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Physics 1A
Solution 4
November 11, 2019
Problem 4: Revenge of the Elevator
a) The elevator falls downward, and gravity acts in the downward direction, so the work is just the force of
gravity x the distance moved:
W = Fx d=925kg x 10m/s x 28.5m = 263625J
b) All of this energy gets converted into kinetic energy:
W = 263625J = AKE = m ,
v = V2 x 263625J/925kg = 23.87m/s
c) We need to consider the whole energy now. There is no kinetic energy at the end, so AKE is zero. And
we have to add the compression distance x to our height change.
AKE + APE = 0
kx2
2
+mg(-(h + x)) =0
40000x2- 9250x - 263625 = 0
Using the quadratic equation, we can solve this to find x = 2.69 m.
d) In this case, since the elevator is not accelerating, the forces on it must be balanced. Therefore:
mg = kx
x =
my - 0.116m
K
Problem 5: Rollercoaster
a) At each point, we know that all of the potential energy from the change in height, gets converted to
kinetic energy, so we have mgAh = mv?/2 - v = v(2gAh). We can use this formula to find the speed
at points 2, 3 and 4:
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- JeffreyHutchinson