Average magnitude of what ? Do you mean of stopping force ? Add up the total mass. I Newton of Force = Mass (Kg) x Acceleration (m/s^2) Note the acceleration in this case is the rate of stopping (deceleration)but it makes no difference, just changes the direction of the force. (26.5 - 0) m/s in 4.73 s is equivalent to 25.5/4.73 m/s^2 acceleration = 5.60 m/s^2 Then Mass Kg x 5.60 = average force in Newtons that will stop the car over 4.73 seconds. In passing it is interesting to note that Newton was alleged to have discovered gravity by being hit on the head with a falling apple that was being accelerated towards his head by a force of about 1 Newton. If both his head and the apple were totally inelastic objects it would stopped in almost zero distance and have hit his head with a force approaching infinity, and he would never have discovered gravity. An interesting exercise is to figure out if a chair which is designed just to exactly support your weight (ie your mass time gravitational acceleration) would survive you sitting on it. Answer...NO... because the act of sitting involves moving most of your mass down on to the seat at a non zero velocity (or else your bum would never get there) and then coming to a stop in a very short distance. Net force, your mass times a quite large acceleration. That's why your granma didn't like you flopping onto the chairs when you were a kid, and why chairs should be designed to support several times your weight.