Practice Applying Newton's Third Law

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  • 0:04 What Is Newton's Third Law?
  • 1:36 Examples
  • 4:48 Lesson Summary
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Lesson Transcript
Instructor: Michael Blosser

Michael has a Masters in Physics and a Masters in International Development. He has over 5 years of teaching experience, teaching Physics, Math, and English classes.

This lesson will introduce the reader to Newton's Third Law and give real world examples. We'll also look at example problems that show how we can use Newton's Third Law and other equations to solve for unknown forces.

What Is Newton's Third Law?

How do birds fly? Why does a diving board fling you into the air? How do fish swim in the ocean? All of these actions are possible due to Newton's Third Law. Newton's Third Law will be explored in this lesson and applied with real world problems and calculations.

Isaac Newton created his revolutionary three laws of motion in the 17th century. Newton's Third Law states, 'For every action (in nature) there is an equal and opposite reaction.' Newton's Third Law is also known as the Law of Interaction or the Law of Action-Reaction.

There are numerous real world examples of Newton's Third Law. A bird flies because its wings are pushing down on the air, while the air is pushing up on the bird's wings with equal force, pushing the bird up. Similarly, fish swim in the ocean by pushing the water back with their fins, while the water pushes the fish forward with equal force.

An important thing to remember about Newton's Third Law is that two objects apply the same force on each other, no matter how more massive or small one object is in relation to the other object. For example, a small car that gets into a crash with a huge trailer truck applies the same amount of force to the truck as the truck applies to the car. The result of the collision affects the smaller car more because, although they apply the same amount of force to each other, their masses are vastly different. Since we know Newton's Seconnd Law, F = m * a, this means that the smaller car has a lot greater acceleration in the collision, causing more damage to the smaller car than the bigger truck.


For example, let's say a 50 kg car is moving with a constant velocity of 50 m/s. It crashes into a wall with 5000 Newtons of force. The collision causes the car to come to a complete stop, but the wall does not move. How much force did the wall deliver to the car?

In this problem, an object is in motion, hits another object, and comes to a complete stop. We could use all three of Newton's Force Laws to help us solve this problem for a variety of unknown variables. However, this problem is only asking for the amount of force that the wall delivers to the car.

We can use Newton's Third Law to help solve this problem. Although it may seem that the wall delivered more force to the car than the car did to the wall as the wall did not move, we know that with Newton's Third Law that for every action there is an equal and opposite reaction. So, since the car delivered 5000 Newtons to the wall, the wall also delivered 5000 Newtons to the car.

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