A bullet fired from a gun makes a clean whole in a window  whereas a stone pebble thrown brakes the whole window.why?

Hi,
Lets us understand the physics behind this phenomenon.
First we should know why the glass or window breaks when an object is thrown at it.
This occurs because of the vibration caused when an object transfers its energy to it.
Bullet and stone case:
As the stone hits the window, the glass vibrates.
As the bullet hits the window, the glass vibrates.

This vibration causes the window to break.

The frequency of the vibration depends on the elasticity of the collision (how the transfer of energy occurs). The elasticity of the collision depends on the structure of the object that collides with the glass.
 
To answer the question we must consider the fact that how much energy transfer takes place between the two objects and the vibration caused.

The bullet is very small and very smooth. The shape of the bullet is aerodynamic, so, it will not lose much energy as it moves through the glass. When the bullet collides with the glass at a very high speed, it passes through the glass without releasing much energy. So, the glass does not vibrate very much.

The stone is much larger that the bullet. The stone is not as smooth as the rock. The shape of the stone is not aerodynamic, so, the stone will release much energy as it moves through the glass. When the stone collides with the glass at a very low speed, releasing much if its kinetic energy, the energy will cause the glass to vibrate enough to break into pieces.

Hope this helps,
Good luck.

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