d=rt-5t^2 , i think this my formula to use, need help, thx

Question

with an underhand volleyball serve at 1 m above the floor and velocity of 7 m per second, how high above the floor will it be in 0.3 seconds? and if the ball is untouched, when will it be back level to when it was served? and when will it hit the floor?

Answer 1

Use the formula

 

d = -4.9t² + 7t + 1

 

 

To find the height of the ball in 0.3 seconds, evaluate d when t=0.3.   This part you can do on your own.

 

To find when the ball will be back at the level it was served, evaluate t when d=1.

 

-4.9t² + 7t + 1 = 1

 

-4.9t² + 7t = 0

 

 

Solving for t,

 

t(-4.9t + 7) = 0

 

-4.9t + 7 = 0

-4.9t = -7

      t = 1.429

 

The ball returns to serve level after 1.429 seconds.

 

 

To find when the ball will hit the floor, set d=0 and solve for t.

 

-4.9t² + 7t + 1 = 0

 

 

Use the quadratic formula to solve for t.