Work done and energy transfer: force, F, in newtons, N

The idea

Force is the push or pull that acts on an object, measured in newtons (N). In the work done equation W = Fs, F is the component of force acting along the direction of movement, so a force at an angle transfers less energy than the same force applied straight along the motion. One newton is the force that gives a 1 kg mass an acceleration of 1 m/s². For example, pushing a box with 20 N across 3 m does W = 20 × 3 = 60 J. If the force is perpendicular to the motion, no work is done by that force. Always identify the force doing the work, check its direction, and convert units such as kN to N before substituting into the equation.

What earns the marks

• State that force is measured in newtons (N) and is a vector quantity with both magnitude and direction.
• Identify F in W = Fs as the force component acting along the displacement, not any arbitrary force on the object.
• Convert units correctly, for example 2.5 kN = 2500 N, before calculating work done.
• Recognise that a force perpendicular to the displacement transfers no energy, so W = 0 J for that force.
• Use W = Fs to calculate force as F = W ÷ s when work done and distance are known.

Common mistakes and how to put them right

• Using the total force on an object instead of the component along the direction of travel.

  Put it right by resolving the force or using only the parallel component.

• Forgetting to convert kilonewtons to newtons.

  Put it right by multiplying kN by 1000 before substitution.

• Treating force as a scalar and ignoring direction.

  Put it right by stating the direction and checking whether the force aids or opposes motion.

Exam tips

• Write the equation W = Fs, rearrange it if needed, then substitute values with units before calculating.
• Check that the force and distance are in the same line of action; if not, state that only the parallel component does work.
• Give the unit N with every force value and show conversion steps clearly to secure method marks.