Draw and use free body force diagrams

The idea

A free body force diagram isolates one object and shows every force acting on that object as an arrow starting from a point or box representing it. Each arrow's direction shows the direction of the force and its label names the force, such as weight, normal contact force, friction, tension or thrust. Draw only forces acting on the chosen object, not forces it exerts on other things. Use the diagram to reason about the resultant: if opposing arrows are equal the object is in equilibrium and moves at constant velocity or stays at rest; if they are unequal there is a net force causing acceleration.

For example: A box on a table shows weight downwards and normal contact force upwards; a falling object with air resistance shows weight down and drag up, with a net downward force while accelerating.

What earns the marks

• Isolates a single object and represents it with a point or simple shape.
• Draws each force as an arrow showing its direction and labels the type of force.
• Includes only forces acting on the object and excludes forces the object exerts elsewhere.
• Compares opposing arrows to deduce whether a net force acts and whether the object is in equilibrium.
• Uses the diagram to link an unbalanced force to acceleration or a balanced set to constant velocity.

Common mistakes and how to put them right

• Includes forces the object exerts on other bodies.

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

• Omits the label on an arrow so the force type is unclear.

  Put it right: Name each force such as weight or friction.

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

  Put it right: Size arrows to reflect relative size of forces.

• Claims a moving object must have a net force.

  Put it right: Recognise that constant velocity means balanced forces.

Exam tips

• Start by drawing the object as a dot or box, then add one arrow at a time and label it immediately.
• Check each arrow by asking whether that force acts on the object, not by the object.
• State explicitly whether the forces balance or give a resultant, and link this to the motion described.