# What length will I measure?

1. Feb 17, 2013

### satyudhi

I take a rod and attach to its ends a another rod parallely at a distance r. The second rod oscillates about the first one through 180 degrees with some time period 't'. Now I am in a frame in which this system is moving at a speed 'v' with respect to me. I keep some sand in the way of this system. So when the rod passes over this sand, there will be 2 imprints left, one is the straight line which is the trajectory of our fixed rod while the other is the imprint of the osscilating rod when it struck the sand. This imprint will be of the same lengh as the oscillating rod as the whole (osscillating) rod struck the sand at once. But the oscillating rod is of the same length as the length of the fixed rod, as it it tied at the ends. So in principle the length of the imprint must be equal to the proper length of rod(system). But then this metod of length measurement gives me proper length in all frames, rather than a contracted length. So what will eb the length of the imprint actually?

2. Jun 4, 2013

### Henriamaa

In your reference frame the rod does not have proper length. Anyways, you need to be precise by what you mean "struck the sand at once". In special relativity you will need two clocks (in your reference frame)at different positions measuring the end points at the same time.