Starting from rest, a scooter travels with a constant acceleration...

During the time interval $\Delta t_1$, the scooter accelerates with an acceleration $a_1$. The forces acting on the man are his weight $W = 500 \text{ N}$ acting downwards and the force from the seat. Since the man is at rest relative to the scooter, he accelerates with $a_1$. The force exerted by the seat has a vertical normal component $N = 500 \text{ N}$ to balance gravity and a horizontal frictional component $f = m a_1$ to provide the acceleration.

The total force exerted by the seat is $F_1 = \sqrt{N^2 + f^2} = \sqrt{500^2 + (m a_1)^2}$, which is greater than $500 \text{ N}$.

During the time interval $\Delta t_2$, the scooter moves with a constant velocity, so the acceleration is zero. The horizontal force is zero, and the force exerted by the seat is only the vertical normal force $N = 500 \text{ N}$.

During the time interval $\Delta t_3$, the scooter decelerates with an acceleration $a_3$. The force exerted by the seat again has a horizontal component $f' = m a_3$ and a vertical component $N = 500 \text{ N}$. The total force is $F_3 = \sqrt{500^2 + (m a_3)^2}$, which is greater than $500 \text{ N}$.

Thus, the force exerted by the seat on the man is $> 500 \text{ N}$ for time $\Delta t_1$ and $\Delta t_3$, and $500 \text{ N}$ for $\Delta t_2$.