How to determine the torque of a rotating disc?

In summary: If so, this would be wrong, as the discs are at a speed of 20 m/s, and the ball has an initial velocity of 0 m/s. Lastly, what is the power of a motor? I'm sorry if I'm not understanding something here. In summary, to determine the power of a motor, you need to find the torque due to the rotating disc. You can do this by finding the moment of inertia of the object, which in this case is a ball. You can then use the formula to find the power.
  • #1
MechaMZ
128
0
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
Hi MechaMZ! :wink:

Show us what you've tried, and where you're stuck, and then we'll know how to help. :smile:
 
  • #3
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.

where
I =K.M.R2
M is the mass
R is the radius of the object from the center of mass (in some cases, the length of the object is used instead.)
k is a dimensionless constant called the inertia constant that varies with the object in consideration.

Inertial constants are used to account for the differences in the placement of the mass from the center of rotation. Examples include:

* k = 1, thin ring or thin-walled cylinder around its center,
* k = 2/5, solid sphere around its center
* k = 1/2, solid cylinder or disk around its center.

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

and followed by the formula,
torque= I.α
where α is angular acceleration

but what is the time referring to?
 
  • #4
MechaMZ said:
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?

The mass is the mass of whatever you are working out the moment of inertia of. :smile:
but what is the time referring to?

What time? :confused:

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

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? :confused:

Can you please write out the whole question for us? :smile:
 
  • #5
hi tim, actually my problem is quite similar with this
https://www.physicsforums.com/showthread.php?t=76293

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:
  • #6
this is what i did..

E= [tex]\tau \vartheta[/tex]
mgh + 0.5mv2 = [tex]2\tau(\pi/2)[/tex]
since there are 2 motors
(0.18 x 9.81 x 1.2) + (0.5 x 0.18 x 202) = [tex]\tau \pi[/tex]
[tex]\tau = 12.13Nm[/tex]
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) = [tex]\tau[/tex] x 2[tex]\pi[/tex] x N / 60000
= 3.23Kw

the answer seems like not correct though =(
 
  • #7
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?
 

1. How can I measure the rotational speed of the disc?

The rotational speed of a disc can be measured using a tachometer, which measures the revolutions per minute (RPM) of the disc. Another method is to mark a point on the disc and use a stopwatch to time how long it takes for the mark to make one full rotation.

2. What is the formula for calculating torque?

The formula for calculating torque is T = F x r, where T is torque, F is the force applied, and r is the distance from the axis of rotation to the point where the force is applied.

3. How do I determine the mass and radius of the disc?

The mass and radius of the disc can be determined by using a scale to measure the weight of the disc, and a ruler to measure the diameter or radius of the disc. The mass and radius should be measured at the same point on the disc where the force is applied.

4. What is the difference between static and dynamic torque?

Static torque is the torque required to keep an object in a fixed position, while dynamic torque is the torque required to change the rotational speed of an object. In the case of a rotating disc, static torque would be the force needed to keep the disc spinning at a constant speed, while dynamic torque would be the force needed to increase or decrease the disc's speed.

5. How does the shape and mass distribution of the disc affect the torque?

The shape and mass distribution of the disc can affect the torque by changing the location of the center of mass and the distribution of the mass around the axis of rotation. A disc with a larger diameter or a mass that is concentrated towards the outer edges will require more torque to rotate than a disc with a smaller diameter or a more evenly distributed mass.

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