Question

A ball is thrown straight up from a student's hand. Her hand is 2.0 m above the ground. The ball leaves the student's hand with a speed of 15 m/s How long is the ball in the air before it hits the ground?

Answer 1

x=15tcos90° y=2+15tsin90°-4.9t^2
x=0 y=2+15t-4.9t^2
To find t when y=0, 0=-4.9t^2+15t+2

Quadratic formula makes t=.3135 or -7.8135
Clearly, the logical answer is the positive one. The ball is in the air .3135 seconds before it hits the ground.