A 20 foot ladder slides down a wall at 5ft/sec. At what speed is the bottom sliding out when the top is 10 feet from the floor (in ft/sec).

Printable View

- Apr 17th 2008, 11:13 AMDINOCALC09Speed
A 20 foot ladder slides down a wall at 5ft/sec. At what speed is the bottom sliding out when the top is 10 feet from the floor (in ft/sec).

- Apr 17th 2008, 11:36 AMTheEmptySet
- Apr 17th 2008, 11:44 AMDINOCALC09
i got 5/sqrt(3)

Taking the derivatives of the pythagorean theorem,

$x\left(\frac {dx}{dt}\right) + y\left(\frac {dy}{dt}\right) = z\left(\frac {dz}{dt}\right)$

But $\left(\frac {dz}{dt}\right) = 0$ so

$x\left(\frac {dx}{dt}\right) = - y\left(\frac {dy}{dt}\right)$

Then from pythagoras, $x^{2} + y^{2} = z^{2}$. We are looking for $x$ at $y = 10$ so $x = \sqrt {400 - 100} = \sqrt {300}$.

Then we have

$\sqrt {300}\left(\frac {dx}{dt}\right) = - 10(5)\Rightarrow\left(\frac {dx}{dt}\right) = \frac {50}{\sqrt {300}} = \frac {5}{\sqrt {3}}$

The negative sign just gives direction so it isn't really important.