If Hannah slows down from a speed of 12 m/s to 8 m/s and has a mass of 45kg, how much force has been exerted on her to cause this deceleration?

Since force = mass x acceleration, acceleration = 4 as the change in velocity is 4 m/s and m = 45 as the mass of Hannah is 45 kg and hence the force required to be exerted on her is 4*45 which is 180N.