Calculating Relative Motion Using Time and Distance Equations

[uberbandgeek6]

A suspicious-looking man runs as fast as he can along a moving sidewalk from one end to the other, taking 2.50 s. Then security agents appear and the man runs as fast as he can back along the sidewalk to his starting point, taking 8.00 s. What is the ratio of the man's running speed to the sidewalk's speed? (running speed / sidewalk speed)

Thanks

 

[Kurdt]

Where is your attempt at the solution?

 

[uberbandgeek6]

I'm not sure how to go about it. I set up the velocities as V(with)=length/2.5 and V(against)=length/8, but I'm not sure what to do from there.

 

[uberbandgeek6]

How do I work around the fact that i don't know the length of the sidewalk?

 

[Kurdt]

You know that both times he travels the same distance. What you need to do is find an expression that would give you the distance for run 1 and run 2. You can then equate them because you know the distance should be the same and you should be able to find the answer from there.