Supposing you weigh 50 kg in your weighing machine, then what is your force when you are standing on the weighing machine?
We know, f= m*a, so your force is 50 * 10 m/s2 (approx. of gravity which is 9.8m/s2) = 500 N.
So, even though you weigh 50 kg on earth or moon, your force on earth is 500 N and your force on moon would be different as the gravity of moon is lesser than earth!
Another point to ponder is you are exerting 500 N on earth and the earth is exerting that equal amount of force (500 N) as a ground reaction force on you. Both the forces cancel out and you are standing blissfully unaware of whatever is going on. If the ground is not exerting an equal amount of force on you then you would pierce against the ground with your force like a metal coin in a glass of water.
So, what exactly is a force?
Remember Newton’s I law? Force is that which alters the state of rest or of uniform motion of an object.
Balanced forces result in equilibrium.
Here comes another interesting question. What is then a moment?
Whenever a force acts at a distance from a joint/pivot, then it creates a torque or moment that tends to rotate the object around that joint/pivot.
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Moment = force * distance |
The classical example would be a seesaw. You tend to rotate with respect to the pivot up or down. Going back to our previous example, when you are standing on the ground, you are exerting your force (in this case 500 N) to the ground. Where exactly are you exerting? Is the force goes throughout the whole feet or hypothetically at some point? Remember center of mass of an object? Brush up your center of gravity and line of gravity before you read the following paragraphs. Supposing the center of mass or center of pressure of your foot is mid-point between the foot and your nearest joint/pivot is your ankle joint, then you do have a moment generated here, isn’t it? The force is say, 3-4 cms away from your ankle joint, and then the moment that is created would be
M=500 N * .03m = 15 N.m.
i.e., a moment of 15 N.m is trying to rotate your ankle joint up (dorsiflexion). But again there you are, standing blissfully unaware of what is happening at your foot. So, what is preventing your rotation? The action of your plantarflexors! Yes, that’s right; they produce an opposing plantar flexion moment that opposes your ankle rotation up.
Next time when you are standing for a long time and wondering why you get calf pain, you know the answer at least…
Now, coming out of the foot segment, let us concentrate on the whole body. Where do you think you have your center of gravity and line of gravity? The center of gravity is approximately located anterior to S2. And following are the moments that result due to the position of the LOG either in front or behind the joint, which are opposed by the moments developed by the opposite groups of muscles/ligaments.
