Describe, using free body diagrams, examples of the special case where…

The idea

A free body diagram shows one chosen object as a point or simple shape with an arrow for every force acting on that object only. Each arrow points in the direction of the force, and arrow length represents magnitude. When the arrows cancel — equal and opposite along each line — the resultant force is zero. This is the balanced-force special case, and it is qualitative at Higher tier: you reason about balance and direction, not about calculating a value. Example: a book resting on a table has weight acting downwards and the normal contact force from the table acting upwards; equal magnitudes give a resultant of zero, so the book stays at rest. Example: a car travelling at constant velocity along a straight level road has driving force balanced by resistive forces and weight balanced by the normal reaction.

What earns the marks

• Identifies the single object whose forces are being shown and draws only the forces acting on that object.
• Draws each force as an arrow whose direction shows the direction of the force.
• Represents the magnitude of each force by the length of its arrow.
• Shows weight acting vertically downwards from the object.
• Shows the normal contact force acting perpendicular to the supporting surface.
• Pairs equal and opposite forces along the same line to show that they cancel.
• States that a resultant force of zero means the object stays at rest or moves with constant velocity.
• Distinguishes balanced forces from an absence of forces acting on the object.

Common mistakes and how to put them right

• Includes forces that the object exerts on other bodies.

  Put it right: Show only the forces acting on the chosen object.

• Draws weight as a horizontal arrow.

  Put it right: Draw weight vertically downwards towards the centre of the Earth.

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

  Put it right: Accept that constant velocity also corresponds to zero resultant force.

• Assumes balanced forces mean no forces act.

  Put it right: Recognise that several forces can act and still cancel.

• Draws arrows of arbitrary length with no relation to magnitude.

  Put it right: Scale arrow length to the size of each force.

Exam tips

• Name the object first, then list each force acting on it before drawing any arrow.
• Check each pair of arrows for equal length and exactly opposite direction along the same line.
• Link the zero resultant explicitly to the motion: at rest or constant velocity, never acceleration.
• Use the phrase 'resultant force is zero' rather than 'no forces' when forces cancel.