(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

A bullet hits balk of 5kg on a rope of 2m lenths. Bullets mass 2gr, velocity unknown. Plastic collission, balk goes upat 45 degrees.

2. Relevant equations

When i solve it with energy equations i get

h=2-2cos45=0.58m

V(of balk and bullet) = sqrt(2gh)=3.38 m/s. (potential energy of total mass)

Now i need to find a velocity of bullet at t(0).

Here is the trick

when i solve it using momentum i get:

mv=(m+M)V

v=8450 m/s

But!! when i solve it using energy i got

m*sqr(v)/2=(m+M)*g*h

v=168.5 m/s.

Thinking about that we used the potential energy to calculate a balk speed and taking into account an absolute plastic collision i dont know why is the differense?

Thanks for any help.

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# Energy vs Momentum

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