Moments Flashcards

All 19 cards in this deck

What is the moment of a force?

The turning effect of a force about a pivot (turning point).

What is the equation for the moment of a force?

moment (N m) = force (N) × distance normal to the direction of the force (m)

Name two everyday situations in which a force causes rotation about a pivot.

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

True or false? A force whose line of action passes through the pivot will make the object rotate.

False. The normal (perpendicular) distance is zero, so the moment is zero and there is no turning.

True or false? If a force acts at an angle to a spanner, its moment is the force multiplied by the length of the spanner.

False. You must use the distance normal (perpendicular) to the direction of the force, which is less than the spanner's length.

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

e.g. a 20 N force applied at right angles to a spanner, 0.5 m from the pivot

1. Find the distance normal to the force: $0.5$ m
2. Multiply force × distance: $20 \times 0.5$
3. Moment $= 10$ N m

State the principle of moments.

For rotational forces in equilibrium, the sum of the clockwise moments = the sum of the anticlockwise moments about the same pivot.

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

e.g. 10 N acting 0.4 m to the left of the pivot, balanced by force $F$ acting 0.2 m to the right

1. Anticlockwise moment: $10 \times 0.4 = 4$ N m
2. Clockwise = anticlockwise: $F \times 0.2 = 4$
3. $F = 20$ N

What are the steps to find an unknown distance on a balanced beam?

e.g. 5 N acting 0.6 m to the left of the pivot, balanced by 10 N at distance $d$ to the right

1. Anticlockwise moment: $5 \times 0.6 = 3$ N m
2. Clockwise = anticlockwise: $10 \times d = 3$
3. $d = 3 \div 10 = 0.3$ m

True or false? Two objects of equal weight hanging on opposite sides of a pivot always balance the rod.

False. They balance only if they hang at equal distances from the pivot, as moment depends on distance too.

What are the steps to decide, using moments, whether a person can lift a rock with a bar?

e.g. person pushes down with 600 N at 0.5 m from the pivot; rock weighs 1800 N at 0.2 m from the pivot

1. Moment of person: $600 \times 0.5 = 300$ N m
2. Moment of rock's weight: $1800 \times 0.2 = 360$ N m
3. $300$ N m $<$ $360$ N m, so the rock does not lift

What is a lever?

A rigid bar that turns about a pivot, used to transmit the turning effect of a force.

Where must the effort be applied on a lever for it to act as a force multiplier?

Further from the pivot than the load, so a small effort at a large distance gives the same moment as a large load at a small distance.

True or false? Moving the pivot of a lever closer to the load makes the load easier to lift.

True. The load's moment decreases and the effort acts at a greater distance, increasing the effort's moment.

Which way does the second gear turn in a pair of interlocking gears?

In the opposite direction to the first gear; the teeth push each other round.

What happens to turning effect and speed when a small gear drives a larger gear with more teeth?

The larger gear turns more slowly but with a greater turning effect (larger moment).

What are the steps to find how many turns per second a driven gear makes?

e.g. a 48-tooth gear turning 2 times per second drives a 12-tooth gear

1. Teeth passing per second are the same for both gears: $2 \times 48 = 96$
2. Divide by the teeth on the driven gear: $96 \div 12$
3. $8$ turns each second

What are the steps to find an unknown force on a rod with several loads hanging from it in equilibrium?

e.g. rod pivoted at its centre, with 2 N at 0.3 m and 4 N at 0.1 m on the left, and force $F$ at 0.5 m on the right

1. Add the moments on one side: $(2 \times 0.3) + (4 \times 0.1) = 1.0$ N m anticlockwise
2. Set clockwise = anticlockwise: $F \times 0.5 = 1.0$
3. Rearrange: $F = 1.0 \div 0.5 = 2$ N

True or false? When several objects hang from a rod in equilibrium, the moments of all the objects on the same side of the pivot are added together.

True. Their moments all turn the rod the same way, so the sum of clockwise moments equals the sum of anticlockwise moments.