Principle of moments

Turning forces Students need to be introduced to the idea of turning forces, by giving examples of levers and leverage. The idea that the turning force depends on the moment of the force where MOMENT OF A FORCE = FORCE(N) X PERPENDICULAR DISTANCE FROM FULCRUM (m) ( or the point in question )

Moment Moment = Force (N) x Distance (cm or m). The moment of a force is given by the relationship: Moments are measured in Newton centimetre (Ncm) or Newton metre (Nm). moment Fxd

Students should be able to be able to calculate the moment in different situations, (initially for one force) from diagrams supplied, giving the correct unit and whether the moment is acting in a clockwise or anticlockwise direction. There are good examples of powerpoints such as ABSORB PHYSICS. Students should be able to explain how and why the turning effect changes as a cyclist pushes against a pedal. (The perpendicular distance alters).

Moment Moment = Force (N) x Distance (cm or m). The moment of a force is given by the relationship: Moments are measured in Newton centimetre (Ncm) or Newton metre (Nm). moment Fxd

Students should also be able to work out the resultant moment acting in more complicated examples where several turning forces are acting. Students can then state if the system is in equilibrium, or whether it will rotate in a clockwise or anticlockwise direction.

pivot 500 N 0.5 m Gina weighs 500 N and stands on one end of a seesaw. She is 0.5 m from the pivot. What moment does she exert? moment = 500 x 0.5 = 250 Nm Click for solution Moments calculation

Possible investigations Using a suspended meter ruler to find the mass/weight of an object by applying the principle of moments. Students could take five sets of readings and calculate an average, ignoring any anomalous results. They could check the mass and calculate their % error

Principle of moments The green girl exerts an anti- clockwise moment equal to... her weight x distance from pivot. The yellow girl exerts a clockwise moment equal to... her weight x distance from pivot. pivot

If the two moments are equal then the seesaw is balanced. This is known as the principle of moments. When balanced Total clockwise moment = total anti-clockwise moment “c.m.” = “a-c.m.” Principle of moments pivot

Finding the mass of a meter ruler by suspending it around the 25cm mark, with a piece of string and suspending a mass of similar size to the ruler near the zero mark. The string can be adjusted until the ruler is balanced. This experiment could be used introduce the concept of centre of mass.

Why don’t cranes fall over? Tower cranes are essential at any major construction site. load arm trolley loading platform tower Concrete counterweights are fitted to the crane’s short arm. Why are these needed for lifting heavy loads? counterweight

Once students understand the idea of centre of mass, they could use a plumbline to find the centre of mass of a simple lamina, such as a cardboard map of Britain. The lamina is freely suspended so that it can rotate from at least three different places. A line is drawn along the plumbline on the lamina from each point of suspension. The centre of mass will be the point where all the lines cross.

Why don’t cranes fall over? Using the principle of moments, when is the crane balanced? moment of = moment of load counterweight If a N counterweight is 3 metres from the tower, what weight can be lifted when the loading platform is 6 metres from the tower? 6 m6 m 3 m3 m N ?

Various toys, and items such a Bunsen, racing car, lifeboat and ball could be used to demonstrate the importance of a low centre of gravity and a wide base in certain situations. The terms stable, unstable and neutral equilibrium could be explained