(d) >500 N for time ∆t1​ and ∆t3​ and 500 N for ∆t2​.During the time interval ∆t2​, the scooter is moving with a constant velocity, which implies that the force exerted by the seat on the man is 500 N (for balancing the weight of the man).During the time interval ∆t1​ and ∆t3​, the scooter is moving with constant acceleration and deceleration, which implies that a frictional force is also applied. Therefore, the net force exerted by the seat on the man should be >500 N.