Mrs. Napholtz's Math Site
Projectile Problems
When a ball is thrown upward with an initial speed of 14.7 m/s, physicists tell us that the height of the ball after "t" seconds is given by the equation
h = 14.7t - 4.9t2
At what two times will the ball be 9.8 m high?
9.8 = 14.7t - 4.9t2 Add 4.9t2 and subtract 14.7t
4.9t2 - 14.7t + 9.8 = 0 Factor out 4.9.
4.9(t2 - 3t + 2) = 0
4.9(t - 1)(t - 2) = 0
t - 1 = 0 OR t - 2 = 0
t = 1 OR t = 2
Answer: The ball will be 9.8 meters high 1 second after the ball is thrown (on its way up) and also 2 seconds after the ball is thrown. (on the way down)