# Impulse and pulley

## Homework Statement

Two blocks of mass(4 Kg and 0.1 Kg ) are connected by a mass less string that passes over a friction less pulley.The 4 Kg mass rests on the ground and the 0.1 Kg is hanging above the ground.Find the minimum distance from which the 0.1 Kg mass can be lifted and drop so that it just lifts the 4 Kg mass off the ground.

## The Attempt at a Solution

In order to lift the 4kg mass the tension on the block must be greater than or equal to 40 N. Therefore when i drop the smaller block from a particular height h it should cause an impulse of 40 N along the string
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Last edited:
Delta2

You will need to post your attempt at understanding this before anyone can help you with it.

haruspex
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an impulse of 40 N
40 N is a force, not an impulse. What do you know about impulses?

Delta2
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Hmmm, shouldn't we also know the stretching of the string or the time duration of the "collision" (the sort duration process during which the small mass changes its velocity from ##\sqrt {2gh}## to something smaller or even zero (I suppose zero, cause that's what it means by "just lift the 4kg mass"). If the stretching and the time duration are zero, this means that the tension force becomes infinite, doesn't it?

Last edited:
haruspex
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Hmmm, shouldn't we also know the stretching of the string or the time duration of the "collision" (the sort duration process during which the small mass changes its velocity from ##\sqrt {2gh}## to something smaller or even zero (I suppose zero, cause that's what it means by "just lift the 4kg mass"). If the stretching and the time duration are zero, this means that the tension force becomes infinite, doesn't it?
Yes, there is something odd here. Maybe it is a trick question, or maybe some fact has been omitted, like a spring constant.

40 N is a force, not an impulse. What do you know about impulses?
sorry, yes it is the Force.

Hmmm, shouldn't we also know the stretching of the string or the time duration of the "collision" (the sort duration process during which the small mass changes its velocity from ##\sqrt {2gh}## to something smaller or even zero (I suppose zero, cause that's what it means by "just lift the 4kg mass"). If the stretching and the time duration are zero, this means that the tension force becomes infinite, doesn't it?
say t=0.001
will it be possible to solve the problem if i am not given the time of collission?

Delta2
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say t=0.001
will it be possible to solve the problem if i am not given the time of collission?
I don't see how it can be solved if we are not given the duration of the collision or the stretching of the string.
But if I understand you, this is a problem you have made by yourself and not found in some book right?
if ##\Delta t=0.001## then we simply solve the system of equations ##F\Delta t=mv_0## and ##v_0=\sqrt{2gh}## for ##v_0## and ##h##.

haruspex
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we are not given the duration of the collision or the stretching of the string.
Even knowing the duration would not suffice. The question is the peak tension, so you would have to assume, say, an ideal spring.
Or, if we take it as inextensible, then the answer is any nonzero height.

Delta2
Delta2
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Gold Member
Even knowing the duration would not suffice. The question is the peak tension, so you would have to assume, say, an ideal spring.
Or, if we take it as inextensible, then the answer is any nonzero height.
Maybe we can assume that the tension remains approximately constant during the "collision", though I know this doesn't look like a very realistic assumption.