What is a free-body diagram?

A free-body diagram shows all the forces acting on a single isolated object, with each force represented as a labelled arrow indicating magnitude and direction.

Define the moment of a force about a point.

The moment of a force about a point is the product of the force and the perpendicular distance from the line of action of the force to the point.

State the equation for calculating the moment of a force.

Moment = Fx, where F is the force and x is the perpendicular distance from the line of action to the pivot point.

A couple consists of two equal and opposite parallel forces whose lines of action do not coincide, producing a pure rotation with no net translational force.

Define the torque of a couple.

The torque of a couple is the product of one of the forces and the perpendicular distance between the lines of action of the two forces.

OCR A-Level Physics A H556

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Forces in Action

Moments, couples and equilibrium

3 things to know
Papers 1 and 3

Overview

Recall questions on free-body diagrams, moments and the principle of moments, couples and torque, centre of mass and centre of gravity, finding the centre of gravity of a lamina, and the conditions for equilibrium.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Numbers

Write the equation for the torque of a couple.

Key term

When finding the centre of gravity of an irregular lamina, what is done first?

Numbers

Calculate the moment when force = 354 N, perpendicular distance = 1.15 m.

Questions from this subtopic

Each one with the model answer underneath.

1
What is a free-body diagram?
Model answer
A free-body diagram shows all the forces acting on a single isolated object, with each force represented as a labelled arrow indicating magnitude and direction.
2
Define the moment of a force about a point.
Model answer
The moment of a force about a point is the product of the force and the perpendicular distance from the line of action of the force to the point.
3
State the equation for calculating the moment of a force.
Model answer
Moment = Fx, where F is the force and x is the perpendicular distance from the line of action to the pivot point.
4
Calculate the moment when force = 354 N, perpendicular distance = 1.15 m.
Model answer
Moment = 354 × 1.15 = 407 N m.
5
What is a couple?
Model answer
A couple consists of two equal and opposite parallel forces whose lines of action do not coincide, producing a pure rotation with no net translational force.
6
Define the torque of a couple.
Model answer
The torque of a couple is the product of one of the forces and the perpendicular distance between the lines of action of the two forces.

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Moments, couples and equilibrium

Overview

What you need to know

Write the equation for the torque of a couple.

When finding the centre of gravity of an irregular lamina, what is done first?

Calculate the moment when force = 354 N, perpendicular distance = 1.15 m.

Questions from this subtopic

What is a free-body diagram?

Define the moment of a force about a point.

State the equation for calculating the moment of a force.

Calculate the moment when force = 354 N, perpendicular distance = 1.15 m.

What is a couple?

Define the torque of a couple.

Other ways to work this

What is inside Levo

The 3-step method, run for you

Cram a whole paper in one sitting

Blank Blitz makes facts automatic

Build It gets the exam shape right

Test It catches misconceptions

Brain Breaks between sessions

Every question in Forces in Action, on a schedule.