# Newton's second law and a weight and string

1. Jan 28, 2008

### rejz55

1. The problem statement, all variables and given/known data

suspended a heavy weight from a light string and attach a similar string below it. if you pull on the lower string with a steadily increasing force, the upper string will break; if you pull the lower string with a jerk, the lower string will break. please explain to me both cases why it happens? thanks!

2. Jan 28, 2008

### PhanthomJay

This one's a bit tricky to answer, but if this is a homework question, you must show some of your attempt or thoughts on this before we can assist. You might want to think about how Newton's laws might apply for each case.

3. Feb 24, 2008

### zumulko

I'm not quite sure about the case of pulling the lower string with a jerk, but I think the force exceeding lower string's strength is less than the weight of the heavy block. Because of the inertia of the block the force applied to the block by the lower string is still not enough to move it, so the lower string will break. The upper string almost doesn't feel the force applied to the lower one. Am I right? And if yes, why do I feel it is possible to explain it more ellegant? :)

4. Feb 27, 2008

### zumulko

Couple of days ago I hit the tree with my bike and became wiser ;)
If you pull the lower string with a jerk, you give the block relatively large acceleration directed downwards, so large inertia force due to the mass of the block is directed upwards. So the upper string feels just the weight of the block (like before pulling it), whereas the lower string feels the force of pulling which is greater than its strength. Hence the lower string will break.
If you pull on the lower string with a steadily increasing force, you give the block relatively small acceleration, so small inertia force due to the mass of the block is directed upwards. Now the upper string feels the weight of the block enlarged by the force you pull it down, which in total is greater than its strength, whereas the lower string feels the tension of the upper one decreased by the weight of the block (i.e. if the weight of the block is nearly as the strength of the upper string, than you can break the upper string using just a tiny (steadily increasing) force directed downwards). Hence the upper string will break.

5. Feb 27, 2008

### Pythagorean

read about inertia in your text book... and impulse, maybe...