Equilibrium of Forces
Physics · 50 practice questions
Equilibrium of coplanar forces, triangle and polygon of forces, Lami's theorem; principles of moments (moment of a force, moment of a couple/torque); conditions for equilibrium of rigid bodies under parallel and non-parallel forces (resolution and composition of forces, resultant and equilibrant); centre of gravity and stability (stable, unstable and neutral equilibrium).
- Apply the conditions for equilibrium of coplanar forces, and use triangle/polygon laws and Lami's theorem to solve problems.
- Analyse the principle of moments and determine moment of a force and a couple.
- Apply conditions for equilibrium of rigid bodies, resolve forces into two perpendicular directions and determine resultant and equilibrant.
- Differentiate between stable, unstable and neutral equilibrium.
Try a few questions
- Practice question 1
The triangle law of forces states that if three forces acting at a point are in equilibrium, they can be represented by
- A.only two sides of a triangle
- B.the three sides of a triangle, taken in order
- C.a single straight line
- D.the diagonal of a rectangle
Show the answer
B. the three sides of a triangle, taken in order
This graphical law lets three balanced forces be represented as the sides of a closed triangle.
- Practice question 2
In a system of coplanar forces in equilibrium, resolving all the forces into two perpendicular directions allows
- A.two independent equilibrium equations to be formed
- B.only a single equilibrium equation to be formed
- C.no meaningful equilibrium equations to be formed at all
- D.the system to be analysed only if all forces are already parallel
Show the answer
A. two independent equilibrium equations to be formed
Resolving into perpendicular directions (commonly horizontal and vertical) yields two separate equations describing equilibrium.
- Practice question 3
For a rigid body in equilibrium under non-parallel coplanar forces, the conditions for equilibrium require that
- A.only the net moment about one specific point needs to be zero
- B.only the vector sum of forces needs to be zero, with no reference to moments
- C.the vector sum of all forces is zero, and the net moment about any point is also zero
- D.the forces must all act in exactly the same direction
Show the answer
C. the vector sum of all forces is zero, and the net moment about any point is also zero
General coplanar equilibrium needs both zero net force and zero net moment, regardless of the forces' directions.
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