Explain examples of the forces acting on an isolated solid object or a…

The idea

This statement asks you to explain forces in two settings. First, an isolated solid object: name each force acting on it, such as weight acting downwards and the normal contact force from a surface acting upwards, and say what each force is and which object exerts it. Second, a system where several forces act and do not cancel: add the forces as vectors, taking direction into account, to find the resultant force, then state its size and direction and the effect it has, such as acceleration in that direction. The special case is balanced forces: when the resultant force is zero, the forces cancel, so the object stays at rest or keeps a constant velocity. A worked example is a box pushed right with 30 N while friction acts left with 10 N, giving a resultant of 20 N to the right.

What earns the marks

• Names each force acting on the isolated object and states the direction in which each force acts.
• Identifies the object exerting each force so the pair of forces on the object is clear.
• Adds forces as vectors, taking account of opposite directions, to obtain the resultant force.
• States the size and direction of the resultant force and links it to the resulting acceleration.
• Recognises that a resultant force of zero means the forces are balanced and the object stays at rest or moves at constant velocity.

Common mistakes and how to put them right

• Adds the sizes of opposing forces instead of subtracting them.

  Put it right: Subtract the smaller force from the larger and keep the direction of the larger.

• Treats a moving object as always having a resultant force.

  Put it right: Recognise that constant velocity means zero resultant force.

• Lists only the weight of an object and ignores contact forces such as normal reaction or friction.

  Put it right: Name every force acting on the object.

• Gives the resultant force without a direction.

  Put it right: State the direction as well as the size.

Exam tips

• Draw a free-body diagram with arrows whose lengths show relative sizes before choosing an answer.
• Check whether the question describes an isolated object or a system with several forces, because the reasoning differs.
• For balanced-force options, test whether the object is at rest or at constant velocity, since both fit a zero resultant.