- #1

potatogirl

- 8

- 1

- Homework Statement
- Consider a projectile (0.1 kg) fired into a ballistic pendulum (0.5 kg, r_cm = 0.3m), the resulting collision is inelastic and the pendulum swings to a maximum angular deflection of 25º. Assume r_cm is the same with or without the projectile attached. What is the change in potential energy for the pendulum due to the collision?

- Relevant Equations
- ΔPE = mgΔh

ΔPE = ΔKE

I really can't find anything in my textbook about how PE relates to 2D collisions, but here is what I do know...

ΔPE = mgΔh

And I know that since it is a completely inelastic equation kinetic energy is not conserved but momentum is conserved. The change in kinetic energy should be equal to the change in potential energy. So I thought about using:

ΔPE = ΔKE

And then solving for PE...but I don't know how to go about that since ΔKE is 1/2mv^2 - 1/2mv_i^2 and I don't have the velocity (or time variable to solve for it).

There is a given of r_cm = 0.3m which I thought would be the center of mass, and already resolved for x and y components. Is that wrong?

Finally, there are some additional equations about the conservation of momentum, where final momentum equals initial momentum. Since momentum is conserved, I could totally see how that may apply. But again, without knowing velocity I wouldn't know how to apply that.

Anyways...I have a feeling this is really simple but I am missing something. The question also says "see figure 2" but there is no figure 2 included in the document.

Edit: Also, if the answer is just ΔPE = mgΔh - do I simply just plug in total mass (0.6 kg) times gravity (-9.8m/s^2) time Δh (25º)? That doesn't feel like the way....

Thank you for any insight! This isn't a homework problem, just a "pre-lab" question.

ΔPE = mgΔh

And I know that since it is a completely inelastic equation kinetic energy is not conserved but momentum is conserved. The change in kinetic energy should be equal to the change in potential energy. So I thought about using:

ΔPE = ΔKE

And then solving for PE...but I don't know how to go about that since ΔKE is 1/2mv^2 - 1/2mv_i^2 and I don't have the velocity (or time variable to solve for it).

There is a given of r_cm = 0.3m which I thought would be the center of mass, and already resolved for x and y components. Is that wrong?

Finally, there are some additional equations about the conservation of momentum, where final momentum equals initial momentum. Since momentum is conserved, I could totally see how that may apply. But again, without knowing velocity I wouldn't know how to apply that.

Anyways...I have a feeling this is really simple but I am missing something. The question also says "see figure 2" but there is no figure 2 included in the document.

Edit: Also, if the answer is just ΔPE = mgΔh - do I simply just plug in total mass (0.6 kg) times gravity (-9.8m/s^2) time Δh (25º)? That doesn't feel like the way....

Thank you for any insight! This isn't a homework problem, just a "pre-lab" question.