Forces that make objects turn

1. A force does not always move an object in a straight line, because an object held at one fixed point will turn about that point instead.
2. That fixed point is the pivot, and identifying it is the first step in every turning question.
3. Pushing the handle of a door swings the door about its hinges, which act as the pivot.
4. Pulling on a spanner turns the nut about the centre of the nut.
5. Pushing down on one end of a seesaw turns the plank about the central support.
6. Pushing a bicycle pedal turns the crank about the centre of the chain wheel.
7. Turning a tap, a steering wheel, a wheelbarrow handle or the lid of a jar are the same idea applied to different pivots.
8. Whether a force actually turns the object depends on its line of action, the straight line drawn through the force in the direction in which it acts.
9. If the line of action passes through the pivot, the force produces no turning effect at all, because the distance from the pivot to that line is zero.
10. Pushing a door exactly along the line through its hinges is the everyday version of this, because the door will not move however hard it is pushed.

Calculating the moment of a force

1. The size of the turning effect is called the moment of a force.
2. The equation is $moment\; of\; a\; force=force\times distance\; normal\; to\; the\; direction\; of\; the\; force$, written in symbols as $M=Fd$.
3. The symbols mean the following.
   1. $M$ is the moment, measured in newton metres, $N\; m$.
   2. $F$ is the force, measured in newtons, $N$.
   3. $d$ is the distance from the pivot measured normal to the direction of the force, in metres, $m$.
4. Normal means at $90^{\circ }$, so $d$ is the perpendicular distance from the pivot to the line of action of the force, and not simply the length of the handle.
5. When the force is applied at right angles to a spanner, that perpendicular distance is the whole length from the nut to the hand, which is why pushing at $90^{\circ }$ is the most effective way to use one.
6. Tilting the force away from $90^{\circ }$ shortens the perpendicular distance, so the moment falls even though the size of the force has not changed.
7. The moment reaches zero when the force acts straight along the handle, because the line of action then runs through the pivot.
8. There are two ways to produce a larger moment.
   1. Apply a larger force.
   2. Apply the same force at a greater perpendicular distance from the pivot.
9. This is why a long spanner loosens a stiff nut that a short one cannot, and why a door handle is fitted at the edge furthest from the hinges.
10. The equation rearranges to $F=\frac{M}{d}$ and to $d=\frac{M}{F}$, which are the forms needed when the moment is the quantity given.
11. Distances must be converted into metres before substituting, so $30cm$ becomes $0.30m$.
12. A moment is written in $N\; m$ and never in joules, because a moment is a turning effect and not an amount of energy transferred.