Moments Flashcards – AQA A-level Physics (7408)

What does the moment of a force about a point measure?

The turning effect of the force about that point.

What is the formula for the magnitude of the moment of a force about a point?

$M = Fd$, where $F$ is the force and $d$ is the perpendicular distance from the point to the force’s line of action.

What is the line of action of a force?

The straight line along which the force acts.

What is the unit of moment?

The newton metre, $\mathrm{N\,m}$.

What are the two possible directions of a moment about a point?

Clockwise and anticlockwise.

When does a force have zero moment about a point?

When its line of action passes through the point.

What is a couple in mechanics?

Two equal and opposite coplanar forces acting along different parallel lines of action.

What is the overall effect of a couple?

Its resultant force is zero, but it produces a turning effect.

What is the formula for the moment of a couple?

$M = Fd$, where $F$ is the magnitude of either force and $d$ is the perpendicular distance between their lines of action.

How does the moment of a couple depend on the point about which it is calculated?

It is independent of the chosen point.

What is the principle of moments for a body in rotational equilibrium?

About any point, the total clockwise moment equals the total anticlockwise moment. Equivalently, the resultant moment is zero when opposite directions have opposite signs.

Besides balanced moments, what condition is required for complete equilibrium?

The resultant force must also be zero.

What is the centre of mass of a body?

The point representing the mean position of the body's mass.

Where may a body's weight be treated as acting in a uniform gravitational field?

At its centre of mass.

Where is the centre of mass of a uniform regular solid?

At its geometric centre.

What does it mean for a solid to be uniform?

Its mass is distributed evenly throughout the solid.