Forces come in pairs

The idea

Whenever one object exerts a force on a second, the second simultaneously exerts an equal and opposite force on the first; such forces always occur in pairs, act on two different objects, and arise for contact and non-contact forces alike.

Pushing a wall

You push a wall with your hand. You feel your hand being pushed back.

What do you think the wall does when you push it?

• It pushes back on your hand
• It does nothing
• It pushes in the same direction as you

Look at three pairs

Choose each pair and change the strength.

Equal and opposite

In every pair, the arrows had equal lengths and opposite directions, but each acted on a different object. Walking, paddling a canoe and the pull between the Earth and a fruit all work in this way, whether the objects touch or not.

Whenever one object exerts a force on a second, the second simultaneously exerts an equal and opposite force on the first; such forces always occur in pairs, act on two different objects, and arise for contact and non-contact forces alike.

Notes

Forces come in pairs: equal and opposite, on two different objects, for contact and non-contact forces alike.

Check yourself

You push a wall with a force of 30 N (an example). With what force does the wall push you?

When you walk, you push the ground backward with your foot. What makes you move forward?

Does the third law apply to non-contact forces, such as gravity?

A canoeist pushes the water backward with a paddle. The water…

• 0 N, because a wall cannot push. Whenever one object exerts a force on a second, the second exerts an equal and opposite force on the first.
• 60 N, twice as much. The two forces are equal in magnitude, not double.
• 30 N, in the opposite direction — correct. Yes!
• The ground pushes your foot forward with an equal force — correct. Yes!
• Your push on the ground acts on you. Your push acts on the ground. The force that acts on you is the force of the ground on your foot.
• The air pushes you forward. The ground applies a force on your feet in the direction opposite to your push.
• No, it applies to contact forces alone. Newton's third law applies to all types of forces, contact or non-contact.
• Yes, it applies to contact and non-contact forces — correct. Yes!
• No, gravity pulls in one direction only. The Earth pulls the fruit, and the fruit pulls the Earth with an equal and opposite force.
• does not push, because it is soft. The water pushes the paddle forward with an equal force.
• pushes the paddle forward with a smaller force. The two forces are equal in magnitude.
• pushes the paddle forward with an equal force — correct. Yes!