# Atwood machine problem

#### tigigi

I'm actually grading this stuff, and have a question. Ideally there's no friction and no kinetic energy of pulley. If we take them into account, which one is more important? I wonder how to analyze this? Thanks a lot.

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#### mfb

Mentor
You can compare the energy lost from friction to the energy of the kinetic energy of the pulley and rope.
Which one will win depends on the specific setup.

#### tigigi

Thanks. And if there's initial pull to give a bit speed in the beginning, how to know the friction from this experiment?

#### mfb

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I don't understand that question at all. What do you know, what do you want to find?

#### tigigi

The situation was, two objects with same mass release at the same height without initial velocity. But now it gives the object a little pull ( I guess it means it gives it some initial velocity) then how can you find / decide friction force from this situation.

#### mfb

Mentor
You have to know more about the system - like the force of this pull and the resulting acceleration. To distinguish between friction and the kinetic energy in the pulley, you'll need even more data, like measurements with different external forces.

#### tigigi

Thanks. I'll see the manual again on Monday, and I'll post the exact complete situation and question from it. It seems to be something simpler, no need to know the exact force pulling it.

#### Sharath K Menon

By kinetic energy on the pulley, if you mean rotational kinetic energy and that the pulley has a mass then we have to solve the problem in totally different way.
try considering the toque acting on the pulley due to the tension (if the two hanging masses are different) and then solving a problem

PLease note:- NO ONE asks question on atwood machine when the rope slips over the pulley. So even if there is friction, it wont make ANY difference as the rope doesnt slip on the pulley.
The only difference comes when the pulley is not massless and then we have to use the concept of torque to solve it.

PS:- if you meant by kinetic energy that the pulley is also moving i.e the pulley is attached to another pulley, then try using constrained motion relationships and solve the problem

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#### tigigi

Thanks for these answers. I take a look at the question and figure you can find total energy loss from the data, graph, and from there find friction, which cause the energy loss. The initial pull (more conceptual) means overcomeing static friction, so the energy loss is due to only kinetic friction.

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