Torque Moments Levers

Torque, Moments, Axis and Levers - now this is the meat of podiatric biomechanics.

Torque and moment are used to describe rotational force, i.e. force applied to a lever about some axis of rotation.

Moment is the more usual term in biomechanics and moments of force are in the units of Newton.meters Nm. Why? because Newton is the basic unit of force and metre is the measure of the length of the lever.

So the unit of mass is kilogram and gravity (on earth) acting on 1kg is equal to 9.87Newtons, why? Because as we've seen before the acceleration of gravity is 9.87m/s^2 and if you remember Force = Mass x Acceleration (f=ma)

Example: a lever 500mm long with a force applied of 250N results in a moment of 125Nm
Simple enough of course.

There are classically 3 types of lever, 1) force-fulcrum-load, 2) Force-load-fulcrum 3) Load-force fulcrum and they can all be seen in the body (https://www.slideshare.net/spanglerscience/muscle-leversppt-presentation

 Although a lever is a rigid beam with load. fulcrum and force you should know that in mechanics we tend to consider the 'lever arm' or 'moment arm' and the moment arm is the distance from the applied force to the axis of interest. So what we are usually interested in is the 'moment arm' rather than the lever.

 Now also an axis is usually, but not necessarily, a physical point of rotation like say, the knee joint or the MPJs. An axis can be any point of consideration. So the moment caused by a force applied at the foot could be considered about the mid tibia or the hip joint and they might be very different in magnitude. The moment arm determines the moment .

So the moment arm from force to load is about half that from force to fulcrum. If there is no rotation of the lever then this is a static problem and the net moment about the fulcrum will be Zero. Net means 'the sum of' so while there will be a moment about the fulcrum applied by the load and a moment about the fulcrum applied by the force, because the forces they apply are in opposite direction and the moment of force by each is equal then the sum is zero.

How can the moments of force sum up to zero? if we consider the load about the fulcrum: The load is 5kg and so the force of the load is 5 x 9.87(gravity) = 49.35N, the moment arm from load to fulcrum is 400mm. Therefore the moment is 0.4 x 49.35 = 19.74Nm.

So what is the force that achieves a zero net moment about the fulcrum? The moment arm from the fulcrum to the applied force is 800mm and the moment to balance the load moment should be 19.74N
Therefore the is the moment arm is twice as long and moment = force x moment arm then force = moment / moment arm - let's try that  - f = 19.74/0.8 =24.675Nm or half the force applied by the the load.

Here's 3 questions to think about

What is the force magnitude and direction applied by the lever to the fulcrum?

What is the net moment at the load point?

Why didn't we consider inertial force in the example?

I'm gonna leave you with those questions for a while - good luck!