Skip to main content

Difficulty of being useful and correct

Hmmm! I'm realising as I get into this blog writing, how difficult it is to be both useful in concept and be entirely correct without using advanced physics and maths that would be confusing.

So, the topic on CoM and moment of inertia was meant to convey the concept of how the effort require to rotate an object, in this case a leg, becomes greater the further away the CoM is from the axis of rotation, e.g. the hip joint, even tho the total mass remains the same. In fact the maths of that is I (moment of inertia) = mass (CoM) x radius squared. Which means that the effort required to rotate (change its angular velocity) an object increases by the square of the radius of rotation of the centre of mass about the axis of interest.

This is demonstrable practically: take a sledge hammer and hold it at the head, its fairly easy to hold up. But, hold the sledge hammer at the far end of the handle and extend it so the handle is parallel to the ground and then it becomes very difficult indeed to hold it up.

Why!

Because the hammer head represents most of the mass so in the 2nd condition the CoM is a long way from the rotational axis ie your wrist. The radius of rotation is much larger on the same CoM position. The muscular effort required is much greater to stop the rotation because the CoM has a greater lever and the mass is being accelerated (angular change in velocity) by gravity and you are trying to resist this acceleration in the opposite direction.

Look here for a good intuitive explanation

Comments

Post a Comment

Popular posts from this blog

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 ...
5 comments
Read more

Three Cs Part Two - Centre of Pressure and Centre of Force

Two very important concepts to understand in biomechanics are Centre of Pressure and Centre of Force. These are often used interchangeably but are actually different, mostly though, by convention, it is CoP that is used when considering how normal forces act over an area. Normal force means that force perpendicular to the surface of interest. When 2 objects act on each other with force then there will always be an interface that has some area and the forces acting over that area are called pressure. Pressure if Force divided by area and the unit of pressure is the Pascal, however 1 pascal is very small at 1N/m^2 or 100grams spread over 1million sq millimeters = 0.00001 grams / mm^2. So it is common to use kilopascals (1000 pascals) or KPa as the standard unit of pressure. Go here for a PPT presentation on CoF Vs CoP
7 comments
Read more
Presentation on analysis and interpretation of characterisation of pressure mat data This is an embedded Microsoft Office presentation, powered by Office Online . or go here   pressure mat systems
Post a Comment
Read more