Thread: Average speed

please help me on the following problem:

A rock released from rest to fall on a small airless planet falls y=gt^2 m in t sec, g a constant. Suppose that the rock falls to the bottomof a crevasse 20 m below and reaches the bottom in 4 sec.

a) find the value of g.
b) find the average speed of the fall.
c) With what speed did the rock hit the bottom?

Originally Posted by macgyver

please help me on the following problem:

A rock released from rest to fall on a small airless planet falls y=gt^2 m in t sec, g a constant. Suppose that the rock falls to the bottomof a crevasse 20 m below and reaches the bottom in 4 sec.

a) find the value of g.
b) find the average speed of the fall.
c) With what speed did the rock hit the bottom?

a) Substitute t = 4 and y = 20 into y = gt^2. Solve for g.

b) Average speed = distance/time = .....

c) t = 4 s, s = 20 m, u = 0 m/s, v = ?
Substitute the data into s = (u + v)t/2 (one of the usual straight line motion under constant acceleration formula).

Alternatively, t = 4, s = 20, a = g = answer to (a), v = ?
Substitute the data into s = vt - at^2/2 (one of the usual straight line motion under constant acceleration formula).