I will be FOREVER grateful if someone can tell me the answer to this: involves KE+PE!

1. Oct 31, 2012

riseofphoenix

c, d, and e I don't know how to solve

For d I tried doing

Vf = √(2*9.8*2.3) = 6.71 m/s

Assuming the inclined plane is frictionless
Since m2 is roped to m1, it's velocity up the ramp will be 6.71m/s too

The total increase in KE is then 0.5[8+6]*6.71^2 = 315.16 J

^

"INCORRECT"

Right now I need a genius to tell me how to get the answer ASAP -.-

Since (d) is wrong, so is e. -.-
Story of my life.

2. Oct 31, 2012

Staff: Mentor

Re: I will be FOREVER grateful if someone can tell me the answer to this: involves KE

For (c), how did you get that number? Isn't it just the sum of the two ΔU values?

3. Oct 31, 2012

riseofphoenix

Re: I will be FOREVER grateful if someone can tell me the answer to this: involves KE

m1 falls 2.3m and therefore has -6*9.8*2.3 = -135.24 J difference.

m2's PE increases 8*9.8*sin 30° = +39.2 J difference.

The total change then is 39.2 - 135.24 = -96.04 J

^ That was my previous answers, but I got that wrong too...
So I'm pretty much screwed.

4. Oct 31, 2012

Staff: Mentor

Re: I will be FOREVER grateful if someone can tell me the answer to this: involves KE

You left off the 2.3 for m2...

5. Oct 31, 2012

riseofphoenix

Re: I will be FOREVER grateful if someone can tell me the answer to this: involves KE

(c)

m1 falls 2.3m and therefore has -6*9.8*2.3 = -135.24 J difference.

m2's PE increases 8*9.8*sin 30°*2.3 = +90.16 J difference.

The total change then is 90.16 - 135.24 = -45.08 J

Ok so how does this relate to part d and e though? How would I solve for those two last ones?

(d)

(e)

6. Oct 31, 2012

Staff: Mentor

Re: I will be FOREVER grateful if someone can tell me the answer to this: involves KE

(d) asks for the change in KE. Since the system is frictionless...

7. Oct 31, 2012

riseofphoenix

Re: I will be FOREVER grateful if someone can tell me the answer to this: involves KE

But I already tried solving for it and i got it wrong... (???????)

8. Oct 31, 2012

Staff: Mentor

Re: I will be FOREVER grateful if someone can tell me the answer to this: involves KE

But you had the wrong answer for (c) before. Now you have that right.

What is the total energy of a system in terms of the potential and kinetic energies? If the system is lossless (no friction), what can you say about the total energy over time...?

9. Oct 31, 2012

riseofphoenix

Re: I will be FOREVER grateful if someone can tell me the answer to this: involves KE

KEinitial + PEinitial = KEfinal + PEfinal
KEinitial + PEinitial = 0 + 0 (No friction)
KEinitial + PEinitial = 0?

10. Oct 31, 2012

Staff: Mentor

Re: I will be FOREVER grateful if someone can tell me the answer to this: involves KE

Rearrange so you show the relationship between ΔKE and ΔPE...

11. Oct 31, 2012

riseofphoenix

Re: I will be FOREVER grateful if someone can tell me the answer to this: involves KE

KE = -PE
KE = -mv???

12. Nov 1, 2012

Staff: Mentor

Re: I will be FOREVER grateful if someone can tell me the answer to this: involves KE

If you put Δs in front of your first equation, you would be on the right track...

13. Nov 1, 2012

Perpendicular

Re: I will be FOREVER grateful if someone can tell me the answer to this: involves KE

Consider the total energy over time which is a constant. Hence change in PE must balance out change in KE. The rest is obvious.

14. Nov 1, 2012

Perpendicular

Re: I will be FOREVER grateful if someone can tell me the answer to this: involves KE

As for (e) consider that you have a final change in KE, as well as the masses. You can now plug that in to solve for velocity.

15. Nov 1, 2012

Staff: Mentor

Re: I will be FOREVER grateful if someone can tell me the answer to this: involves KE

Oops, Looks like riseofphoenix is on a temporary vacation from the PF for a different thread.