The Physics
Hypertextbook
Opus in profectus

# The Nature of Waves

## Practice

### practice problem 1

A freight train at rest starts moving forward. There is enough slack in the couplings between cars that, after the first car starts moving, there is a slight delay before the second car starts to move. Then, after the second car starts moving, there is a slight delay before the third car starts to move. Likewise, after the third car starts moving (keep repeating this procedure over and over again) until finally the last car starts to move. This phenomenon satisfies the definition of a wave. Is the wave I just described a…
1. mechanical or electromagnetic wave?
2. transverse, longitudinal, complex, or torsional wave?
3. pulse or a periodic wave?
4. standing or traveling wave?

#### solution

The question didn't ask for an explanation, but I will provide one anyway.

1. This is a mechanical wave since it requires a material medium to propagate through (the cars of the train). It's also not an electromagnetic wave since it isn't anything on the standard list of electromagnetic waves with special names (radio waves, microwaves, infrared, light, ultraviolet, x-rays, gamma rays).
2. Each car is yanked forward. The wave propagates backwards through the line of train cars. Forward and backward are parallel, so this is a longitudinal wave.
3. Each car starts moving forward only once, so this is a pulse. If it were periodic, each car would have to be yanked repeatedly and the time between yanks would have to have a characteristic value.
4. This is a traveling wave because it travels from the front of the train to the back. Don't say it's a traveling wave because trains travel. That's faulty reasoning.

### practice problem 2

Write something else.

### practice problem 3

Write something different.