# Homework Help: Torque, energy, and average power

1. Jan 3, 2006

### schrodinger

Calculate the torque (in N*m), energy (in J), and average power (in W) required to accelerate the Earth in 3 days from rest to its present angular speed about its axis.

2. Jan 3, 2006

3. Jan 3, 2006

### schrodinger

t = I*a

rotational inertia of a solid sphere = 2/5*m*r^2
mass of earth = 5.9742*10^24 kg
radius of earth = 6378.1 km
rotational inertia of the earth = 9.72126*10^31

a = delta v / delta t
angular speed of the earth = 7.27*10^-5 rad/s
3 days = 259200 seconds
a = (7.27*10^-5)/(259200) = 2.80478*10^-10

torque = (9.72126*10^31)*(2.80478*10^-10) = 2.7266*10^22 N*m

But this answer is wrong, I don't know why... please help me. Am I on the right track, am I using the correct equations?

4. Jan 3, 2006

### Gamma

Check this one. Should be ~ 9.7*10^37 kgm^2

Also you have not written the units for some quantities. Units matters.

Gamma.

5. Mar 28, 2008

### ukidhen

You probably didn't change the radius of the earth to meters. :)