# Circular Motion, Revolutions from diameter, time and speed

1. Oct 6, 2012

### SherBear

1. The problem statement, all variables and given/known data
A car is moving at a speed of 20 m/s in 4 seconds and the car's tire is 38cm which is its diameter. How many revolutions did your tires make?

2. Relevant equations

Relevant Equations, T=2∏r / v

3. The attempt at a solution

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?
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Oct 6, 2012

### ehild

This is the correct solution. The number of revolution was asked.

No. But nobody asked the number of revolutions in one second.
An other note: Do not use more significant digits in the result than the number of the significant digits in the data.

ehild

3. Oct 6, 2012

### SherBear

Thank you very much ehild, so 67.22 REV is correct?

4. Oct 6, 2012

### ehild

Yes, but better to write 67 rev only.

ehild

5. Oct 6, 2012

### SherBear

Thank you very much! Is there a way I can rate you as excellent?

6. Oct 7, 2012

Thank you

ehild