Explain how we measure g using an electronic timer in the laboratory. (3) What does this tell us. (1) What equation of motion must we use to find g (1) What are sources of uncertainty (4)

A steel ball-bearing is held by an electromagnet When the current is switched off, ball begins to fall, and electronic timer starts. Ball falls through trap door and breaks circuit, stopping timer Tells us time taken for ball to fall from rest trough distance h between bottom of baal and trapdoor H=1/2gt^2 Electromagnet may retain some magnetism when it is switched off, can slow balls fall Time t recorded by timer may be longer than if ball were to fall completely freely - From h=1/2gt^2 if t is to great, experimental value of g will be too small Measuring height h is awkward and value of h will be given ±1 mm at best