Homework Help: Explain Why we get hut by kicking a brick

1. Jan 10, 2010

area51

Explain Why we get hurt by kicking a brick

1. The problem statement, all variables and given/known dataYou can easily move a brick lying on a floor by gently pushing it by your foot. But
you hurt your foot when you try to kick the brick. Explain why? (in 60 to 80 words)

2. Relevant equationsTheory Question

3. The attempt at a solution Explanation: When we gently push the brick with foot, The Force we apply on the brick by pushing and friction by floor makes it move. But when we kick the brick lying on the floor, From Newtons third law of motion: Every action has equal and opposite reaction. the brick applies the same opposite force that we apply on the brick by kicking and we get hurt.

is it right?

Last edited: Jan 10, 2010
2. Jan 10, 2010

HallsofIvy

No, that's not right. Newton's third law of motion applies in both cases. Even when slowly moving the brick, the brick applies the same force to your foot.

The difference is the amount of the force. If you move a brick slowly, you are applying a very slight acceleration to get it moving, then almost no acceleration so, since F= ma, very slight force, then almost no force. When you kick a brick quickly, you apply a lot of acceleration so a large force- which, by Newton's third law, the brick also applies to your foot.

By the way, "friction on the floor" does NOT make the brick move. It slows the brick- friction always acts against motion. The friction is the reason that, once the brick is moving, you must apply "almost" no force. If there were no friction, once the brick started moving, it would continue to move with no additional force.

3. Jan 10, 2010

area51

so in ur words, can u answer it in 60-80 words the above question

4. Jan 10, 2010