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Force, energy and momentum: 13/1/19 (Centre of mass (The centre of mass of…
Force, energy and momentum: 13/1/19
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Moments
The moment about a point is the force acting on the object x the perpendicular distance from the line of action of the force to the pivot
Couple
A couple is a pair of equal and opposite forces that do not act in the same line - this means they cause rotation
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Moment of a couple: one of the forces x perpendicular distance between lines of action of the forces
Principle of moments
For an object in equilibrium, the total acw moments about a point must be equal to the total cw moments about a point
Centre of mass
The centre of mass of an object is defined as the point at which we can consider the whole mass of an object to be located
The centre of gravity of an object is defined as the point at which we can consider the whole weight of an object to act
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Axis method works because an object will always come to rest with its centre of mass directly below the line of action of its weight
Stability
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Stable equilibrium
If an object is displaced from equilibrium it can return to equilibrium - e.g. a ruler hanging from a pivot
Unstable Equilibrium
If an object is displaced from equilibrium it cannot return to equilibrium - e.g. a pen falling over
This is because its centre of mass is no longer above its base, and so there is a clockwise moment causing it to topple
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