When the ball hits the ground, the height is 0. Substitute 0 for
h
.
h
=
−
16
t
2
−
10
t
+
200
0
=
−
16
t
2
−
10
t
+
200
−
16
t
2
−
10
t
+
200
=
0
This equation is difficult to solve by factoring or by completing the square, so solve it by applying the Quadratic Formula,
x
=
−
b
±
√
b
2
−
4
a
c
2
a
. In this case, the variable is
t
rather than
x
.
a
=
−
16
,
b
=
−
10
, and
c
=
200
.
t
=
−
(
−
10
)
±
√
(
−
10
)
2
−
4
(
−
16
)
(
200
)
2
(
−
16
)
Simplify. Be very careful with the signs.
t
=
10
±
√
100
+
12800
−
32
=
10
±
√
12900
−
32
Use a calculator to find both roots.
t
is approximately
−
3.86
or
3.24
.
Consider the roots logically. One solution,
−
3.86
, cannot be the time because it is a negative number. The other solution,
3.24
seconds, must be when the ball hits the ground.
Answer
The ball hits the ground approximately
3.24
seconds after being thrown.