# Homework Help: What does KE equal in this impossible physics problem?

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. -.-

-----------------------------------

EDIT:

(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)

KE = -PE
KE = -mv????????

(e)

2. Oct 31, 2012

### Staff: Mentor

You pinned down the change in potential energy in your EDIT, so that's good.

Since energy is conserved, the change in PE must be balanced by an equivalent change in KE (equal in magnitude, opposite in sign).

What's the formula for KE of a moving mass? How much mass is moving in the problem?

3. Oct 31, 2012

### riseofphoenix

KE = 1/2mv2

And m1 = 6 kg is moving..

What next?

4. Oct 31, 2012

### Staff: Mentor

So you're saying that m2 is not moving? Isn't it connected to m1 via a rope? Don't they move at the same time? Didn't the gravitational potential of both change?

5. Oct 31, 2012

### riseofphoenix

Oh yeah m1 and m2 are both moving.
And yes (?) the gravitational potential changed (?)

I need to put the conceptual stuff into a formula tho and find the total KE of the system though.... :(

6. Oct 31, 2012

### Staff: Mentor

You already did that when you found the change in PE. Conservation of energy: the sum PE + KE is a constant. If PE goes down, KE goes up by the same amount.

7. Oct 31, 2012

### riseofphoenix

So KE = mv???

8. Oct 31, 2012

### Staff: Mentor

Nooooo! mv is momentum.

You've already found the system's change in PE. The change in KE must be equal in magnitude (but opposite in sign).

9. Oct 31, 2012

### riseofphoenix

ΔKE = -ΔPE
ΔKE = -(-45.08)
ΔKE = 45.08

??

10. Oct 31, 2012

### Staff: Mentor

So what is the total mass that's moving?
Right, the gravitational potential of BOTH masses changed (yielding a net change in total gravitational potential energy). But the important thing is that BOTH masses are moving.
PE + KE = constant.

So ΔKE = -ΔPE

11. Oct 31, 2012

### Staff: Mentor

Yes. Surely the underlying theme of the chapter you're currently studying must be energy conservation for gravitational PE and KE?

12. Oct 31, 2012

### riseofphoenix

Ohh ok...
And yeah it is!

So what about the last one, part (e) for final velocity of the system?

ΔKE = -ΔPE
ΔKE = -(45.08)
ΔKE = 45.08
0.5mv2 = 45.08
0.5(m1 + m2)v2 = 45.08
0.5(6+8)v2 = 45.08
7v2 = 45.08
v2 = 6.44
v = 2.53 m/s ??

13. Oct 31, 2012

### Staff: Mentor

Yes, that looks good.

14. Oct 31, 2012

Thanks!!!