# Homework Help: Average Speed = distance/time question.

1. Jul 5, 2009

### rdmfresno

1. The problem statement, all variables and given/known data
A race car races on a circular racetrack of radius (r).
(a) Write an equation for the carâ€™s average speed when it travels a complete lap in time (t).
(b) The radius of the track is 400m and the time to make a lap is 40s. Show that the average speed around the track is 63m/s.

2. Relevant equations
Average Speed = Total Distance/ Travel Time

3. The attempt at a solution
(a) speed = distance/time ?

(b) 400m x 3.14 = 1256m.
63m/s = 1256m/40s.. this is where I get lost. This obviously isn't right. I'm getting an average speed of 31.4m/s.

2. Jul 5, 2009

### negitron

400m x 3.14 = 1256m is not quite right. What's the formula for circumference again?

3. Jul 5, 2009

### rdmfresno

3. The attempt at a solution
(a) speed = distance/time ?

(b) 400m x 2 = 800m = diameter
800m x 3.14 = 2512 = distance
63m/s = 2512m/40s = 62.8m/s

Last edited: Jul 5, 2009
4. Jul 5, 2009

### rdmfresno

Ty again!

It's been awhile ;(

5. Jul 5, 2009

### negitron

Yes, you have it now.