# How can I find an objects # of revolutions as it accelerates

1. Mar 2, 2015

### Kacie

The crankshaft in a race car goes from rest to 3180rpm in 2.4s .

a) What is the crankshaft's angular acceleration? >>Which I found to be ~140rad/s^2

b) How many revolutions does it make while reaching 3180rpm ?

How can I find the answer to b) ? I'm not even sure where to start..

2. Mar 2, 2015

### Staff: Mentor

Hi Kacie,

Welcome to Physics Forums.

In future please be sure to use the formatting template provided in the editing window whn you post a question here in the homework areas. It's in the forum rules.

For part b, you have an angular acceleration and a time. How many radians (angular distance) will the shaft rotate during that time?

It may be helpful to compare this to a linear analogy where you have acceleration and time. What formula would you use to find the distance that an object moves if it starts from rest and accelerates at a constant rate a for time t?

3. Mar 2, 2015

### HallsofIvy

Staff Emeritus
??? For (a), if you accelerate at 140 revs per second per second for 2.4 seconds, you will go from 0 to 140(2.4)= 336 revs per second. 336(60)= 20160 revs per minute, not 3180. How did you get 140?

Assuming that increase is at constant "acceleration", then we can take an average revs per minute as the average of the initial and end values: (0+ 3180)/2= 1590 revs per minute. How many revolutions would it make at 1590 revs per minute in 2.4 s?

4. Mar 3, 2015

### Staff: Mentor

Kacie's acceleration result had units of radians per second per second, not rotations/s/s.