If you are pushing on a heavy door trying to push it open and your friend behind you push how has th net force changed

The force will change as the net force applied will increase.

Explanation:

If you are pushing on a door really hard, trying to make it slide open and if your friend is also stand behind you and contributes to help you push towards the door, then the overall force will change as the net amount of force applied will increase.

This is an example of Newton's second law according to which you need more force to move an object with heavier mass.

F = ma

Therefore, the force applied by you and your friend will add and the net force applied will be increased.

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