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

• dl447342
In summary, the conversation discusses the angular acceleration and torque of a system that is released from rest. The gravitational force is the primary force acting on the ruler, but there may also be other forces such as the normal force exerted by the pivot. The location of the pivot, specifically above the weights, is important for balancing. The existence of a torque is uncertain and should be determined through the use of equations and not assumptions.
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?

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

haruspex said:
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.

dl447342 said:
What are those other forces?

dl447342 said:
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?

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

dl447342 said:
Well of course there's the normal force exerted by the pivot
Yes. Where is that force exerted?

dl447342 said:
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?

PeterDonis said:
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.

Lnewqban
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

Last edited:
dl447342 said:
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.

dl447342 said:
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.

## What is torque about pivot on ruler?

Torque about pivot on ruler is a measure of the force that causes an object to rotate around a fixed point, known as the pivot, on a ruler. It is also known as moment of force.

## How is torque about pivot on ruler calculated?

Torque about pivot on ruler is calculated by multiplying the force applied to the object by the distance between the pivot and the point where the force is applied. The unit of torque is Newton-meters (Nm) or foot-pounds (ft-lb).

## What is the relationship between torque and force?

Torque and force are directly proportional to each other. This means that the greater the force applied to an object, the greater the torque and the greater the distance between the pivot and the point of force, the greater the torque.

## How does the position of the pivot affect the torque about pivot on ruler?

The position of the pivot affects the torque about pivot on ruler as it determines the lever arm, which is the distance between the pivot and the point where the force is applied. The longer the lever arm, the greater the torque.

## What are some real-life applications of torque about pivot on ruler?

Torque about pivot on ruler is used in many everyday objects, such as wrenches, door handles, and seesaws. It is also used in more complex machines, such as car engines and bicycles, to create rotational motion and perform work.

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