Torque: Flywheel problem

• XwyhyX
In summary, the conversation discusses finding the angular acceleration and torque of a flywheel in the form of a uniform disk. The student asks for help with their solution and compares it to the solution given by their teacher. The expert points out that the given value is for torque, not net force, and there may have been a typo in the problem statement. The student then realizes their mistake and understands the solution.

XwyhyX

1. Homework Statement
A flywheel in the form of a uniform disk (I = ½ MR2) 5.0 ft in diameter, weighs 650 lb. What will be its angular acceleration if a net force of 225 ft-lb acts it upon? If the disk is rotating at 1200 rev/min, what torque is required to stop it in 30 minutes?

Homework Equations

T=I$\alpha$=Fr

The Attempt at a Solution

I need help in the first question, because apparently I arrive with a different answer from the solution that my teacher gave me. I just wanted to know if I got it wrong or my teacher forgot something

So using the definition of torque, I solve for alpha which is

alpha = FR/I

Defining I in the equation gives

alpha = FR/(1/2)MR2

Since the given is weight, I still have to define mass in the equation

alpha = FR/(1/2)(W/g)R2

Substituting the values, I'll have

alpha = (225)(2.5)/(1/2)(650/9.8)(2.5)2

The solution in my note was

alpha = 2g(Torque)/WR2

which results to 3.54rad/s2Am I missing something? Or is everything just right?

XwyhyX said:
alpha = FR/(1/2)(W/g)R2

Substituting the values, I'll have

alpha = (225)(2.5)/(1/2)(650/9.8)(2.5)2
Two problems:
(1) Even though it says 'net force', what's really given is the net torque.
(2) Careful with units: g ≠ 9.8

Oh I see, one question though, How can I tell whether its net force or net torque?

XwyhyX said:
Oh I see, one question though, How can I tell whether its net force or net torque?
The units given (ft-lb) are for torque, not force. Check the problem statement; I suspect there was a typo somewhere.

Thanks! I got my english units rambled up, I usually think of pounds as mass, so I was thinking of what the english unit is for force. Nevermind, I understand it now. :D

What is torque?

Torque is a measure of the twisting force that causes rotation. It is commonly measured in newton-meters (Nm) or foot-pounds (ft-lb).

What is a flywheel?

A flywheel is a mechanical device that stores rotational energy for later use. It consists of a heavy wheel mounted on an axle and is used to reduce fluctuations in rotational speed.

How does torque affect a flywheel?

The amount of torque applied to a flywheel affects its rotational speed and the amount of energy it can store. A greater torque will result in a faster rotation and more energy stored, while a smaller torque will result in a slower rotation and less energy stored.

What are some real-life applications of torque and flywheels?

Flywheels and torque are commonly used in machines that require a steady output of power, such as engines, turbines, and electric motors. They are also used in vehicles to store energy and reduce fuel consumption.

How do you calculate torque in a flywheel problem?

The formula for torque is T = r x F, where T is torque, r is the radius of the flywheel, and F is the force applied. In a flywheel problem, the torque can be calculated by multiplying the force applied to the flywheel by the radius of the flywheel.