A 7-g bullet is fired into a 1.5-kg ballistic pendulum. The bullet emerges from the block with a speed of 200 m/s, and the block rises to a maximum height of 12 cm. Find the initial speed of the bullet.
conservation of momentum
The Attempt at a Solution
First I tried to find the initial momentum of the bullet as if it did't penetrate the block
so .007vi = (1.507)vf
because I have two unknowns I used the conservation of mechanical energy to solve for vf
1/2(1.507)(vf2) = (1.507)(9.8)(.12)
vf = 1.54 m/s
then plug that in to find the initial velocity
vi = 435.29 m/s
initial momentum = 3.05
Then I tried to find the momentum of the bullet exiting the block and add it to the initial momentum to get the total initial momentum.
200(.007) = 1.4
1.4 + 3.05 = 4.45
and finally I divided that by the mass to get the initial velocity 635.75 m/s. The correct answer is 529 m/s.