Physics Word Problem?

Question

An object is propelled vertically upward with an initial velocity of 20 meters per second. The distance s (in meters) of the object from the ground after t seconds is s = -4.9t^2 + 20t.

(1) When will the object be 15 meters above the ground?

(2) When will the object strike the ground?

Answer 1

You have two quadratic equations:
For (1), s = 15 = -4.9t^2 + 20t, so -4.9t^2 + 20t -15 = 0
For (2), s = 0 = -4.9t^2 + 20t, so -4.9t^2 + 20t = 0
Use the standard method for solving these.

Answer 2

42