Walt wrote:
VtSkier wrote:
Walt wrote:

... torque is force applied at some distance,

Yes, but isn't that what WORK/ENERGY is?


Work is force acting though a distance: W = F(dot)d. With torque, the
force is perpendicular to the distance vector , so when you compute the
dot product you get zero.

Torque, like distance and force, is a vector quantity while energy
(work) is a scalar. To compute torque, you take the vector cross
product of force and position vector: T = F x d. This gives a vector,
unlike the dot product above which gives a scalar.

To say a vector (torque) is equal to a scalar (energy) is like saying
Wednesday is equal to cheese. It makes no sense.


//Walt

Whether or not torque and work are directly
equivalent, they are use interchangeably in
many ways.

By adding the element of time to work we get the
concept of "horsepower". When James Watt observed
that a horse, on average, could lift (I'm sure he
used "lift" in some way so that he didn't have
(much) friction to deal with) 550 pounds one foot
in one second, he decided that moving 550 pound,
one foot in one second would be a measure of power
called "One Horsepower".

This is all well and good. The "work" is linear
and the time it takes to do the work is one second.

When talking about engines which produce useful
work in a rotational way, we speak of torque and
horsepower. Horsepower being basically the speed
at which the torque is produced. We don't need to
get into the various uses of that power which
requires maximum horsepower to be produced at
low speed or high speed, that's the subject of
another discussion and is not germain to this one.

The "horsepower" unit was devised for the speed of
linear work. It is very useful in describing the
speed of rotational work.

Torque is not a vector, it is work applied in a
vector (rotational) direction. Some of your
earlier argument suggested that torque can be
measured if the the object that the torque is
being applied to does not move. I suggested that
while the *potential* torque can be measured there
is no work/torque performed until the object moves.

Yes, you can apply a lot of force and still not
have the nut move. You can also apply a lot of
force trying to move a rock and no work is
accomplished until the rock does move, in a
linear way. This is directly analogous to to
the nut not moving in your example. The reason
that torque and work/energy use the same units
is that they really are the same thing.

If the thing doesn't move, there is only force,
no work or torque. If the thing moves over a
distance, linear or rotational, there is work/
torque. If you measure the rate at which this
work or torque is taking place, the unit you
use to describe either linear or rotational
power is *horsepower*.

The argument I'm making here is that the only
difference between force/work and torque is
the direction the force is applied. Linear for
force/work, rotational/vector for torque.