According to Newton's third law of motion, when you lean on a brick wall what is happening?

• A. You are pushing on the wall.
• B. The force of the wall pushing is equal to the force of you pushing.
• C. The wall is pushing back on you.
• D. All of the answers are correct.

Answer: (D)

When you lean on a wall, you create a pair of forces that Newton’s third law describes: forces come in equal and opposite pairs acting on two different objects. You push on the wall, and the wall pushes back on you with the same magnitude in the opposite direction. That reaction force on you is what you feel as the wall pressing against you. Since the two forces are equal in size and opposite in direction, and they act on different bodies, they are the complementary action–reaction pair. So all the described aspects are true: you push on the wall, the wall pushes back with equal force, and the wall’s push is in the opposite direction to your push.