All the cars are fitted with seatbelts. A driver with a mass of 80Kg is moving at 15m.s when the car is involved in a collision. His seatbelt stops him in 0.3s. Calculate the average force that the seatbelt exerts on his body.

momentum = 80 kg * 15 m/s = 1200 kg m/s

stops so final momentum = 0
force = change of momentum /change of time ( or mass * acceleration)
how long to stop? 0.3 s
so
force = 1200 / 0.3 = 4,000 Newtons (about 5 times his weight)