# Homework Help: Circular Motion, Revolutions

1. Oct 6, 2012

### SherBear

A car is moving at a speed of 20 m/s in 4 seconds and the car's tire is 38cm around which is its diameter. How many revolutions did your tires make?

Relevant Equations, T=2∏r / v

My attempt at a solution:

To get the distance travelled=20 m/s (4 sec.) = 80 m

2∏(0.19m)= 1.19 m

80m / 1.19 m = 67.22 Rev

67.22 rev / 4 sec. = 16.805 rev/s ?

is this wrong?

2. Oct 6, 2012

### nasu

Is this the actual text of the problem or your interpretation?
I won't assume that "38 cm around" means diameter. Diameter is rather "across" that "around".

3. Oct 6, 2012

### SherBear

My interpretation it says 38 cm = Diameter

4. Oct 6, 2012

### nasu

OK, then the only thing is that the question (as you wrote it) asks how many revolutions and not how many per second.

5. Oct 6, 2012

### SherBear

is 67.22 rev correct then?

6. Oct 6, 2012

### nasu

It looks OK. If this is what the problem requires.

7. Oct 6, 2012

### SherBear

Thank you very much for your time and patience nasu =-)