Moments, Levers & Gears Flashcards

All 17 cards in this deck

What is the moment of a force?

The turning effect of a force about a pivot.

What is the equation linking moment of a force, force and distance, and what are the units of each quantity?

$M = F \times d$

Moment $M$ in newton-metres (N m), force $F$ in newtons (N), distance $d$ in metres (m).

True or false? In $M = F \times d$, $d$ is the distance from the pivot to the point where the force is applied, measured along the direction of the force.

False. $d$ is the perpendicular distance from the pivot to the line of action of the force.

What are the steps to calculate the moment of a force?

e.g. a force of $2.0$ N acts at a perpendicular distance of $7.5$ cm from the pivot

1. Convert the distance to metres: $7.5\ \text{cm} = 0.075\ \text{m}$
2. Substitute into $M = F \times d$: $M = 2.0 \times 0.075$
3. Give the answer in N m: $M = 0.15$ N m

Give two everyday examples in which a force causes an object to rotate.

Pushing a door handle to open a door; using a spanner to turn a nut.

(Also: turning a bicycle pedal, using a screwdriver.)

How can the moment about a pivot be increased without increasing the size of the force?

Apply the force further away from the pivot.

e.g. push on a spanner nearer its end.

State the principle of moments.

If an object is balanced, the total clockwise moment about a pivot equals the total anticlockwise moment about that pivot.

What are the steps to find an unknown force acting on a balanced object?

e.g. a $10$ N force acts $0.2$ m to the left of the pivot; force $F$ acts $0.5$ m to the right

1. Find the known moment: $10 \times 0.2 = 2$ N m (anticlockwise)
2. Set clockwise moment = anticlockwise moment: $F \times 0.5 = 2$
3. Rearrange for the unknown: $F = 2 \div 0.5 = 4$ N

True or false? If a seesaw is balanced, the forces on the two sides must be equal.

False. The moments must be equal, so a smaller force further from the pivot can balance a larger force close to it.

True or false? A larger force always produces a larger moment about a pivot.

False. The moment also depends on the perpendicular distance from the pivot.

What happens to an object if the total clockwise moment about its pivot is greater than the total anticlockwise moment?

It is not balanced, so it rotates clockwise about the pivot.

What do a simple lever and a simple gear system both do?

They transmit the rotational effects of forces.

How does a simple lever act as a force multiplier?

The effort is applied at a large perpendicular distance from the pivot, giving a large moment, which produces a larger force on the load acting at a smaller distance from the pivot.

In a pair of meshed gears, how does the direction of rotation of the second gear compare with the first?

It rotates in the opposite direction.

What happens to the moment and the speed of rotation when a small gear drives a larger gear?

The larger gear turns with a larger moment but rotates more slowly.

Its teeth are further from its axle, so the distance is greater.

True or false? A gear system can change the direction of rotation but cannot change the size of the moment.

False. Gears of different sizes also change the size of the moment transmitted.

True or false? If the perpendicular distance from the pivot is measured in centimetres, it must be converted to metres before calculating a moment in N m.

True. Moment in N m needs force in N and distance in m, e.g. $7.5$ cm $= 0.075$ m.