A scooter, starting from rest, moves with a constant acceleration for a time $\Delta t_1,$ then with a constant velocity for the next $\Delta t_2$ and finally with a constant deceleration for the next $∆t_3$ to come to rest. A $500 \mathrm{N}$ man, sitting on the scooter behind the driver, manages to stay at rest with respect to the scooter without touching any other part. The force exerted by the seat on the man is