# Calculate Torque Needed to Spin an LP Turntable 648 Degrees

• mikefitz
In summary: I was a little unclear on how the torque equation fits into all of this.The torque equation relates torque to angular acceleration. If you know the angular displacement and angular acceleration, you can calculate the torque.
mikefitz
An LP turntable must spin at 3.51 rad/s to play a record. How much torque must the motor deliver if the turntable is to reach its final angular speed in 1.8 revolutions, starting from rest? The turntable is a uniform disk of diameter 31 cm and mass 0.26 kg.

I know that 1.8rev is equal to 648 degrees. This means that the disc must spin 648 degrees to reach the required 3.51 rad/s for the record.

But I don't know how to begin to calculate the torque required to achieve that angular velocity. Where do I begin> Thanks

mikefitz said:
An LP turntable must spin at 3.51 rad/s to play a record. How much torque must the motor deliver if the turntable is to reach its final angular speed in 1.8 revolutions, starting from rest? The turntable is a uniform disk of diameter 31 cm and mass 0.26 kg.

I know that 1.8rev is equal to 648 degrees. This means that the disc must spin 648 degrees to reach the required 3.51 rad/s for the record.

But I don't know how to begin to calculate the torque required to achieve that angular velocity. Where do I begin> Thanks
Begin by expressing everything in radians rather than degrees, and by reviewing the equations of rotational kinematics. They are directly analogous to liner kinematics.

so, total rotation (1.8 rev) is equal to 11.3097 rad. It must spin at 3.51 rad/s.

How do I use this information to calculate a torque? thanks again

mikefitz said:
so, total rotation (1.8 rev) is equal to 11.3097 rad. It must spin at 3.51 rad/s.

How do I use this information to calculate a torque? thanks again
Look at your rotational kinematics equations. Find one that relates a change in angular velocity to an angular displacement and a constant angular acceleration. Find another one that relates torque to angular acceleration.

There are other paths to the solution (ther usually are) but these two equations will be a fairly direct route.

equations my teacher has given me:

KErot = .5 I w^2
I = .5 m r^2
Torque = F r sin(theta)

I guess I am unsure which equation relates all these different elements together?

mikefitz said:
equations my teacher has given me:

KErot = .5 I w^2
I = .5 m r^2
Torque = F r sin(theta)

I guess I am unsure which equation relates all these different elements together?
Look here and scroll down to the table showing the linear equations and their angular analogs.

http://online.cctt.org/physicslab/content/PhyAPC/lessonnotes/rotationalmotion/kinematics.asp

You should have seen all of these before. If you have not ssen them, you are seeing them now. Give particular attention to the last one. There is also a rotational analog of Newton's second law

F=ma <> Torque = I*alpha

See what you can do with these.

great, thanks for the help OlderDan - no I had not seen that last equation until I clicked your link.

## 1. What is torque and how is it related to spinning an LP turntable 648 degrees?

Torque is a measure of the force that is applied to an object to cause it to rotate. In the context of spinning an LP turntable 648 degrees, torque is the force that is needed to overcome the friction and resistance of the turntable's components to achieve the desired rotation.

## 2. How do you calculate the torque needed to spin an LP turntable 648 degrees?

The torque needed to spin an LP turntable 648 degrees can be calculated by using the formula: torque = moment of inertia x angular acceleration. The moment of inertia is the resistance of the turntable to changes in its rotational motion, and the angular acceleration is the rate at which the turntable is rotating. Both of these values can be determined using the turntable's specifications and the desired rotation angle of 648 degrees.

## 3. What factors can affect the torque required to spin an LP turntable 648 degrees?

The main factors that can affect the torque required to spin an LP turntable 648 degrees include the turntable's weight and size, the quality of its bearings and components, and the level of resistance from the record and stylus. External factors such as temperature and humidity can also impact the torque needed.

## 4. How can you reduce the torque needed to spin an LP turntable 648 degrees?

To reduce the torque needed to spin an LP turntable 648 degrees, you can make adjustments to the turntable's components, such as replacing old or worn bearings, using a lighter and more balanced tonearm, and ensuring that the record and stylus are clean and free of debris. Additionally, regularly maintaining and lubricating the turntable can help reduce friction and resistance, ultimately reducing the required torque.

## 5. Is there an ideal torque for spinning an LP turntable 648 degrees?

The ideal torque for spinning an LP turntable 648 degrees will vary depending on the specific turntable and its components. It is important to find the right balance of torque that is sufficient to overcome resistance and friction but not so high that it causes damage to the turntable's components. It is recommended to consult the turntable's manual or a professional for guidance on the ideal torque for your specific turntable.

• Introductory Physics Homework Help
Replies
16
Views
3K
• Introductory Physics Homework Help
Replies
6
Views
2K
• Introductory Physics Homework Help
Replies
7
Views
9K
• Introductory Physics Homework Help
Replies
4
Views
4K
• Introductory Physics Homework Help
Replies
4
Views
7K
• Mechanics
Replies
7
Views
1K
• Introductory Physics Homework Help
Replies
5
Views
1K
• Introductory Physics Homework Help
Replies
9
Views
24K
• Introductory Physics Homework Help
Replies
15
Views
2K
• Mechanical Engineering
Replies
5
Views
3K