An industrial robotic arm slowly lowers a powerful permanent magnet vertically towards a small iron bolt resting on a horizontal conveyor belt.
Explain why the bolt remains stationary on the belt at first, but then suddenly jumps upwards to meet the magnet only when the magnet is lowered to a certain critical height.
30 exam-style questions on Edexcel GCSE Physics 12.1 Magnets and magnetic fields. Each one has a worked solution and a mark scheme showing where the marks go.