Retrieval Practice
Moments, Couples & Equilibrium — AQA A-Level Physics
Q1. Define the moment of a force.
- Moment = Force times the perpendicular distance from the pivot to the line of action of the force.
- Unit: N m.
Q2. State the principle of moments.
For a system in equilibrium, the sum of clockwise moments about a point equals the sum of anticlockwise moments about the same point.
Q3. What three conditions must a couple satisfy?
Equal in magnitude, opposite in direction, and separated by a perpendicular distance (lines of action do not coincide).
Q4. Why does a couple produce rotation but no linear acceleration?
- The two forces are equal and opposite, so the resultant force is zero (no net force means no linear acceleration by Newton's second law).
- But they are separated, so they create a net moment.
Q5. Where is the centre of mass of a uniform rectangular object?
At the geometric centre, where the diagonals cross.
Q6. What makes an object more stable?
A wider base and a lower centre of mass.
Q7. When do centre of mass and centre of gravity differ?
- In a non-uniform gravitational field.
- Near the Earth's surface, they are the same because g is effectively constant.
Q8. If a force is not perpendicular to the distance from the pivot, how do you calculate the moment?
Use the component of the force that is perpendicular: M = F d cos(theta), where theta is the angle between the force and the perpendicular direction.