Is this Power/Work problem Possible

Dec 2016
2
0
I am currently in AP Physics C. I was given a homework problem that I don't think is possible with the given info.

A 20 kg rock slides on a rough horizontal surface at 8 m/s and eventually stops due to friction. The coefficient of kinetic friction between the rock and the surface is 0.200. What average thermal power is produced as the rock stops? (Answer: 157 WATTS) (first get the work done!)

Here I am given mass, velocity, force of friction( (20 * 9.8) * .2 = 39.2), AND THAT's IT. I tried getting work first, but I don't have a distance. I suppose I could solve for acceleration since friction is the only force in the X direction, which is -1.96, but even then, how would I get a distance from that. If I had a period of time this took place over I could find distance, but I don't. Could somebody explain this? Thanks
 
Jun 2010
422
33
NC
Similar Problem...

Hi LPB,

Is it (v2)(v2) - (v1)(v1) = 2 ax? Does that get "x."

Interesting problem. I wonder if it is the average "frictional" power that happens?
Here's a friction loss problem. Not your problem but something to look at.

Crate Pushed Uphill | THERMO Spoken Here!

Good Luck, JP
 
Dec 2016
2
0
Hi LPB,

Is it (v2)(v2) - (v1)(v1) = 2 ax? Does that get "x."

Interesting problem. I wonder if it is the average "frictional" power that happens?
Here's a friction loss problem. Not your problem but something to look at.

Crate Pushed Uphill | THERMO Spoken Here!

Good Luck, JP
I don't think that's it, but seeing that formula made me remember what to do. It's a kinematics equation. Vfinal (0) = vinitial(8) + acceleration(1.96)*time. From there I can solve for time and get the answer, thanks.
 

ChipB

PHF Helper
Jun 2010
2,367
292
Morristown, NJ USA
Right, the initial KE is all converted to heat, so the average power is

Power =\(\displaystyle \frac {\Delta KE} t = \frac {\frac 1 2 mv^2 }{t} \)

The time for the rock to stop comes from \(\displaystyle \Delta v = at \). You just need to determine 'a' from \(\displaystyle F_r = \mu mg = ma\).