple exert equal forces on each other, the Earth's massive mass makes it much harder to accelerate, so its movement is imperceptible compared to the apple.

How does mass affect acceleration when forces are equal?

Objects with greater mass experience less acceleration when the same force is applied due to their greater inertia, while smaller mass objects accelerate more easily.

What does the script say about the misconception that bigger forces cause greater acceleration?

The script clarifies that while the forces are equal, the resulting accelerations differ because smaller mass objects experience more acceleration, even if the force is the same.

Why can't the equal and opposite forces in Newton's Third Law cancel each other out?

The equal and opposite forces do not cancel because they act on different objects. Forces on one object are separate from the forces on the other object.

What is the problem with the terms 'action' and 'reaction' when explaining Newton's Third Law?

The terms 'action' and 'reaction' can be misleading because they suggest one force happens before the other, but in reality, both forces occur simultaneously and with the same duration.

How does Newton's Third Law apply in a real-world example, like a fish swimming?

When a fish pushes against the water with a force, the water pushes back with an equal and opposite force, which propels the fish forward. This is an example of Newton's Third Law in action.

How does a balloon flying off demonstrate Newton's Third Law?

As air is pushed out of the balloon, the balloon exerts an equal and opposite force on the air, which propels it in the opposite direction, making the balloon fly upwards.

What is the key difference between the forces acting on Batman and the ground?

The gravitational force pulls Batman down, and the ground pushes Batman up. These are not equal and opposite forces of Newton's Third Law; instead, they involve the Earth and Batman interacting with each other.

Can the equal and opposite forces in Newton's Third Law be applied to more complex systems, like rockets?

Yes, rockets operate on a similar principle. The rocket exerts a force on the gases it expels, and the gases exert an equal and opposite force on the rocket, propelling it upward.