# How Does a Flywheel Begin to Move with Constant Speed After Adding a Mass?

• physea
In summary: So, you need to calculate the torque due to the weight of the hanging mass, and then calculate the torque due to the friction at the axle. The net torque should be zero for constant velocity.
physea
Member advised to use the homework template for posts in the homework sections of PF.

For this problem, I don't understand how can the system be at rest and immediately after placing the mass, to move with constant speed. But anyways, can you tell me please how to proceed?

On the hanging mass, there is mg acting and T the tension of the rope. On the flywheel, the tension of the rope is acting and the friction of the bearings.

I have no clue what the tension of the rope is. I wanted to say that opposite to the mg, is the friction and the inertial force of the flywheel, but I am not familiar with calculations of the inertial force.

Any idea?

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Last edited:
physea said:
View attachment 100425

For this problem, I don't understand how can the system be at rest and immediately after placing the mass, to move with constant speed. But anyways, can you tell me please how to proceed?

On the hanging mass, there is mg acting and T the tension of the rope. On the flywheel, the tension of the rope is acting and the friction of the bearings.

I have no clue what the tension of the rope is. I wanted to say that opposite to the mg, is the friction and the inertial force of the flywheel, but I am not familiar with calculations of the inertial force.

Any idea?
I would not be concerned about the ' initially at rest ' phrase.

If the 0.25kg hanging mass has constant velocity, then what is the net force on this mass ? The answer to this should give you the tension for this case.

That should get you started.

physea said:
I don't understand how can the system be at rest and immediately after placing the mass, to move with constant speed.
Can you understand how a box may be at rest on a horizontal floor, yet when pushed with a constant horizontal force F moves at constant velocity?

haruspex said:
Can you understand how a box may be at rest on a horizontal floor, yet when pushed with a constant horizontal force F moves at constant velocity?

OK got it. But the tricky bit is what happens during the time that speed is zero and the very next moment is some constant value. I thought that in order to attain that constant speed, there must be an acceleration. It may indeed be within fractions of second, but in that time, dV/dt is not zero, it has some value! Anyway, as for the problem, the friction cancels the effect of the weight. But do I have to calculate them as torques? because I don't know if the friction is exerted at the same line as the weight of the falling mass

physea said:
what happens during the time that speed is zero and the very next moment is some constant value
You are right that there is a small question of how the motion gets started. You have to assume it was given some nudge to get it moving.
Otherwise, if you steadily increase the force until motion starts it will switch from static to the lower kinetic friction and therefore accelerate.
physea said:
do I have to calculate them as torques? because I don't know if the friction is exerted at the same line as the weight of the falling mass
The friction consists of a torque at the axle.

## What is flywheel moment of inertia?

Flywheel moment of inertia is a physical property of a rotating object that determines its resistance to changes in its rotational motion. It is a measure of how mass is distributed around the axis of rotation.

## How is flywheel moment of inertia calculated?

The flywheel moment of inertia is calculated by multiplying the mass of the object by the square of its distance from the axis of rotation. It can also be calculated by integrating the mass distribution of the object over its entire volume.

## Why is flywheel moment of inertia important?

Flywheel moment of inertia is important because it affects the performance and stability of rotating systems. Higher moment of inertia means more energy is required to change the rotational speed of the object, making it more stable and resistant to external forces.

## How does flywheel moment of inertia affect a flywheel's performance?

A higher flywheel moment of inertia means that the flywheel can store more kinetic energy, making it more useful for applications such as energy storage or as a stabilizing component in machines. It also helps maintain a more consistent rotational speed, reducing vibrations and fluctuations.

## How can flywheel moment of inertia be changed?

The moment of inertia of a flywheel can be changed by altering its mass or its distribution. Increasing the mass or moving it farther from the axis of rotation will increase the moment of inertia. Conversely, decreasing the mass or moving it closer to the axis will decrease the moment of inertia.

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