Question

When a bullet passes through a glass window, it makes a hole in it . while ,when a stone is thrown on window, whole glass breaks. Why ??​

Answer 1

1. 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?

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.

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.

Answer 2

Just because of high velocity the bullet makes hole. When bullet comes in touch for a very short time compared to the stone so glass does not get a chance to come in motion.

There's something called the "Impact" or "Impulse" factor basically 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.

Try the exact same thing with a small stone, it doesn't happen due to less momentum but you maybe able to see cracks from the point of contact, due to the spread of the impact force.

Example : Easier to cut a piece of a fruit with a knife than a spoon . Also please note, it again depends on the kind of glass as well.