Question

A skier starting from rest skis straight down a slope 50 meters long in 5.0 seconds. What is the magnitude of the acceleration.

Answer 1

His average speed all the way is

(50 meters) / (5 sec) = 10 m/s .

But, if the acceleration is uniform all the way down, then

Average speed = (1/2) (start speed + end speed)

Start speed = 0.
So
Average speed = (1/2) (0 + end speed)

= (1/2) of end speed .

10 m/s = (1/2) of end speed

End speed (at the bottom) = 20 m/s .

Magnitude of acceleration = (change in speed) / (time for the change)

= (20 m/s) / (5 sec)

= 4 m/s² .