# Pulley problem

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In picture 3, $A$ has touched the ground, so it has stopped. But $B$ has a velocity upward. So in this situation, the cable is not straight. What will be the tension of the cable and what will be the accelertion of $B$ in picture 3 ? Please explain 'why' in details.

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Philip Wood
Gold Member
What do YOU think the tension will be in picture 3?

What do YOU think the tension will be in picture 3?
I think, it should be 0, from my experience. But, how to explain this by the laws of physics?

I am not sure, what the tension is , just at the moment, when $A$ touches the ground. The cable is straight right at this moment.

Philip Wood
Gold Member
Cable has no tension when slack. I suppose that, if you wanted laws of Physics, you'd need to look at forces between particles in the rope.

Nugatory
Mentor
I think, it should be 0, from my experience. But, how to explain this by the laws of physics?
it is zero. You should be able to show this by considering the acceleration (known, for both masses) and the forces on them.

I am not sure, what the tension is , just at the moment, when $A$ touches the ground. The cable is straight right at this moment.
At the exact moment that the cable touches the ground, it won't behave like the idealized perfectly inextensible cable that we've been assuming throughout the problem, so you cannot solve for the tension at that exact moment without knowing a lot more about the detailed physical characteristics of the cable. In problems of this sort, your best bet is to solve for the tension a split second before the weight touches the ground, a split second after, and not worry about what happens during the arbitrarily small time in between.