Moments

Motion

What is a moment of a force?

The turning effect of a force about a pivot or axis of rotation.

What is another name for the turning effect of a force?

The moment of a force.

What is a pivot?

The point or axis about which an object can rotate.

What is the equation for the moment of a force?

Moment = force × perpendicular distance from the pivot to the line of action of the force.

What is the symbol equation for moment?

M = Fd.

What is the unit of moment?

Newton metre (N m).

What does F represent in M = Fd?

The force in newtons (N).

What does d represent in M = Fd?

The perpendicular distance from the pivot to the line of action of the force, in metres (m).

Which distance must be used when calculating a moment?

The perpendicular distance from the pivot to the line of action of the force.

What is the line of action of a force?

An imaginary line extending in the direction in which the force acts.

How can the moment of a force be increased?

Increase the force or increase the perpendicular distance from the pivot.

Why is it easier to open a door by pushing near the handle rather than near the hinges?

The force acts further from the pivot, producing a larger moment.

What is a clockwise moment?

A moment that tends to rotate an object clockwise.

What is an anticlockwise moment?

A moment that tends to rotate an object anticlockwise.

What is the principle of moments?

For an object in rotational equilibrium, the total clockwise moment about a pivot equals the total anticlockwise moment about the same pivot.

What is the equation for the principle of moments?

Sum of clockwise moments = sum of anticlockwise moments.

What must be true about the pivot when comparing clockwise and anticlockwise moments?

All moments must be calculated about the same pivot.

What is rotational equilibrium?

A situation where the resultant moment on an object is zero.

What is equilibrium?

A situation where an object has zero resultant force and zero resultant moment, so it has zero acceleration.

What conditions are required for complete equilibrium?

Resultant force = 0 and resultant moment = 0.

What happens to a stationary object in equilibrium?

It remains stationary.

What happens to a moving object in equilibrium?

It continues moving at constant velocity.

What happens to a rotating object in rotational equilibrium?

It continues rotating at the same speed in the same direction.

Can an object have zero resultant force but still rotate?

Yes, if the moments acting on it are not balanced.

Can an object have balanced moments but still move?

Yes, if there is a non-zero resultant force.

How do you calculate the total clockwise moment when several forces act?

Calculate each clockwise moment separately using M = Fd, then add them together.

Why should forces and distances not simply be added before calculating moments?

Each force may act at a different distance from the pivot, so each moment must be calculated separately.

What is the centre of gravity?

The point through which the entire weight of an object appears to act.

What causes the weight of an object?

The gravitational attraction between the Earth and the particles in the object.

Where does the weight of an object appear to act?

Through its centre of gravity.

Where is the centre of gravity of a symmetrical object?

At the intersection of all its lines of symmetry.

Where is the centre of gravity of a uniform circle?

At its centre.

Where is the centre of gravity of a uniform rectangle?

At the intersection of its lines of symmetry.

Is the centre of gravity of a triangle always halfway up the triangle?

No. For a uniform triangle it lies closer to the base.

What determines the centre of gravity of an irregular object?

The distribution of mass throughout the object.

Where is the centre of gravity likely to be if one part of an object contains more mass?

Closer to the part containing more mass.

What is the centre of mass?

The point at which the mass of an object can be considered to be concentrated.

When can centre of mass and centre of gravity be treated as the same point?

For objects that are small compared with the size of the Earth.

How can the centre of gravity of an irregular rod be estimated?

Balance the rod on a narrow pivot; the centre of gravity lies directly above the pivot when it balances.

Why does an object balance when supported directly below its centre of gravity?

The perpendicular distance between the weight's line of action and the pivot is zero, so the moment due to its weight is zero.

How can the principle of moments be used to find the centre of gravity of an irregular rod?

Balance the rod using a known counterweight and use clockwise moment = anticlockwise moment to calculate the position of its centre of gravity.

What happens if the line of action of an object's weight does not pass through its pivot?

The weight produces a moment and causes the object to rotate.

What happens if the line of action of the weight passes directly through the pivot?

The perpendicular distance is zero, so the weight produces no moment.

How do you convert a distance in centimetres before using M = Fd?

Divide by 100 to convert it to metres.

If a 10 N force acts 2 m from a pivot, what is its moment?

20 N m.

If a 5 N force acts 0.4 m from a pivot, what is its moment?

2 N m.

If an object is balanced and has a 20 N m clockwise moment, what must the total anticlockwise moment be?

20 N m.