Skip to main content

Been a while

Hi to anyone who's been following

It been a while since my last post, sorry about that I've had a few projects to get on with lately, so  I thought I'd give younthis little snippet.

What's the difference between Centre of Mass and Centre of Gravity?

We often see CoM and CoG usqed interchangably as if they were the same thing but are they?

Simply put No!

Remember that mass is not dependent on gravity, a body has mass even when not in a gravitational field and the positionnof CoM is constant whatever the variation in the gravitation field.

Centre of Mass is the theoretic centre of the distribution of all the particles of mass in a body. That is, the point where the sum of all the distrubuted particles would equal zero. It is the point where, if a force is applied the body would move in a straight line without any rotation of the body. The CoM is not necessarily found within the mass of the body of interest, in a hoop or a football the COM is in the centre of the space that their shape surrounds.

CoG is dependent on the gravitational field, if there is no gravity then there is no Centre of Gravity. CoG is the point in a body where gravity can be thought to act. So, in a gravitational field, gravity acts on all the particles of mass and so if gravity on all the particles is equal then we find that the CoG acts at the CoM.

Of course as we are on earth and earth is absolutely huge compared to us or any object that we might consider, then gravity is constant and acts equally on all the particles of mass in a body of interest.

Therefore, practically, CoG and CoM can be thought of as the same and used interchangeably.
However, if we could have a building many miles high, then the CoG would be slightly nearer the ground than the CoM. WHY? because the acceleration of gravity is greater when nearer to the centre of the earth. And so since f=ma and also f=mg then the force acting on particles of the tall building nearer to the ground with be greater then the force acting on the particle furthest away from the ground.

But in less you are working with a person 10 miles tall then you don't need to worry about the difference.

It is very useful to know the difference though and also very convenient that we can find the CoM of a body of interest because it balances at the CoG, which is at the same location. Hoooray!

Lessons

Comments

  1. Unknown2 April 2022 at 17:35

    Merkur Gold Strike Safety Razor - FEBCASINO
    Merkur's Gold Strike Safety wooricasinos.info Razor, Merkur https://febcasino.com/review/merit-casino/ Platinum Edge Plated Finish, German, Gold-Plated, Satin 토토 사이트 Chrome Finish. Merkur casino-roll.com has a mens titanium wedding bands more aggressive looking,

    ReplyDelete

Post a Comment

Popular posts from this blog

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

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

Moment of inertia and Centre of Mass

Sorry to scare you but as we've just taken a look at inertia I thought I'd do this little snippet on 'moment of inertia' its important and useful in biomechanics, why? Well I'll tell you: As you'll remember, inertia is a body's resistance to change in velocity and inertial force is the ficticious force that manifests in the presence of acceleration, proportional to the acceleration ( times the mass) but in the opposite direction. Considering the inertia in this 1st case it is often quite convenient to just ignore inertial force. So say you want to push something in a straight line, like a sledge across the ice with your fat mate sitting on it. The sledge and your mate weigh 200kg and you apply a force of 500N to the centre of mass then the acceleration will be what? Get out the handy equation f=ma - rearrange to find the acceleration a=f/m plug in the numbers we know 500/200 and viola a=2.5m/s/s. After a couple of seconds you stop pushing and he's goi
1 comment
Read more