Question

A bullet of mass 100 g moving with velocity of 100 m/s just pierces through a plank of 100 cm thick .If it is 50 cm thick ,it emerges with velocity of?​

Answer 1

When thickness is 100 cm then it just comes out means it emerged with zero velocity

So change in kinetic energy = final kinetic energy - initial kinetic energy

= 0 - 1/2 * 100/1000kg *(100)^2 = - 500

Now suppose a frictional force acts in the negative direction = - f

So work done by it = (-f)*(100/100)m= - 500

So f = 500 N

Now you know the force

Since friction force depends on the nature of the surface only

So, f will be same even when thickness changes

So work done in second case = (-f) *(50/100) m = final kinetic energy bhi- initial kinetic energy

= 1/2 * (100/1000)*V^2 - 500

So,

- 250 = V^2 / 20 - 500

250 = V^2 / 20

V^2 = 5000

V = 50(root2)

Where have you posted it?

I am not getting

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