Describe the motion of an object that is in equilibrium.

It either stays at rest or continues to move in a straight line at constant velocity, because there is no resultant force to accelerate it.

Two coplanar forces act on a point object that is in equilibrium. What must be true of them?

They must be equal in magnitude, opposite in direction and act along the same line of action.

Three coplanar forces act at a point and hold it in equilibrium. What shape do the three vectors form when set out tip to tail?

They form a closed triangle, known as the triangle of forces.

Outline the method of resolving used to solve an equilibrium problem involving several coplanar forces.

Every force is resolved into components along two chosen perpendicular directions, the components in each direction are set equal to zero, and the resulting simultaneous equations are solved for the unknowns.

CCEA A-Level Physics 1210

Physical Quantities, Scalars and Vectors

Topic overview Physical quantities and SI base units Prefixes and standard form Scalars and vectors Resolving vectors and finding resultants Coplanar forces in equilibrium

Moments and Equilibrium

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Linear Motion and Dynamics

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Physical Quantities, Scalars and Vectors

Coplanar forces in equilibrium

3 things to know
Unit AS 1

Overview

Recall questions on the condition for equilibrium and what it says about the motion, the closed triangle of forces for three coplanar forces, why their lines of action must meet at a point, the resolving method, and worked problems on a hanging picture, a block on a slope and a string pulled aside.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Numbers

A block of weight 150 N rests on a frictionless slope at 25° to the horizontal. Calculate the normal contact force from the slope.

Key term

State the condition for a point object acted on by several coplanar forces to be in equilibrium.

Key term

Explain why a closed triangle of forces shows that a point object is in equilibrium.

Questions from this subtopic

Each one with the model answer underneath.

1
State the condition for a point object acted on by several coplanar forces to be in equilibrium.
Model answer
The vector sum of the forces must be zero, which means the components resolved along any two perpendicular directions must each add to zero.
2
Describe the motion of an object that is in equilibrium.
Model answer
It either stays at rest or continues to move in a straight line at constant velocity, because there is no resultant force to accelerate it.
3
Two coplanar forces act on a point object that is in equilibrium. What must be true of them?
Model answer
They must be equal in magnitude, opposite in direction and act along the same line of action.
4
Three coplanar forces act at a point and hold it in equilibrium. What shape do the three vectors form when set out tip to tail?
Model answer
They form a closed triangle, known as the triangle of forces.
5
Explain why a closed triangle of forces shows that a point object is in equilibrium.
Model answer
The resultant is the vector from the tail of the first arrow to the tip of the last, so if the triangle closes that vector has zero length and the resultant force is zero.
6
Outline the method of resolving used to solve an equilibrium problem involving several coplanar forces.
Model answer
Every force is resolved into components along two chosen perpendicular directions, the components in each direction are set equal to zero, and the resulting simultaneous equations are solved for the unknowns.

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