Develop Understanding of Newton's Third Law

Imagine that you are pushing a pram. 

 

 

As you apply a force to push the pram forward, you experience a force from the pram pushing back towards you! That's because forces come in pairs. 

 

The fact that forces come in pairs is described by Newton's Third Law. Let's learn about this law in more detail, and check out some examples of it in action!

 

Isaac Newton described three laws of motion. 

Newton's Third Law states that when two objects interact, they cause equal and opposite forces on each other. We say that forces come in pairs. 

 

For our example of the pram from earlier, we started with a push force from you forwards on the pram. The pram exerts a push force back towards you. 

 

What are the properties of these Third Law pairs?

 

1.  The forces are always the same size.

2.  The forces always act in the opposite direction. 

3. The forces are always the same type (e.g. both push forces, or both gravity).

4. The forces always act on different objects. 

 

How do these rules apply to our pram example?

 

 

In this example, you exert a forwards push force, and the pram exerts a backwards push force. The forces are the same size, in opposite directions. It is the same type of force (a push force) and the forces act on different objects (on the pram, and on you). 

 

What are some other examples?

 

 

The Earth exerts the force of gravity. That force pulls the moon towards the Earth. But, because forces come in pairs, the moon pulls back on the Earth with its own gravitational force! The forces are the same size and act in opposite directions. They are the same type of force (gravity) and act on different objects (the Earth and the moon). 

 

Imagine queuing to buy some groceries. Your weight is acting downwards. What is the Third Law pair of this force?

 

 

The Third Law pair is that your gravity is pulling on the Earth! The force from you is the same size as your weight, and it acts in the opposite direction to your weight. The forces are the same type (gravitational) and act on different objects (you, and the Earth!)

So why doesn't the Earth move up towards you? Because the Earth is so massive, and on that scale, you wouldn't be able to measure any effect of this force. 

 

Someone might say that the Newton's Third Law pair in this case is actually the normal reaction force from the floor acting upwards to balance your weight. This is not true. 

Yes, there is a normal reaction force from the floor, but it isn't paired with your weight.

 

Remember those rules from earlier, that tell us the conditions for a Third Law pair? A normal reaction force is a different type of force from weight, so it doesn't count. As well as that, both forces affect the same object (the person standing), and that's another reason that it can't be a Third Law pair. 

 

 

It can be a bit confusing, but the best way to understand this is to try some questions!