Question

A stone thrown on a glass window, smashes the window pane to pieces, but a bullet from the gun passes through making a clear hole. Why?

Answer 1

For solving this question, we need to understand the force acting on the glass window by both the stone and the bullet. For this we need to consider two things. First, the change in the momentum and the time during which the change of momentum takes place.

Complete answer:
Let us first consider the case when a stone is thrown on the glass window. We know that the velocity of the stone is much lesser than that of the bullet. Due to this the stone remains in contact with the window pan for a long time. This results in the higher momentum transfer from stone to the glass window. This applies the force on the glass window which tends to break it. Thus, a stone thrown on a glass window, smashes the window pane to pieces.

In the case of a bullet, the velocity is much higher than that of the stone. This causes very less time of contact between bullet and glass window. Therefore, momentum change in this case will be very small. Hence, the bullet from the gun passes through making a clear hole.

Note: In other words, we can answer this question by using the impact or impulse factor which a measure of force multiplied by the time of application of force. A bullet has very less impulse factor due to its time of contact, but has enough momentum to break the glass. A stone on the other hand has huge momentum and has enough contact time with the glass for its force to be spread around the glass from its point of contact, thereby shattering it starting from the point of contact.