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vadbol
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Homework Statement
A bullet hits balk of 5kg on a rope of 2m lenths. Bullets mass 2gr, velocity unknown. Plastic collission, balk goes upat 45 degrees.
Homework 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 don't know why is the differense?
Thanks for any help.