Homework Help: Kinetic friction and lost mechanical energy

1. Feb 12, 2015

hunbogi

An Object slides with a consistent velocity down an incline that has a 30 degree angle.
I'm trying to find the kinetic friction and the ratio that the mechanical energy is lost on the way down.

I think I have already calculated µ with F(friction)= µmgcos(θ)=mgsin(θ) out comes Tan(θ) = µ
I think I did that correctly..
I'm struggling with the other part about the rate that the mechanical energy is lost. I would guess that it is not possible to get the answer in a number.
3.
I tried to us PE = mgh -friction = k2.
mgh - µmghcos(θ) = ½mv^2þ
not sure what my answer might look like in the end.

2. Feb 12, 2015

mnmman

I agree with you that u= tan(theta). As for the ratio- the ratio of mechanical energy lost to what? Thermal energy maybe?

3. Feb 12, 2015

hunbogi

It only says find the mechanical energy that is lost because of friction on the way down the incline.
I think U =mgs is the potential energy. energy on the end of the incline is K = ½mv^2
so I think the diffrent between the two is the friction force.

4. Feb 12, 2015

mnmman

Ok so there is no ratio. I think your getting force and energy confused also. Be aware that force has units of Newtons= kg*m/s^2 and energy has units of Joules= N*m= kg*m^2/s^2. The mechanical energy lost is equal to the thermal energy generated. So if T is thermal energy then using conservation of energy set ΔKE+ΔPE+T= 0 and solve for T. btw this should probably not be posted in advanced physics

5. Feb 12, 2015

hunbogi

Okey thanks I will try that. New here so I didn't know where to post

6. Feb 13, 2015

BvU

Well, hello hunbogi, and welcome to PF :)

I suppose the consistent velocity is a constant velocity and the ratio is the rate, i.e. the mechanical energy loss per unit time.
There are two ways to find that rate:
One is friction force times displacement per unit time
Two is from potential energy loss per unit time

7. Feb 13, 2015

nasu

The rate of energy dissipation will depend on the velocity. Is the velocity given?

8. Feb 15, 2015

hunbogi

no it only gives the angle 30 degree and says the velocity is consistent

9. Feb 15, 2015

nasu

Consistent velocity does not make much sense. Is this a translation from another language?
If it's constant velocity, it can have any value, including zero. In which case there is no energy dissipation.
Something is missing and it may be due to translation.

10. Feb 15, 2015

hunbogi

Sorry it is constant velocity. mixed up in translation

11. Feb 15, 2015

nasu

Can you post the entire problem, as it is? Not just a summary.

12. Feb 15, 2015

hunbogi

3.
An object slides with a constant velocity down an 30 degree incline
a) Find the friction force and find the ratio of the mechanical energy that is lost on the way down

13. Feb 15, 2015

nasu

Oh, I think you want to say "the fraction of mechanical energy". This could make sense.
It may ask "What fraction of the mechanical energy is lost due to friction". Like, one half or 1/3 or 0.2.

Still does not seem completely alright. It is not clear what mechanical energy they are talking about.
The kinetic energy does not change. So the decrease in potential energy will be equal to the work of the friction force.