# Pulley problem

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1. Dec 28, 2014

### arpon

[moderator's note: moved to homework forum so no template]

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.

Last edited by a moderator: Dec 28, 2014
2. Dec 28, 2014

### Philip Wood

What do YOU think the tension will be in picture 3?

3. Dec 28, 2014

### arpon

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.

4. Dec 28, 2014

### Philip Wood

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.

5. Dec 28, 2014

### Staff: Mentor

it is zero. You should be able to show this by considering the acceleration (known, for both masses) and the forces on them.

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.