physics
posted by Oisin on .
A constant force of 20N is applied tangentially to a string wound on the rim of a 40 cm diameter wheel, initially at rest.
(i) How much work does this force do as it turns the wheel through 45„a?
(3 %)
(ii) If the wheel has mass 8.0 kg and radius of gyration 15 cm, how long does it take the wheel to rotate through this angle?

(i) 20N * 0.2 m * (angle in radians).
I cannot interpret your angle symbol, which shows up here as ,,a.
(ii) Angular acceleration rate = (Torque)/(Moment of inertia)
= (Torque)/[(Mass)*Rg^2]
The torque is 4 N*m
Angular acceleration =
4 N*m/(8*.15^2 kg m^2) = 22.2 rad/s^2
Time required to rotate angle theta =
2*(theta)/(angular acceleration rate) 
so for the first part 20N * 0.2m * 2pi is that correct?
also how did you get your torque?
(iii) What is the angular momentum of the wheel after this time? 
You never said the angle turned was 2 pi. You typed 45,,a
Torque is force times lever arm (in this case, wheel radius)
(iii) Angular momentum after time T is
(Torque)*(Time) =
(Moment of inertia)*(angular velocity) 
no i mean to convert 45degrees into rads you multiply by 2pi
thanks for your help 
45 degrees is pi/4 radians. You multiply by pi/180 radians per degree, NOT 2 pi

how did u find the angular velocity?
im tryin to use v = wr but i don't know where to find the normal velocity of the rim