# Interesting question from AP student

1. Apr 25, 2012

### Drew boxer

I'm a physics teacher and one of my AP students posed the following question: "if there was a steel bar 2 light years long and you moved it forward a couple inches, would the other end move forward a couple inches immediately or would there be a delay?". We'd been discussing electric fields, so that got him thinking. I thought it was interesting and wondered how others would have answered.

Thanks,

2. Apr 25, 2012

### Khashishi

There would be a delay. The movement would propagate down the bar at the speed of sound in the bar, which is $\sqrt{Y/\rho}$, square root of Young's modulus over density.

3. Apr 25, 2012

### ZxcvbnM2000

So if what you say is true then the bar would be a couple of inches shorter for a little while ? I fail to understand how that can happen :S

4. Apr 25, 2012

### Khashishi

It's not surprising that the length of the bar will change. If you strike a bar, it will compress a little and send a compression wave down the bar. It's no different if you move it gently--the compression wave is just much smaller in amplitude.

5. Apr 25, 2012

### Travis_King

Not to mention the inertia of a 2 lightyear long steel bar!