Algebra Demystified 2nd Ed

408 algebra De mystif ieD

when h = 0, so h 0 = −16t^2 + 1600 becomes 0 = −16t^2 + 1600. We solve this

for t.

016 1600

16 1600

100

100

10 10

2

2

2

=− +

=

=

=±

==−

t

t

t

t

tt(isnotasolution)

The object will hit the ground 10 seconds after it is dropped.

A ball is dropped from the top of a four-story building. The building is

48 feet tall. How long will it take for the ball to reach the ground?

Because the object is dropped, the initial velocity, v 0 , is zero: v 0 = 0.

The object is dropped from a height of 48 feet, so h 0 = 48. The formula

h = − 16 t^2 + v 0 t + h 0 becomes h = − 16 t^2 + 48. The object hits the ground

when h = 0.

ht

t

t

t

tt

=− +

=− +

=

=

==−

16 48

01648

16 48

3

33

2

2

2

2

(isnotaasolution)

t≈ 17 3.

The ball will reach the ground in about 1.73 seconds.

PRACTICE

1. An object is dropped from over a bay. If the ball is dropped 56 feet
   above the surface of the water, how long will it take for the object to
   reach the water?


2. An object is dropped from the top of a 240-foot-tall observation tower.
   How long will it take for the object to reach the ground?


3. A ball is dropped from a sixth floor window at a height 70 feet. When
   will the ball hit the ground?


4. An object falls from the top of a 100-foot communications tower. How
   long will it take for the object to hit the ground?

PRACTICE

1. An object is dropped from over a bay. If the ball is dropped 56 feet
   above the surface of the water, how long will it take for the object to

PRACTICE

1. An object is dropped from over a bay. If the ball is dropped 56 feet