# Ballistic bullet pendulum conservation of momentum problem

#### fifipaws

1. The problem statement, all variables and given/known data
A 15 g bullet is fired into a 2.55 kg block that is hanging by a 2.0 m long string. The speed of the bullet is 425 m/s.

Calculate the angle when the system reaches its maximum height?

2. Relevant equations
m1v1=(m1+m2)vf

(1/2)mv^2=mgh

3. The attempt at a solution

(0.015 kg)(425 m/s) = (0.015 kg + 2.55 kg)vf
vf=2.49 m/s

(.5)(2.565 kg)(2.49 m/s)^2=(2.565)(9.8)h
h=.316 m

Now that I have my height, I am not sure what to do. I think it should be a simple trig function at this point, but I cannot get the correct answer which is 32.6 degrees. I've also tried using L-Lcosθ=h, but have not had luck with that.

Thanks in advance.

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#### ehild

Homework Helper
1. The problem statement, all variables and given/known data
A 15 g bullet is fired into a 2.55 kg block that is hanging by a 2.0 m long string. The speed of the bullet is 425 m/s.

Calculate the angle when the system reaches its maximum height?

2. Relevant equations
m1v1=(m1+m2)vf

(1/2)mv^2=mgh

3. The attempt at a solution

(0.015 kg)(425 m/s) = (0.015 kg + 2.55 kg)vf
vf=2.49 m/s

(.5)(2.565 kg)(2.49 m/s)^2=(2.565)(9.8)h
h=.316 m

Now that I have my height, I am not sure what to do. I think it should be a simple trig function at this point, but I cannot get the correct answer which is 32.6 degrees. I've also tried using L-Lcosθ=h, but have not had luck with that.

Thanks in advance.
Your work is correct so far, and yes, use L-Lcosθ=h. What is your problem with it?

ehild

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