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?
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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)
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