# Application of derivatives

• September 29th 2007, 07:03 AM
Joyce
Application of derivatives
A light shines frm the top of a pole 50ft high.A ball is dropped frm the same height frm a point 30ft away frm the light.How fast is the shadow of the ball moving along the ground 1/2sec later?(Assume the ball falls s=16t^2ft in t sec)
• September 29th 2007, 07:22 AM
Joyce
hello
why nobody help?
• September 29th 2007, 07:28 AM
earboth
Quote:

Originally Posted by Joyce
A light shines frm the top of a pole 50ft high.A ball is dropped frm the same height frm a point 30ft away frm the light.How fast is the shadow of the ball moving along the ground 1/2sec later?(Assume the ball falls s=16t^2ft in t sec)

hello,

I've attached a drawing of the situation.

You are dealing with 2 similar triangles. You can set up the proportion:

$\frac s{30} = \frac{30-s}y$. Solve for y because that's the length which the shadow of the ball is away from the pole:

$y=\frac{900}s - 30$ . You are told that $s = 16t^2$ . Substitute the variable s and you'll get the equation:

$y(t) = \frac{900}{16t^2}-30$

You know that speed is the first derivative wrt t of the length:

$y'(t) = -\frac{900}{8t^3}$

The speed at $t = \frac12$ is:

$\rm{speed} = y'\left(\frac12 \right) = -\frac{900}{8 \cdot \frac18} = -900$
• September 29th 2007, 07:32 AM
earboth
Quote:

Originally Posted by Joyce
why nobody help?

I'm an infirm old man and need some time to type the solution and to make a nice drawing. I'm now really breathless and exhausted :D