Some angular momentum problems

[nns91]

Homework Statement

1. A spaceship is rotating about its longitudinal axis at 6 rev/min. The occupants wish to stop this rotation. They have small jets mounted tangentially, at a distance R=4m from the axis and can eject 10 g/s if gas from each jet with a nozzle velocity of 800 m/s. For how long must they turn on these jets to stop of the rotation ? The rotational inertia of the ship around its axis (assumed to be constant) is 4000 kg*m^2

2. True or false: If the net torque on a rotating system is zero, the angular velocity of the system cannot change.

Homework Equations

Torque=dL/dt

The Attempt at a Solution

1. Can anyone suggests me how to this problem ?

2. I know it's false. I try to explain by using dL/dt=0 since net torque is zero. Thus, d (I*w)/dt=0. Thus I* dw/dt + w*dI/dt=0. Since dI/dt= 0, I*dw/dt must be 0 so I can be 0 also, not necessarily dw/dt must be 0. However, my teacher said it's wrong. How do I fix it ??

 

[LowlyPion]

Determine the Torque that the jets can create.

You know

T = I * α

so you can figure α.

After that it's easy-peasy nice and easy simple kinematics.

http://hyperphysics.phy-astr.gsu.edu/hbase/mi.html#rlin

 

[nns91]

1. I have 1 equation Torque= I*alpha but I have 2 unknowns, Torque and alpha so how do I solve for alpha ?

How about the conceptual question ?

 

[LowlyPion]

1. I have 1 equation Torque= I*alpha but I have 2 unknowns, Torque and alpha so how do I solve for alpha ?

How about the conceptual question ?

Can't you figure the Force and the distance from the center of rotation - the moment arm?

 

[nns91]

oh yeah, I forgot it. T=F*lever arm.

How about the conceptual question ?

 

[LowlyPion]

How about the conceptual question ?