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

Hint: Use the relation between the velocity of the stone and bullet and the change in momentum of the glass window due to the collision with the stone and the bullet. Also consider the factor time for which the glass and bullet remains in contact with the glass window.

Formula used:
The force \[F\] acting on an object is given by
\[F = \dfrac{{\Delta P}}{{\Delta t}}\] …… (1)
Here, \[\Delta P\] is the change in momentum of the object and \[\Delta t\] is the time interval during which the momentum changes.

Complete step by step answer:
From equation (1), it can be concluded that the force acting on an object is directly proportional to the change in momentum of the object and is inversely proportional to the time interval.

When the stone is thrown on the glass window, the momentum of the stone is transferred into the glass window and since the velocity of the stone is not too large, it will remain in contact with the glass of the window for a long time.

As the stone remains in contact with the glass for a large time, the change in momentum of the glass is also large.

The change in the momentum of the glass window causes a force to act on it and bend it. The glass bends upto a certain limit and then breaks into pieces due to large change in momentum in it.

When the bullet is fired from a gun on the glass window, the velocity of the bullet being very large, the bullet remains in contact with the glass for a very short time.

Due to the bullet fired on the glass, the momentum of the glass changes but the change in momentum is not as large as the bullet is in contact with the glass window for a very short time. Hence, the bullet makes a clear hole in the glass and passes through it.

Note:
The students may think that the bullet is in contact with the glass for a short time so the force should be large on the glass and the glass should be broken into pieces. But the area of the tip of the bullet is very small and force due to it on the glass is very small, hence, it makes a hole in glass and passes instead of breaking the glass.