Represent forces as vectors (to include: drawing free body force diagrams…

The idea

A vector has both magnitude and direction, so a force is fully described only when you give its size in newtons and the direction in which it acts. A free body force diagram isolates one object and shows every force acting on that object as an arrow: the arrow's length represents the force's magnitude and the arrowhead gives its direction. Draw each arrow starting from the object or from a labelled point on it, and label each force (for example weight, normal contact force, friction, tension, thrust). Choose a scale so lengths are comparable, and draw opposing forces along the same line. If arrows are equal and opposite the forces balance and the resultant is zero; if not, the resultant is found by vector addition. A moving object at constant velocity still has balanced forces, so no resultant arrow is needed.

What earns the marks

• Identifies that a force vector requires both a magnitude in newtons and a direction.
• Selects the single object under consideration and shows only the forces acting on that object.
• Draws each force as an arrow whose length represents magnitude and whose head shows direction.
• Labels each arrow with the name of the force, such as weight or normal contact force.
• Recognises that equal and opposite arrows on the same line indicate balanced forces and zero resultant.
• States that an object moving at constant velocity has balanced forces and no resultant.

Common mistakes and how to put them right

• Draws arrows for forces the object exerts on other bodies.

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

• Omits the direction of a force and gives only its size.

  Put it right: State both magnitude and direction for a vector.

• Assumes a moving object must show a resultant arrow.

  Put it right: Recognise that constant-velocity motion has balanced forces and zero resultant.

• Draws arrows of arbitrary length with no consistent scale.

  Put it right: Use one scale so relative magnitudes are visible.

Exam tips

• Read the question to identify which single object the diagram should isolate before drawing any arrow.
• Check that every arrow is labelled and that its length is consistent with the stated scale.
• For a constant-velocity situation, confirm the arrows balance rather than adding a resultant.