Torque

Corvus · Post by **Corvus** » Wed Jan 08, 2014 9:29 pm

Can there be torque without movement?

If you'd have asked me not that long ago I'd have said yes, without a second's hesitation, as in this example:

http://www.largiader.com/articles/torque.html

Extract:

"Torque is a twisting force applied to an object, like a wheel or a crankshaft. Note that motion is not required for torque to exist! If you stand on a lug wrench that is on a frozen lug bolt, you are applying a torque to that bolt even though there may be no movement. For our purposes, we will consider that torque is measured in pounds-force feet (lbf-ft) meaning the equivalent of a given force, in pounds, acting on the end of a lever of length in feet. For example, standing with 180 pounds body weight on a lug wrench one foot long yields 180 lbf-ft of torque. A child of 90 pounds standing on a two-foot lug wrench applies the same torque."

I've used the exact same example myself; a seized nut or bolt.

But is that really the case? What differentiates swinging on a seized bolt from a simple fulcrum? Without any movement there is no centre of rotation, as such. Just a three point fulcrum.

Makes you think.

Manfred · Post by **Manfred** » Wed Jan 08, 2014 10:19 pm

Even if there is no movement there is still a centre of potential rotation.

There is still movement on the molecular level, you just don't have the resolution to see it.

If you heat the bolt so it releases, the torque hasn't changed.

The fulcrum is the bolt, the torque is the force applied to it via the lever.
If the torque is too low to make it turn, that doesn't stop the torque existing.

Merecat · Post by **Merecat** » Wed Jan 08, 2014 11:16 pm

I wasn't going to.............but I can't help myself.

I don't see how you can have torque using a fulcrum. A fulcrum is a pivot point.
In its simplest form a seesaw.
Put small child on one end seesaw goes down.
Put large child on the other end, large child goes down, small child goes up.
Ends of seesaw describe arcs in the air but the forces, applied and resultant are in straight lines, so they are plain and simple forces and not torque.

In the seized bolt scenario you mention you put spanner on bolt and stand on spanner, your mass exerts a vertical force on the spanner, which in turn tries to twist the nut off the bolt, that is the torque
The length of the spanner to the centre of the bolt (ft) and your mass (lbs)
If the bolt is well seized, or been cross threaded by some ham fisted oaf it will remain stuck fast despite the applied torque
We may need a longer spanner or save ourself several pages by using a gas axe!!

The long and short of it is by using a fulcrum the resultant motion will be linear.
Put a spanner on a bolt and the resultant motion will be a rotation

tanneman · Post by **tanneman** » Thu Jan 09, 2014 3:30 am

Torque doesn't need angular motion to exit. It needs angular motion to do work and the rate of that work is measured in watt. In the example above the force applied is absorbed by the friction and bending/stretching/shearing forces of the material. Don't forget the gravity acceleration has a very minute impact in this example but becomes a factor the bigger the object and how the force is applied. In this state the forces are in equilibrium until such a moment when the applied force exceeds the resistance to movement. The resultant effect is then that work has been done which is calculated by using the distance of the movement (radians or the arc through which a point has moved) and the time taken to do the work. In the automotive world this is expressed as kW but international standard is watt (W).

Corvus · Post by **Corvus** » Thu Jan 09, 2014 6:57 am

Great replies. Thanks.

How about if I weld the bolt head to whatever surface it is contacting, then put ring spanner on there, then dangle a huge dead weight on the end?

Is that torque? Or just leverage?

Corvus · Post by **Corvus** » Thu Jan 09, 2014 6:59 am

Merecat wrote:I wasn't going to.............but I can't help myself.

I don't see how you can have torque using a fulcrum. A fulcrum is a pivot point.
In its simplest form a seesaw.
Put small child on one end seesaw goes down.
Put large child on the other end, large child goes down, small child goes up.
Ends of seesaw describe arcs in the air but the forces, applied and resultant are in straight lines, so they are plain and simple forces and not torque.

In the seized bolt scenario you mention you put spanner on bolt and stand on spanner, your mass exerts a vertical force on the spanner, which in turn tries to twist the nut off the bolt, that is the torque
The length of the spanner to the centre of the bolt (ft) and your mass (lbs)
If the bolt is well seized, or been cross threaded by some ham fisted oaf it will remain stuck fast despite the applied torque
We may need a longer spanner or save ourself several pages by using a gas axe!!

The long and short of it is by using a fulcrum the resultant motion will be linear.
Put a spanner on a bolt and the resultant motion will be a rotation

I wouldn't say a seesaw's resultant motion was linear? Not unless you push too hard and the child flies off at a tangent.

Corvus · Post by **Corvus** » Thu Jan 09, 2014 7:08 am

Manfred wrote:Even if there is no movement there is still a centre of potential rotation.

There is still movement on the molecular level, you just don't have the resolution to see it.

If you heat the bolt so it releases, the torque hasn't changed.

The fulcrum is the bolt, the torque is the force applied to it via the lever.
If the torque is too low to make it turn, that doesn't stop the torque existing.

Is it the presence of an axis? An axle? This is what I'm driving at with the welded bolt question.

Merecat · Post by **Merecat** » Thu Jan 09, 2014 10:21 am

Corvus wrote: I wouldn't say a seesaw's resultant motion was linear? Not unless you push too hard and the child flies off at a tangent.

Exactly the opposite. If the child flies off at a tangent to the resultant force then the momentum becomes angular

Corvus · Post by **Corvus** » Thu Jan 09, 2014 5:33 pm

The welded bolt head is interesting. With an open ended spanner on it, it seems exactly like a fulcrum. The head doesn't even have to welded, as long as the leverage can't begin to break the friction under the head. Until that point the bolt axis seems irrelevant? The spanner contacts at two points.

So is a fulcrum a torque?

tanneman · Post by **tanneman** » Thu Jan 09, 2014 6:15 pm

You should look at the definition of torque. It states that torque is the tendency of a force applied to rotate an object about its axis. In other words the force need not be applied to a lever (fulcrum), it can be applied directly to the object. Using what ever technique to multiply or not multiply the force applied you will need to calculate the perpendicular force to the direction of movement at any one time for a point at that time. What I'm saying is the distance the force is applied away from the centre of rotation and the angle it forms with a line through the centre of rotation and the point applied needs to be recalculated every time a variable changes. You can therefore see that the distance the force is applied away from the centre of rotation cannot be a lever as the point of force applied may still be within or any point on the object. It is therefore called a lever-arm.

On a fulcrum you are applying leverage, it is the most simplest form of using physics. In the above example the spanner (fulcrum) is used to move the point of where the force is applied away from the surface of the bolt thereby increasing the effectiveness of the force applied. If the bolt doesn't move then the calculation would be a simple one but you are still trying to rotate the object therefore we use the torque formula. To calculate the equal but opposite force then we can assume that torque applied is equally resisted by the friction or use clockwise moments = counter clockwise moments. When movement does happen then the formula for torque holds true and we calculate the angular acceleration using this formula.

In the above example if you are physically trying to rotate the object then use the formula for torque. If you are trying to move the object from one place to another in a linear line then use acceleration formula.

Do not forget that the distance from the centre of rotation is not the same as the length of the spanner. Therefore I can use moments to calculate the opposite force when all the forces are in equilibrium.

Corvus · Post by **Corvus** » Fri Jan 10, 2014 7:13 am

tanneman wrote:You should look at the definition of torque........

.................

I checked in machinery's handbook, 21st edition. Yes, there has to be an axis of rotation.

So I pondered on the difference and similarity between the friction under a bolt head and actually welding the bolt head in place. The relevance of this upon the definition of applying torque. I have come up with this.

The friction under the bolt head wouldn't be there in the first place without the rest of the bolt, namely the thread. The thread, as well as pulling also centres the bolt around an axis, in a concentric, 360 degree sense. Like a helically grooved journal. The welded bolt head, looking the same to me at first, I can now see that you may as well remove the rest of the thread. The head becomes part of the structure. By loading a spanner you are using a fulcrum principle and applying forces to the structure without there being a torque involved. Would appreciate comments on this interpretation.

Going back to the bolt head friction scenario, to doubly check my suggestion. If we apply the same tension induced friction by another method, say a series of strong springs and the bolt head wasn't located in dowel fashion, then by loading a spanner we are not applying a torque, but a fulcrum. Once the bolt head started to shift it wouldn't rotate around a centre point, but would move in sideways fashion.

A lot of air over a poxy bolt? Maybe, but what harm in thinking in naive fashion about something?

(I wish I'd studied harder as a kid!)

Corvus · Post by **Corvus** » Fri Jan 10, 2014 7:17 am

Merecat wrote:
Corvus wrote: I wouldn't say a seesaw's resultant motion was linear? Not unless you push too hard and the child flies off at a tangent.
Exactly the opposite. If the child flies off at a tangent to the resultant force then the momentum becomes angular

A seesaw's motion is an arc all the same. The c of g of a dead weight on one end will follow an arc. A dead weight dangling akin to a plumb bob will follow a straight line, but will only be tangential at one position. To my way of thinking anyway.

bobtail · Post by **bobtail** » Fri Jan 10, 2014 8:40 am

All this talk of torque is making my brain hurt

Merecat · Post by **Merecat** » Fri Jan 10, 2014 1:36 pm

To quote Captain Montgomery Christopher Jorgensen Scott, Jr.

"Ye canne change the laws "o" physics"

Herb · Post by **Herb** » Fri Jan 10, 2014 5:01 pm

I am going to go back to the original post.

'Can you have torque without movement'

Yes, of course.

An electric motor produces maximum torque at zero rpm. Imagine a small motor, spinning a small disc which you grip tighter and tighter until the motor stops rotating.

It is still producing force at the contact point with your fingers, hence it is producing torque, because the force is exerted at a distance from the centre of rotation.