hi, need help

A window washer drops a squeegee from a scaffold 100 m off the ground. The
relationship between the height of the squeegee (h), in metres, and the length of time
it has been falling (t), in seconds, is given by h = 100 − 5t2.
a) When does the squeegee pass a window 30 m off the ground?
b) How long does it take for the squeegee to hit the ground?

thanks!

2. Originally Posted by smmmc
hi, need help

A window washer drops a squeegee from a scaffold 100 m off the ground. The
relationship between the height of the squeegee (h), in metres, and the length of time
it has been falling (t), in seconds, is given by h = 100 − 5t^2.
a) When does the squeegee pass a window 30 m off the ground?
b) How long does it take for the squeegee to hit the ground?

thanks!
For squeegee to be 30 metres above the ground

$\displaystyle h = 30 = 100- 5t^2$

Hence
$\displaystyle 70= 5t^2$

Thus $\displaystyle t^2 = 14$
Hence $\displaystyle t = \pm \sqrt{14}$
Since time cannot be negative its taken as $\displaystyle t= \sqrt{14} s$

For squeegee to hit the ground its height should be 0
so
$\displaystyle h= 0 = 100- 5t^2$

Thus
$\displaystyle 5t^2 = 100$

Hence
$\displaystyle t^2 = 20$

Hence
$\displaystyle t= \sqrt {20}s = 2\sqrt{5}s$

For squeegee to be 30 metres above the ground

$\displaystyle h = 30 = 100- 5t^2$

Hence
$\displaystyle 20= 5t^2$

Thus $\displaystyle t^2 = 4$
Hence $\displaystyle t = \pm 2$
Since time cannot be negative its taken as $\displaystyle t= 2s$

For squeegee to hit the ground its height should be 0
so
$\displaystyle h= 0 = 100- 5t^2$

Thus
$\displaystyle 5t^2 = 100$

Hence
$\displaystyle t^2 = 20$

Hence
$\displaystyle t= \sqrt {20}s = 2\sqrt{5}s$
hey how did you get the 20 in this $\displaystyle 20= 5t^2$

4. Originally Posted by smmmc
hey how did you get the 20 in this $\displaystyle 20= 5t^2$
By mistake