Torque without pivot (Torque with only one force)

In summary, the conversation discusses the concept of angular momentum and its relation to torque. It is explained that although mathematically there is a net torque when dropping a stick, this does not necessarily result in rotation due to other factors. The concept of conservation of angular momentum is also mentioned, and the importance of considering the center of mass when discussing torque and rotation is highlighted.
  • #1
cheahchungyin
49
0
Imagine, I drop a stick with uniform mass horizontally towards the ground. There is only one force acting on it, which is the weight.

By common sense, I would know that the stick will not turn. However, mathematically there is a net torque if I set the pivot point on either side of the stick.

So what's the problem here?
 
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  • #2
cheahchungyin said:
Imagine, I drop a stick with uniform mass horizontally towards the ground. There is only one force acting on it, which is the weight.

By common sense, I would know that the stick will not turn. However, mathematically there is a net torque if I set the pivot point on either side of the stick.

So what's the problem here?
The problem is that you are using an accelerating point as your pivot. About such a point, the rate of change of angular momentum is not simply given by the net torque. There are other terms. (Which end up canceling that torque term, so you are left with no rotation. So your common sense was correct.)
 
  • #3
The problem is, that angular momentum is a different concept than most people realize. If we talk about the conservation of angular momentum we mean the angular momentum off all moving mass around an arbitrary point in space. A point mass (which cannot rotate for more or less obvious reasons) flying past the origin at some distance has an angular momentum. A force acting on this mass will produce a torque changing the angular momentum. If you consider a stick centred at the origin that is hit by this mass it can start spinning an thus gain angular momentum from the point mass which loses it. The reason why we often only speak of torque and angular momentum around the centre of mass of objects is Steiner's theorem (aka. parallel axis theorem) which states that angular movements can be split into the rotation around an origin of an object's centre of mass plus the objects rotation around it own centre of mass.
 

1. What is torque without pivot?

Torque without pivot, also known as torque with only one force, is a measure of the rotational force applied to an object. In other words, it is the amount of force that causes an object to rotate around an axis, without the presence of a fixed point or pivot.

2. How is torque without pivot calculated?

To calculate torque without pivot, you will need to know the magnitude of the force applied to the object, as well as the distance between the point of application of the force and the axis of rotation. The formula for calculating torque without pivot is: Torque = Force x Distance.

3. What are some real-life examples of torque without pivot?

A common example of torque without pivot is opening a door by pushing or pulling on the doorknob. The force applied creates a torque that causes the door to rotate around its hinges. Another example is using a wrench to loosen a bolt. The force applied to the wrench causes a torque that rotates the bolt around its axis.

4. How does torque without pivot differ from torque with a pivot?

In torque with a pivot, the force is applied at a distance from the axis of rotation, creating a perpendicular component that generates torque. In contrast, torque without pivot only has one force acting on the object, and the force itself creates the torque without the need for a perpendicular component.

5. What are the units of measurement for torque without pivot?

The units of measurement for torque without pivot are newton-meters (N∙m) or foot-pounds (ft∙lb). These units are derived from the force (newton or pound) and distance (meter or foot) components in the formula for calculating torque without pivot.

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