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Dec30-08, 05:44 AM
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tiny-tim's Avatar
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Hi saltine!
Quote Quote by saltine View Post
I am confused because u is not a force but just acceleration.
Then it just goes onto the acceleration side of F = ma

you add it to the other (rotational) acceleration.
In Fig 1, there are two forces acting on the ball. The component of mg that is parallel to the rod is cancelled by the normall force from the rod, so the net force at the ball is only mg sinθ.
I didn't understand this at all.

The net force on the ball is mg sinθ plus the force from the rod.

Nothing is cancelled (what is it to be cancelled with?)

the reason why you leave out the mg cosθ is not because its cancelled, but simply because you're only looking at tangential components.
In Fig 5, the situation is similar except that u is now from an external force, Fu = Mu. Is this situation the same? Why is/isn't Fu part of the net force acting on the ball?
Fu isn't "part of the net force acting on the ball" because isn't acting on the ball at all

you must decide which body you're applying Netwon's second law to, and then only use the forces on that body

your book is using the ball as the body, so the only forces on it are mg and the force from the rod

any force on anything else is irrelevant.

Quote Quote by saltine View Post
Net Tangential Force = maT = mg sinθ - mu cosθ

Since aT = α L,

m α L = mg sinθ - mu cosθ.

With small angle approximation:

[tex]mL\ddot{\theta} = mg\theta - mu[/tex]
This is much better you've used the same method as your book, except that you've moved the mu cosθ from one side of the equation to the other, by using the accelerating frame.

(The tangential force is mg sinθ, and the tangential acceleration (times mass) is mαL + mu cosθ)

This isn't necessary

and it's risky, because it's so easy to get the sign wrong.

Your book just uses the stationary frame, and uses only tangential components, so that there's only one component of force on the ball, and the acceleration is split into two parts.