Posted by **Morgan** on Tuesday, March 23, 2010 at 4:02pm.

I;m sorry, but I was stuck on the second part of this problem. Can you please help me?

An automobile engine slows down from 4500 rpm to 1500 rpm in 2.5 seconds.

a) calculate angular accceleration

b)calculate total number of revolutions done in this time.

What I did:

a) I got the answer, and it was right, and it was also negative, but: what confused me was b

b) It is decelerating, this really confused me. I wasn't even sure how to approach this.

- physics -
**bobpursley**, Tuesday, March 23, 2010 at 4:37pm
average speed=4500+1500 divided by 2, or

average speed=3000 rpm

displacement=avgspeed*time

=3000rpm or 3000rev/3600sec *2.5sec

= 7500/3600 rev

- physics -
**Morgan**, Tuesday, March 23, 2010 at 11:02pm
I'm sorry, I tried this and got 2.1 and the computer keeps kicking it out as wrong. Your logic makes sense to me after I thought about it. Is there something I'm doing wrong?

- physics -
**Morgan**, Tuesday, March 23, 2010 at 11:09pm
wait never mind. There are only 60 seconds in a minute!

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