# B A doubt in calculating torque

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1. Oct 11, 2016

### Ajaysabarish97

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

2. Oct 11, 2016

### Staff: Mentor

The axis has to be specified or calculated.

3. Oct 11, 2016

### drvrm

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.

4. Oct 12, 2016

### Ajaysabarish97

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.

5. Oct 12, 2016

### Ajaysabarish97

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

6. Oct 12, 2016

### Ajaysabarish97

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

7. Oct 12, 2016

### CWatters

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.

8. Oct 12, 2016

### CWatters

Yes.

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

9. Oct 12, 2016

### jbriggs444

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.

10. Oct 12, 2016

### Staff: Mentor

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.

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