# Average speed

• Oct 2nd 2008, 07:27 PM
macgyver
Average speed

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?
• Oct 2nd 2008, 08:31 PM
mr fantastic
Quote:

Originally Posted by macgyver

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).