The apparent change in weight that riders experience on coasters is due to the gravitational force.
Step-by-step explanation:
The obvious change in weight is due to the temporary addition and subtraction of opposing normal forces of temporary aid or the force of gravity as you descend you fall into a free fall position where you feel lighter because gravity does not feel as hard it is.
When you are going back, you feel heavy because you are going against the force of gravity, so it is obviously pulling harder than usual.