Torque in a rotating rod

In summary, the question asks for the amount of torque needed to keep a rod of length 65cm and mass 2.0kg from rotating about a point where a pin enters the rod. The torque is calculated about an axis that is perpendicular to the plane of the figure. The solution involves finding the torque due to the hanging mass and the weight of the rod, which act in opposite directions, and then adding them together. The correct pivot point for this problem is the left end of the rod, as using any other point will result in different torque values.
  • #1
aligass2004
236
0

Homework Statement


How much torque must the pin exert to keep the rod from rotating if the rod has a length L=65cm and a mass m=2.0kg? Calculate this torque about an axis that passes through the point where the pin enters the rod and is perpendicular to the plane of the figure.

http://i241.photobucket.com/albums/ff4/alg5045/p13-25alt.gif


Homework Equations





The Attempt at a Solution


I haven't attempted the problem because I'm not quite sure where to start. I know that you have to find Tnet, and I know the mass and the length fit into the equation somewhere. I'm just not sure how to set the equation up.
 
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  • #2
What is the total torque the rod and the 500g mass exert about the pin?

The pin needs to create a torque of the same magnitude but the opposite direction, to cancel the above torque...
 
  • #3
Is it Tnet = mL - w? The w represents the mass hanging.
 
  • #4
aligass2004 said:
Is it Tnet = mL - w? The w represents the mass hanging.

that's the idea, but there are mistakes... weight acts at the center of mass... take clockwise as positive... counterclockwise as negative for signs...

try to fix the expression.
 
  • #5
So is it Tnet = W - mL?
 
  • #6
aligass2004 said:
So is it Tnet = W - mL?

no. take it step by step. what is the torque due to the 0.500kg hanging mass? take clockwise positive counterclockwise negative...

use the definition of torque... what is the force? what is the distance?
 
  • #7
T = -Fr and I think the F is just the weight of the mass. In case you couldn't tell, torque and I don't get along well.
 
  • #8
aligass2004 said:
T = -Fr and I think the F is just the weight of the mass. In case you couldn't tell, torque and I don't get along well.

:) that's ok.

yes, that's the definition you need... here gravity acts downward... so the rod would turn clockwise due to gravity...

so we use a +...

torque due to the 0.500kg = 0.500*9.8*L

try to find the torque due to the weight of the rod... it acts at L/2...
 
  • #9
The torque due to the weight of the rod would be T = (2kg)(9.8)(.325m) = 6.37
 
  • #10
aligass2004 said:
The torque due to the weight of the rod would be T = (2kg)(9.8)(.325m) = 6.37

exactly. you're getting along better already. :)

so the total torque is 0.500*9.8*0.65 + 6.37...

so that's a clockwise torque about the pin... so the pin needs to exert a counterclockwise torque of the same magnitude...
 
  • #11
Got it. T = 9.555. I think I need you to sit beside me during my exam in a couple weeks...just for the torque problems.
 
  • #12
aligass2004 said:
Got it. T = 9.555. I think I need you to sit beside me during my exam in a couple weeks...just for the torque problems.

lol. nahh.. you won't need me.
 
  • #13


I am doing a very similar problem to this one. My question is, why do you have to pick the left end of the rod as your pivot point?

I tried using the center of mass and another point as pivot points and got different answers for the torque from the pin each time. How do you know which is the correct one?
 

1. What is torque in a rotating rod?

Torque in a rotating rod is a measure of the force that causes the rod to rotate around an axis. It is typically measured in units of Newton-meters (N-m) or foot-pounds (ft-lb).

2. How is torque calculated?

To calculate torque in a rotating rod, you need to know the force applied to the rod and the distance from the axis of rotation to the point where the force is applied. The formula for torque is torque = force x distance.

3. What factors affect the torque in a rotating rod?

The torque in a rotating rod is affected by the magnitude of the force applied, the distance from the axis of rotation, and the angle at which the force is applied.

4. Is torque the same as rotational force?

No, torque and rotational force are not the same. Torque is a measure of the twisting or turning force, while rotational force is a measure of the force that causes an object to rotate around an axis.

5. How does torque impact the rotation of a rod?

The torque applied to a rotating rod determines the rate of rotation. The greater the torque, the faster the rod will rotate. If the torque is zero, the rod will not rotate at all.

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