# Elementary questions relating to Newton's laws of motion

1. May 20, 2013

### chipotleaway

i) If a wall breaks when it gets hit by a cannonball, did the wall exert an equal and opposite force on the cannonball?

ii) Would the force exerted by the cannonball on the wall only be the amount that was needed to break it?

iii) Hypothetically, is it possible for the wall to break but also cause the cannonball to rebound?

2. May 20, 2013

### Staff: Mentor

Good questions. What do you think?

3. May 21, 2013

### chipotleaway

i) I think it does and

ii) I also think that the reaction force from the wall would be equal to the force needed to break it. Like if you were to drop a bowling ball on a table, the table wouldn't feel the entire weight of the ball before it broke (assuming the strength is less than the weight of the ball)

iii) Hmm...the wall would have to have sufficient strength to exert a reaction force big enough to change the momentum of the cannonball...I'll try to work out the details later, heh.

4. May 21, 2013

### faldahlawi

To answer your question, I would think about impulse. The longer you exert a force, the greater the momentum. So to break the wall, the cannonball need to exert some amount of force for a period of time. Then, once the wall break and rocks flies in every directions, it's a work problem (force times displacement). Hence, the wall does exert the same force back but not enough impulse to stop the cannonball.