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# Answer to Question #26243 in Mechanics | Relativity for thakshi

Question #26243
A bullet, when fire at a fixed target, has its speed decreased to 50% after penetrating 15cm in to it. What additional thickness it will penetrate before coming to rest?
1
2013-03-15T09:08:06-0400
A bullet, when fire at a fixed target, has its speed decreased to 50%
after penetrating 15cm in to it. What additional thickness it will
penetrate before coming to rest?

Let the velocity of a bullet was V before hitting target. Then the kinetic energy of a bullet was

K1 = mV&sup2;/2.

As after penetrating 15cm into a target bullet&#039;s speed decreased to 50%, its kinetic energy became

K2 = m(V/2)&sup2;/2 = mV&sup2;/8.

Assuming that the bullet&#039;s energy decreases linearly while penetrating a target. Then the amount of energy needed to penetrate one cm of a target is

E = (K1 - K2)/15 = (mV&sup2;/2 - mV&sup2;/8)/15 = mV&sup2;/40.

Therefore the bullet will penetrate

D = K2/E = (mV&sup2;/8) / (mV&sup2;/40) = 5 [cm]

more before coming to rest.

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