This topic accounts for approximately 7% of your exam marks.
stable
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Stable7%
Principle of moments and centre of gravity tested regularly as structured questions.
What a moment is
A moment is the turning effect of a force about a fixed point called the pivot
A force does not always make an object change its straight-line motion; if the object is free to rotate about a hinge or axle, the force instead makes it spin
Common everyday moments include:
the handle of a door rotating about its hinges
a spanner loosening a bolt
a child on one end of a see-saw lifting another child at the far end
a screwdriver prising open a tin lid
a tap being twisted on or off
a wheelbarrow being tipped up to be wheeled
the blades of a pair of scissors closing about the rivet
The rotation may be clockwise or anticlockwise, depending on which side of the pivot the force is applied and which way it points
The equation
M = F × d
where:
M = moment of the force (newton metres, N m)
F = force applied (N)
d = perpendicular distance from the pivot to the line of action of the force (m)
The unit follows from the equation: newton × metre → newton metre (N m). If the lever arm is given in centimetres, the moment can equally be quoted in N cm; convert to metres unless the question asks otherwise
Why the distance must be perpendicular
The lever arm d is the shortest distance from the pivot to the line along which the force acts, measured at right angles to that line
A force pointing straight at the pivot has zero perpendicular distance, so it produces no moment no matter how large it is; it simply tries to push the pivot itself
For a beam lying flat, only the vertical forces (those pointing straight up or straight down) turn it about a pivot on the beam; a force directed along the length of the beam contributes no moment at all
Long lever arms make life easier
For a fixed pivot, a longer lever arm reduces the force you must apply to get the same moment
Real-life consequences:
A door is far easier to push open at the handle (far from the hinge) than near the hinge itself; the same moment comes from a much smaller hand force when d is large
A wheel nut wrench with a long extension bar makes seized nuts shift that a stubby spanner cannot
A short crowbar prises open a crate with a much smaller hand force than fingers alone