# How to determine the torque of a rotating disc?

1. Jan 16, 2009

### MechaMZ

how to determine the torque of a rotating disc in order to find the power of motor?

there is a rotating disc, and a ball will pass through and launched by the rotating disc.
the initial speed is 20m/s, and i could determine the rpm from here.

the information i have now:
the disc diameter is 15cm, and the ball diameter is 0.143m and 0.18kg

thank you

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2. Jan 16, 2009

### tiny-tim

Hi MechaMZ!

Show us what you've tried, and where you're stuck, and then we'll know how to help.

3. Jan 17, 2009

### MechaMZ

i think to find out the power required by the motor, torque should be found before that.
but what should i do to find out the torque due to these limited information =(

i tried to find out the moment of inertia, so i could determine the torque..
and actually there is another roller under the ball, so i assume the vertical forces are balanced.

but the mass is referring to the mass of ball or the roller?

and followed by the formula,
but what is the time referring to?

4. Jan 17, 2009

### tiny-tim

The mass is the mass of whatever you are working out the moment of inertia of.
What time?

Sorry, but you seem to be using words somewhat at random

even with the diagram, I haven't worked out what this problem is about …

is there a disc or a wheel? what is the "roller"? what is the ball "passing through"? how is it launched? is its diameter really 14.3cm?

Can you please write out the whole question for us?

5. Jan 18, 2009

### MechaMZ

hi tim, actually my problem is quite similar with this

but i don't know why the mass(or anything else that related to) of the wheel(rubber) is not considered, as we calculating the torque in order to find the motor power..

thank you =)

Last edited: Jan 18, 2009
6. Jan 18, 2009

### MechaMZ

this is what i did..

E= $$\tau \vartheta$$
mgh + 0.5mv2 = $$2\tau(\pi/2)$$
since there are 2 motors
(0.18 x 9.81 x 1.2) + (0.5 x 0.18 x 202) = $$\tau \pi$$
$$\tau = 12.13Nm$$
height of the ball is 1.2m, and the mass is 0.18kg, the initial velocity is 20m/s

but is this the way to calculate the power required for the motor?
there are 2 rotating wheels with 0.15m in diameter.

at linear velocity 20m/s, rpm = 2546.415rpm
power(kW) = $$\tau$$ x 2$$\pi$$ x N / 60000
= 3.23Kw

the answer seems like not correct though =(

7. Jan 19, 2009

### nvn

MechaMZ: Why do you say the ball has a height of 1.2 m? In your diagram, the ball seems to have an initial and final height of 0 m. Secondly, are you sure the ball has an initial velocity of 20 m/s? Did you mean to say the ball has a final (launch) velocity of 20 m/s, when it exits the rotating disks? Are you saying the initial velocity of the ball entering the disks is v1 = (2*g*h)^0.5 = 4.8514 m/s?