Hard Rotational Kinetic energy question (Flywheel)

[orangeincup]

Homework Statement

A motor imposes 5 Nm on a shaft of a 100kg flywheel which is 80cm(diameter)

What is the rotational kinetic energy after 10 mins? assuming the flywheel was at rest and the mass moment of inertia of the shaft is negligble.

Homework Equations

Ig=1/2 * mr^2
RKE=1/2*Ig*ω^2
T=Ig*Ac
ω=ω0+ Ac*t

The Attempt at a Solution

Can someone look at this and tell me what I should change? I have no idea if it is right or not.

Solving for moment of inertia: Ig= 1/2 * m*r^2, Ig=1/2*(100kg*9.81m/s^2)*(.8m/2)^2
Ig=78.4(not sure units)

Solving for acceleration:
T=Ig*Ac
5/78.4=Ac
0.063=Ac (not sure units)

Solving for ω:
ω=ω0+Ac*t
ω=0+0.063*(10minutes*60)
ω=37.80 rad/s

Solving for RKE:
RKE=1/2*Ig*ω^2
RKE=56010.5 J

 

[Doc Al]

Solving for moment of inertia: Ig= 1/2 * m*r^2, Ig=1/2*(100kg*9.81m/s^2)*(.8m/2)^2
Ig=78.4(not sure units)

Why did you multiply by g? You want the mass, not the weight.

Other than that, your method looks OK.

 

[orangeincup]

Why did you multiply by g? You want the mass, not the weight.

Other than that, your method looks OK.

Opps. Thank you.

 

[SteamKing]

Homework Statement

A motor imposes 5 Nm on a shaft of a 100kg flywheel which is 80cm(diameter)
Solving for moment of inertia: Ig= 1/2 * m*r^2, Ig=1/2*(100kg*9.81m/s^2)*(.8m/2)^2
Ig=78.4(not sure units)

Make sure the information given in the problem matches the information required for your formula.

Is the radius of the flywheel the same as its diameter?

If you are not sure of the units of your answer, calculate them according to the formula you are using.

If inertia = 1/2*m*r^2, then the units will be kg (mass) * m^2 (radius^2), or kg-m^2

Opps. Thank you.

Should be 'Oops'. 'Opps' rhymes with 'tops', while 'oops' rhymes with 'loops'.

 

[Doc Al]

Is the radius of the flywheel the same as its diameter?

It was divided by 2.