# Torque required to rotate a disk

• connor02
In summary, to rotate a disk or diameter 1.4cm, height 1cm, weight 12g with a required torque of 622.6 Newtons, the disk and rod assembly would need to be turned by 2.95 * 10^-7 Newtons.
connor02

## Homework Statement

How much torque is required to rotate a disk or diameter 1.4cm, height 1cm, weight 12g?

## Homework Equations

I think

Iz = mr^2/2

Torque = force x distance

## The Attempt at a Solution

Iz = 2.95 * 10^-7

I do not know where to go from here.

Thank you.

hi connor02!
connor02 said:
How much torque is required to rotate a disk or diameter 1.4cm, height 1cm, weight 12g?

what is the rest of the question?

(that's like asking how much force is required to push a disk of mass 12g)

Hey Tiny-Tim,
Sorry I didn't think any other part would be needed.

You have a disc of diameter 1.4cm and height 1cm. The disc is connected to a rod of diameter 0.8cm and height 7cm. The disc and rod have a density of 7800kg/m^3. The disc and rod have been inserted into a barrel of diameter 1.4cm in order to extrude a fluid. You want this disc and rod to exert a force of 622.6 Newtons on the fluid. What torque should you rotate the disc and rod assembly by?

There is a picture too, let me know if you need it and I will scan it for you.

Thank you.

hey connor02!

i'm still confused …

is the barrel rifled (with a screw thread)?

(the force is given in Newtons, and the cylinder can be pushed, so where does torque come into it?)

would you scan the picture?

Hi Tiny-Tim,
Yes the rod has screw threads at the top so the rod needs to be rotated for the rod and disc assembly to advance down the barrel. My scanner refuses to work so I have drawn the picture instead. The picture is attached here. Thanks.

#### Attachments

• phys_help.jpg
5.7 KB · Views: 474
ahh!

then we need to know the pitch of the screw thread

ie, how many turns per metre?

Hi Tiny-Tim,
That information is not given, does it need to be known? If it must be known, can you assume the pitch is x turns per meter and give the answer in terms of x?

connor02 said:
Hi Tiny-Tim,
That information is not given, does it need to be known?

well, you're asked to turn torque into force (Newtons), so yes
If it must be known, can you assume the pitch is x turns per meter and give the answer in terms of x?

yes

great, so how do I go about calculating it? I have Iz of the disc, now what do I do?

what do you think?

I don't know how to apply torque = force x distance on a disc? And where the Iz value comes in? I only calculated Iz cus the hint of the question gave the Iz formula for the disc.

i don't think the mass or the moment of inertia of the disc is relevant …

it isn't being accelerated, so it doesn't itself need any force or torque

(and I'm going out now)

Okay.

Could someone help me out?

Thanks.

help?

## 1. What is torque and how is it related to rotating a disk?

Torque is a measure of the force that can cause an object to rotate around an axis. In the case of a disk, it refers to the force that is needed to make the disk spin or rotate.

## 2. How is the torque required to rotate a disk calculated?

The torque required to rotate a disk can be calculated by multiplying the force applied to the disk by the distance from the axis of rotation to the point where the force is applied.

## 3. What factors affect the torque required to rotate a disk?

The torque required to rotate a disk is affected by the mass and shape of the disk, the distance between the axis of rotation and the point where the force is applied, and the friction between the disk and its surroundings.

## 4. Is there a difference between the torque required to start rotating a disk and the torque required to keep it rotating?

Yes, there is a difference. The torque required to start rotating a disk is greater than the torque required to keep it rotating. This is because it takes more force to overcome the initial inertia and get the disk moving.

## 5. Can the torque required to rotate a disk be reduced?

Yes, the torque required to rotate a disk can be reduced by decreasing the mass of the disk, decreasing the distance between the axis of rotation and the point where the force is applied, and reducing friction between the disk and its surroundings.

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