Appreciation of the meaning of equilibrium in the context of an object at…

The idea

Equilibrium means the resultant force on an object is zero, so its acceleration is zero. This occurs when the object is at rest or moving with constant velocity. For example, a car travelling at a steady 20 m s⁻¹ along a straight road has driving force balanced by drag and friction, and weight balanced by the normal contact force. A book resting on a table has weight balanced by the normal contact force. In both cases, the vector sum of all forces is zero. Equilibrium does not require the object to be stationary; constant velocity is equally valid. To test equilibrium, resolve forces in two perpendicular directions and check that each component sum is zero, or draw a closed vector triangle. If the resultant is non-zero, the object accelerates in the direction of the resultant force.

What earns the marks

• State that equilibrium occurs when the resultant force on an object is zero.
• Explain that zero resultant force means zero acceleration, so the object remains at rest or continues moving with constant velocity.
• Give a concrete example, such as a car moving at constant velocity with driving force balanced by resistive forces and weight balanced by the normal contact force.
• Use vector addition or resolution to show that the sum of forces in two perpendicular directions is zero.
• Distinguish equilibrium from a stationary state by noting that constant velocity is also equilibrium.
• Apply Newton's first law to link zero resultant force to constant velocity or rest.

Common mistakes and how to put them right

• Thinking that equilibrium only applies to objects at rest.

  Put it right by stating that constant velocity also satisfies equilibrium because acceleration is zero.

• Assuming that if an object is moving, there must be a resultant force in the direction of motion.

  Put it right by explaining that a resultant force would cause acceleration, not constant velocity.

• Confusing balanced forces with equal magnitudes only.

  Put it right by emphasising that forces must be equal in magnitude and opposite in direction, and may need to be resolved into components.

• Forgetting to consider all forces, such as normal contact force or friction.

  Put it right by drawing a free-body diagram and labelling every force.

Exam tips

• When asked to explain equilibrium, always start from the resultant force being zero and link this to zero acceleration.
• Use a free-body diagram to identify all forces and show balanced pairs or resolved components.
• If a question gives a velocity-time graph, constant velocity is shown by a horizontal line, which indicates equilibrium.
• Avoid saying 'no forces act' when describing equilibrium; say 'no resultant force acts'.