What is the force that is responsible for speeding up the roller coaster car as it accelerates down the track?

Gravity

Explanation:

Gravity applies a constant downward force on the cars. The coaster tracks serve to channel this force — they control the way the coaster cars fall. If the tracks slope down, gravity pulls the front of the car toward the ground, so it accelerates

i think it would be 80 km/h because 30 + 50 = 80 (sorry if this not the right answer i'm trying my best)

i think it is non rem sleep