What Causes Angular Acceleration and Torque in a Free Body Diagram?

[dl447342]

Homework Statement

Find the angular and linear accelerations of the ruler in the diagram below.

Relevant Equations

Torque = moment of inertia * angular acceleration = lever arm of force * force magnitude.

I think the angular acceleration is counterclockwise (and thus so is the torque) in the diagram, but what would a free body diagram look like? After the system is released from rest, isn't the only force the gravitational force about the center of mass? And if so, what's causing the angular acceleration and torque? The ends of the ruler?

 

[haruspex]

When released, what forces act on the ruler and where?
What are their torques about mass centre?

 

[dl447342]

When released, what forces act on the ruler and where?
What are their torques about mass centre?

Thanks for offering that hint. I said gravitational force is one, and I think it acts about the center of mass. But what about the others? That's the whole point of asking my question.

 

[Doc Al]

But what about the others?

What are those other forces?

 

[PeterDonis]

After the system is released from rest, isn't the only force the gravitational force about the center of mass?

If gravity were truly the only force acting on the ruler, it would be in free fall towards the center of the Earth, no?

 

[dl447342]

Well of course there's the normal force exerted by the pivot, but what force creates the torque?

 

[PeterDonis]

Well of course there's the normal force exerted by the pivot

Yes. Where is that force exerted?

but what force creates the torque?

You're assuming that there is a torque. What are you basing that on? You said in the OP that you "think" there is a torque; what makes you think that?

 

[dl447342]

Yes. Where is that force exerted?You're assuming that there is a torque. What are you basing that on? You said in the OP that you "think" there is a torque; what makes you think that?

I'm assuming there is a torque because that seems to be implied by the question. If there is no torque, then since the angular velocity was initially zero, it should remain that way; that is, the ruler should remain at rest and not rotate.

 

[hutchphd]

You need to use physics and not psychology to answer physics questions.

If there are forces where do they come from?. Enumerate them, draw them, and solve for their value using Newton's laws (and the similar rotational versions).

Your physics sense is good I think. You need to use the equations to show the result unequivocally. You learn these equations so you can solve problems too complicated to solve by intuition. Psychology will not help for real world physics problems

 

[Lnewqban]

“The pivot passes through the center of the ruler” is key.
There is a reason for the pivot of balances to be located above the weights.

https://st.depositphotos.com/102730...hotos_29992613-stock-illustration-balance.jpg

 

[PeterDonis]

I'm assuming there is a torque because that seems to be implied by the question.

As @hutchphd has pointed out, this is a very bad way to try to answer physics questions.

If there is no torque, then since the angular velocity was initially zero, it should remain that way; that is, the ruler should remain at rest and not rotate.

Yes, that's correct. So is there a torque, or isn't there? Don't try to guess what you think whoever wrote the question is trying to imply. Just think about the physics. All of the necessary physics has already been stated in this thread.