How to Determine Angular Deceleration and Initial Speed of a Coasting Saw Blade?

[Lance WIlliam]

A circular saw blade completes 1200 revolutions in 40s while coasting to a stop after being turned off. Assuming constant deceleration, what are (a) the angular deceleration and (b) the initial angular speed?

Answer in RPM/S
Method I used: Change in theta is 1200
Time: 40s
final Vel.:0
Initial Vel.:?
Alpha:? Eqn Used: (change in theta)=(1/2)(Initial vel.+final vel.)*time
looking for inital speed:
1200=(1/2)(initial vel+0)40seconds
Take the vel. found and place into eqn:
looking for alpha:
final vel. =Initial vel.+alpha*time


I ended up getting 1.5 ...

 

[Doc Al]

looking for inital speed:
1200=(1/2)(initial vel+0)40seconds

This will give you the angular speed in rev/s.

Take the vel. found and place into eqn:
looking for alpha:
final vel. =Initial vel.+alpha*time

That will give you alpha in rev/s/s, not RPM/s like they want. (Convert the speed to RPM--rev/min, instead of rev/s.)