# Does acceleration cause a string to stretch?

1. Feb 18, 2012

### rezihk

if you have a pendulum with a 10kg bob at rest, it causes a stretch in the wire supporting it, but if that same bob were to be let go at a certain angle from the vertical, would would the wire stretch more as the bob reaches the bottom of its swing where the speed is the greatest?

in order words

stretch of wire with acceleration > stretch of wire when at rest?

2. Feb 18, 2012

### Staff: Mentor

i would say so. Consider someone on a swing as they go higher and higher the rope actually slackens and then at midswing its taunt again.

Last edited: Feb 18, 2012
3. Feb 18, 2012

### rezihk

so what other forces would be acting on the bob at the bottom of the swing apart from mg?

by what factor would the tension change at the bottom as compared to the top of the swing?

4. Feb 18, 2012

### rezihk

shouldn't the tension in the string at the bottom of the swing be the same as if it were at rest, and then it be less when it higher?

5. Feb 18, 2012

### Staff: Mentor

since the bob is moving very fast at midswing it wants to go straight ahead but the string is preventing that so there's a component of centripetal force right.

6. Feb 19, 2012

### AlephZero

Do an experiment. Make a pendulum by tying a weight to the end of a piece of elastic. Hang the pendulum above a table, so the weight is just touching the table. Then start it swinging from a horizontal position, and be careful that you are not stretching the elastic when you let the weight go.

The weight will hit the table, showing that the elastic has stretched more than when it was hanging vertically but not swinging.