# Homework Help: Dynamics pulley problem

1. Jul 9, 2008

### devanlevin

on either side of a pulley hung from a ceiling, i hang weights, on the left side, i hang a mass of 4kg,?
on the right side i hang a mass of 4kg and another mass of 2kg,

now there is acceleration towards the right side, correct?

next phase, after a certain amount of time, i disconnect the 2kg mass, leaving 4 on each side,

my question is this, what will happen to the body on the right side,?
since the two sides have equal mass and only mg and t working on them each in opposite directions, i think that they would stay where they are, 0 acceleration,. but common sense would tell me that the right mass would rise and the left mass would drop till they reach the same level.
what is correct?

my attempt at an graphic for the problem
apologies :)
|(-----)|
|.........|
|.........|
|.........|
[]4kg...|
..........|
..........[]4kg
..........|
..........[]2kg

2. Jul 9, 2008

### devanlevin

what i did was applied newtons 2nd law and got that there is no acceleration but it just doesnt seem logical,

pulleys and cords have no mass or friction

3. Jul 9, 2008

### Tom Mattson

Staff Emeritus
You're right about the zero acceleration after the 2kg mass is removed. But the question is, when you disconnect the 2kg mass do you stop the pulley from moving first, or not? If not then it will keep moving at whatever speed it was moving when the mass is disconnected. That's what it means to have 0 acceleration: no change in velocity.

4. Jul 9, 2008

### devanlevin

thanks that helps a lot,never thought of that, so if the 2nd mass falls off the right body will continue falling and the left body rising, despite the fact that their masses are the same.

dont know if there is any connection, but if so, how does a scale work,, thats the 1st thing i thought of when i saw this question,.

5. Jul 10, 2008

### Tom Mattson

Staff Emeritus
That's right.

There is more than one type of scale, but a simple one works with springs. You step on the scale and your weight compresses the springs until they match your weight and hold you still.

That's very different from pulleys.

6. Jul 10, 2008

### tiny-tim

scales

Hi devanlevin!

If you mean an old-fashioned scale, with the pivot well above the pans, then the analogous case would be putting 6kg in one pan, 4kg in the other, and then removing 2kg from the 6kg just as the scale was horizontal … then the scale would keep rotating, until it was high enough to run out of energy, and would then oscillate left and right to that height!