What Axis is Used When Calculating Torque?

[Ajaysabarish97]

When we say torque about a point what axis do we mean,how can rotational motion happen without an axis?

 

[russ_watters]

When we say torque about a point what axis do we mean,how can rotational motion happen without an axis?

The axis has to be specified or calculated.

 

[drvrm]

When we say torque about a point what axis do we mean,how can rotational motion happen without an axis?

Torque involves two physical variables the Force and the radius vector drawn from the fixed point to the force ( point of application of the force)
Does the definition of torque , the vector product of the two defines the direction of an instantaneous axis of rotation transferred to the fixed point?

The effect of torque is motion (rotational /spin motion). Let us try to understand the effect ...i.e. the rate of change of rotational /angular momentum as equivalent to net external torque.
so do you think a pre-fixed axis is necessary to analyze the orbital/spin motion. or a fixed point (may be center of mass of the system) can serve the purpose.

 

[Ajaysabarish97]

Torque involves two physical variables the Force and the radius vector drawn from the fixed point to the force ( point of application of the force)
Does the definition of torque , the vector product of the two defines the direction of an instantaneous axis of rotation transferred to the fixed point?

The effect of torque is motion (rotational /spin motion). Let us try to understand the effect ...i.e. the rate of change of rotational /angular momentum as equivalent to net external torque.
so do you think a pre-fixed axis is necessary to analyze the orbital/spin motion. or a fixed point (may be center of mass of the system) can serve the purpose.

Thank you for replying sir,when we say torque about a point,do we assume the axis is perpendicular to the plane passing through that point?and we study torque and angular momentum to analyse the rotational motion of a body,what is the use of calculating torque and angular momentum about a point when body can't rotate about a point.

 

[Ajaysabarish97]

The axis has to be specified or calculated.

But when we say,torque about a point we don't assume any axis sir.

 

[Ajaysabarish97]

The axis has to be specified or calculated.

But when we say,torque about a point we don't assume any axis sir.

 

[CWatters]

But when we say,torque about a point we don't assume any axis sir.

Yes we do. That's why we say "about a point".

A child on a seesaw (teetertotter) creates a torque about the pivot point of the seesaw. They create no torque about their seat.

 

[CWatters]

Thank you for replying sir,when we say torque about a point,do we assume the axis is perpendicular to the plane passing through that point?

Yes.

and we study torque and angular momentum to analyse the rotational motion of a body,what is the use of calculating torque and angular momentum about a point when body can't rotate about a point.

Why do you think a body cannot rotate about a point?

 

[jbriggs444]

But when we say,torque about a point we don't assume any axis sir.

When discussing torque "about a point", we are working in three dimensions. The torque will not be a scalar quantity which is merely positive or negative (clockwise or counter clockwise). It will be a [pseudo-]vector quantity with direction as well.

The direction of the torque pseudo-vector is perpendicular to both the applied force and to the radius vector. It is determined by the vector cross-product of the two.

 

[russ_watters]

Taking this broader; references are arbitrarily chosen for your convenience. You don't even need there to be any rotation at all to choose an axis/point of rotation and you can use whatever location helps your analysis.

Consider a static analysis of a simple two-dimensional bridge. To analyze it, you sum the torques around one of the supports. And for a more complicated real bridge, you'll analyze the torques around various points along the span of the bridge to ensure the bridge's bending resistance is strong enough.