# Homework Help: Tension in elastic cord at low point

1. Oct 16, 2011

### Opus_723

Problem:

Consider the following scenario:
An elastic band is used to suspend a metal cylinder vertically. The cylinder is given an initial
downward motion so that it moves down, reaches a low point, and then moves back up again.

Your challenge task is to determine the tension in the band at the instant that the cylinder is at its low point. You may use a motion sensor.

Attempt at a solution:

I know that the tension is greater than the weight of the cylinder at the low point, but I'm not sure how to determine by how much. I thought about using kinematic equations to find the acceleration at the bottom and from that the force, but we've only learned how to deal with constant acceleration and I'm sure that the tension in the band varies with the position of the cylinder. The only thing I've thought of is to use the motion sensor to make an acceleration graph somehow and find the acceleration at the lowest x point. But that seems more numerical than what we usually do in this class, so I figure there's probably a mathematical way.

Keep in mind that this is only my first physics class. We've just finished projectile motion and moved on to forces, so we haven't gotten to any special rules for springs or anything.

I appreciate any nudges to get me on the right track, if there is another way to think about this.

2. Oct 16, 2011

### grzz

Think about the forces acting on the cylinder at the lowest point.

3. Oct 16, 2011

### Opus_723

I have. Weight and tension, with the tension being greater than the weight. I just don't see how that helps me determine the tension. I don't know how much greater the tension is, unless I'm missing something.

4. Oct 17, 2011

### grzz

If you manage to measure the acceleration at the lowest point, then you can use
Fnet = ma to find the tension because the net force is given in terms of Tension and Weight as you correctly said.