# Relativity question? I think?

1. Dec 28, 2004

### JackRuby

I was just asked a physics type question that is puzzling me. Perhaps I'm over thinking it.

Q. Suppose that there is a flat bed truck traveling at 30 miles/hour relative to the ground. The truck has an infinitely long bed. A car (sitting) on the bed is traveling at 35 miles and hour relative to the truck bed (per its speedometer). What is the speed of the car irelative to the ground? Assume that the car is traveling the same direction.

My guess is 65 Mi/hr

What is the technical explanation?

2. Dec 28, 2004

### Staff: Mentor

Galilean Relativity

You are correct. It's an example of the "addition of velocities" formula:
$$V_{a/c} = V_{a/b} + V_{b/c}$$.

This also illustrates what is called Galilean relativity, or how to transform from one moving frame to another.

Note that if the speeds involved are high enough, one must use Einstein's special relativity and a different formula for velocity addition to get an accurate answer.

3. Dec 28, 2004

Great!

Thanks....

JR