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sarahjt1
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Homework Statement
A 7.8g bullet moving at 575m/s penetrates a tree trunk to depth of 5.50cm. (a) Use work and energy considerations to find the average frictional force that stops the bullet. (b) Assuming that the frictional force is constant, determine hot much time elapses between the moment the bullet enters the tree and the moment it stops.
Homework Equations
I figure part (a) by:
KEi - Fk = KEf
KEi= kinestic energy initial, Fk=frictional force, and KEf= kinetic energy final
For part (b) I thought that the t=D/r would be sufficient but it's not?? I don't really understand what other equations to use in this scenario... HELP!
The Attempt at a Solution
For part (a)
1/2mV^2 - Fk*d = 1/2mVf^2
1/2(.0078)(575)^2 - .055Fk = 0
1289.44 - .055Fk = 0
-.005Fk = -1289.44
Fk = 2.34e4 N
For part (b)
t= .055/575
t= 9.5e-4 --> which is not correct, book shows 1.91e-4, huh?