Physics Problem (linear collision)

[Towelie]

Ok, I am working on physics homework and I am stuck on one problem :-/ .
"A bullet of mass 10g strikes a ballistic pendulum of mass 2.0kg. The center of mass of the pendulum rises a vertical distance of 12cm. Assuming that the bullet remains embedded in the pendulum, calculate the bullet's initial speed."
I don't see how I can do it without the radius of the pendulum... Anyone have any ideas as to how to solve it?
EDIT: Never mind. I used the conservation of energy (duh!)
EDIT2: Still can't get it though :P

[dark_drake]

Wait.... don't you have to use the conservation of momentum?

[Sidoh]

Remember that gravity is a conservative force (the energy it takes to raise a weight some distance above "the ground" is independent of the path taken if gravity is the only force considered).

Wait.... don't you have to use the conservation of momentum?

I'm pretty sure conservation of energy is the 'correct' way to solve this problem.

[Towelie]

Remember that gravity is a conservative force (the energy it takes to raise a weight some distance above "the ground" is independent of the path taken if gravity is the only force considered).

Wait.... don't you have to use the conservation of momentum?

I'm pretty sure conservation of energy is the 'correct' way to solve this problem.

I have to use both. :P

[Sidoh]

I have to use both. :P

I don't think so.

Solve for the energy that it takes to lift the mass of the bullet plus the pendulum 12cm and set it equal to the kinetic energy of the bullet.  That should enable you to solve for the initial velocity.

[Towelie]

I have to use both. :P

I don't think so.

Solve for the energy that it takes to lift the mass of the bullet plus the pendulum 12cm and set it equal to the kinetic energy of the bullet.  That should enable you to solve for the initial velocity.

I thought of taking kinetic energy RIGHT after the collision and set it equal to the potential energy of the pendulum+bullet at 12cm. After that use the conservation of momentum to find the speed of the bullet required to move the pendulum+bullet at that speed. I got 110m/s and th answer is 1100m/s :(

[Sidoh]

I thought of taking kinetic energy RIGHT after the collision and set it equal to the potential energy of the pendulum+bullet at 12cm. After that use the conservation of momentum to find the speed of the bullet required to move the pendulum+bullet at that speed. I got 110m/s and th answer is 1100m/s :(

If friction is ignored, the kinetic of the energy before it hits the pendulum is exactly equal to the potential energy the pendulum and the bullet gain when they are lifted 12cm.

[dark_drake]

I thought of taking kinetic energy RIGHT after the collision and set it equal to the potential energy of the pendulum+bullet at 12cm. After that use the conservation of momentum to find the speed of the bullet required to move the pendulum+bullet at that speed. I got 110m/s and th answer is 1100m/s :(

This is a handy web page for this . The thing is, it gets 1100 km/h and 308 m/s.

[Towelie]

I thought of taking kinetic energy RIGHT after the collision and set it equal to the potential energy of the pendulum+bullet at 12cm. After that use the conservation of momentum to find the speed of the bullet required to move the pendulum+bullet at that speed. I got 110m/s and th answer is 1100m/s :(

This is a handy web page for this . The thing is, it gets 1100 km/h and 308 m/s.

The answer is 1400m/s*

[Sidoh]

Oh, you can't use just the conservation of energy... the collision isn't elastic.  However, the kinetic energy of the bullet and the pendulum plus the potential energy of them is conserved.

The answer is 1400m/s*

lol, I don't think bullets travel a kilometer and a half a second, even in hypothetical physics problems!  drake's right.  With some rounding errors, I get 307 m/s.

Here's the correct way to do it:

1) Find the velocity of the pendulum + the bullet after the bullet collides with the pendulum.  Use conservation of energy:

2) Use conservation of momentum to solve for the initial speed of the bullet.

[Towelie]

Oh, you can't use just the conservation of energy... the collision isn't elastic.  However, the kinetic energy of the bullet and the pendulum plus the potential energy of them is conserved.

The answer is 1400m/s*

lol, I don't think bullets travel a kilometer and a half a second, even in hypothetical physics problems!  drake's right.  With some rounding errors, I get 307 m/s.

Here's the correct way to do it:

1) Find the velocity of the pendulum + the bullet after the bullet collides with the pendulum.  Use conservation of energy:

2) Use conservation of momentum to solve for the initial speed of the bullet.

I redid it and got the correct answer.
Kinetic initial -> Potential final, v^2=2gh. v=1.53
m1v1=(m1+m2)v
(.01)v1=(2.01)(1.53)
v=307.5m/s.

And now I take a moment to explain that I was an idiot in saying the answer was that, I was looking at the answer to another problem. Thanks for the help though, but I got it before you guys helped ;)

[Sidoh]

And now I take a moment to explain that I was an idiot in saying the answer was that, I was looking at the answer to another problem. Thanks for the help though, but I got it before you guys helped ;)

Meh, I would have gotten it the first time if I had read the whole problem.  I missed the part where the bullet is embedded in the pendulum.  While it's a pretty obvious detail, it's an important hint.