[Gravitational Potential Energy] In a movie stunt, a 65 kg skier...

[LionLieOn]

Homework Statement

[/B]
In a movie stunt, a 65 kg skier starts from a rest position at the top of a hill 30 m high. She slides down the hill to the bottom, where she collides with a 45 kg stationary skier. The collision is completely inelastic. Find the final velocity of the skiers.

Homework Equations

I got the answer, but I feel I did it wrong.

The Attempt at a Solution

Please look at Image.

 

[LionLieOn]

Please ignore the multiple uploads of pictures. I'm new here and I thought it didn't upload the picture. Anyone of the pictures will do.

In the beginning you can see I did

ETF= ETF

Can I do that? My reasoning for doing ETF=ETF is because it's total mech. energy AFTER the collision and Final velocity

 

[gneill]

Moderator note: I've inserted one copy of your image in-line in your post and deleted the extra copies of the attachment.

ETF= ETF

Can I do that? My reasoning for doing ETF=ETF is because it's total mech. energy AFTER the collision and Final velocity

I'm not sure what you mean to imply by saying ETF = ETF. Of course a given quantity is equal to itself. If you mean that the total mechanical energy is conserved from start to finish, then I would disagree (what type of collision is involved? Is energy conserved in that type of collision?).

Only the 65 kg skier starts at the top of the hill, so I don't see why you've included both masses in the change of gravitational potential (your second line in your image).

I'd expect to see an examination of the inelastic collision that occurs when the two collide at the bottom of the hill.

 

[LionLieOn]

Moderator note: I've inserted one copy of your image in-line in your post and deleted the extra copies of the attachment.
I'm not sure what you mean to imply by saying ETF = ETF. Of course a given quantity is equal to itself. If you mean that the total mechanical energy is conserved from start to finish, then I would disagree (what type of collision is involved? Is energy conserved in that type of collision?).

Only the 65 kg skier starts at the top of the hill, so I don't see why you've included both masses in the change of gravitational potential (your second line in your image).

I'd expect to see an examination of the inelastic collision that occurs when the two collide at the bottom of the hill.

Thank you for doing that.Yeah, TME is conserved from start to finish. The collision is completely inelastic.

I included both masses since it's the combined energy of both masses after the collision.

 

[gneill]

Yeah, TME is conserved from start to finish. The collision is completely inelastic.

Energy is NOT conserved in an inelastic collision. What is conserved?

I included both masses since it's the combined energy of both masses after the collision.

But both masses do not go down the hill. Only one gains energy by going down the hill.

 

[LionLieOn]

Energy is NOT conserved in an inelastic collision. What is conserved?

But both masses do not go down the hill. Only one gains energy by going down the hill.

Hmm I don't know how to answer the question. I mean the question

" In a movie stunt, a 65 kg skier starts from a rest position at the top of a hill 30 m high. She slides down the hill to the bottom, where she collides with a 45 kg stationary skier. The collision is completely inelastic. Find the final velocity of the skiers. "

Is all I have to work with, and we were given the answer but they don't show us how the actual work is done. How would you answer this question?

 

[gneill]

There are really two separate parts to this problem. Break the problem into its sequential parts:

i) 65 kg skier slides down hill (30 m height change). What's her speed at the bottom?

ii) Two skiers collide inelastically. The 65 kg skier has the speed from (i) and collides with the 45 kg skier who is initally at rest. What's their speed after the collision?

 

[LionLieOn]

There are really two separate parts to this problem. Break the problem into its sequential parts:

i) 65 kg skier slides down hill (30 m height change). What's her speed at the bottom?

ii) Two skiers collide inelastically. The 65 kg skier has the speed from (i) and collides with the 45 kg skier who is initally at rest. What's their speed after the collision?

So I took your advice and redid the question. This time I feel more confident in my answer than my first attempt. Please check the picture and let me know what you think.

 

[gneill]

Yes, that's much better. The final answer looks good.

You should make it clear that when you say:

that you mean the initial velocity just before the collision.

 

[LionLieOn]

Yes, that's much better. The final answer looks good.

You should make it clear that when you say:
View attachment 109331
that you mean the initial velocity just before the collision.

Ahh true! Thank you so much for your help!
I went back to double check my work (And to make that change that you suggested.) And I feel like an idiot for not getting it in the first place. Again thank you so much!

 

[gneill]

You're welcome!