Imagine a right triangle where the legs represent the horizontal and vertical lengths of the string and the hypotenuse represents the length of the string.
Let us assign some values:
x = horizontal length in feet
50 = vertical length in feet
L = length of the string in feet
Because we are modeling these quantities with a right triangle, we can use the Pythagorean theorem to relate them with the following equation:
L² = x² + 50²
We want to find an equation for the change of L over time, so first differentiate both sides with respect to time t then solve for dL/dt:
2L(dL/dt) = 2x(dx/dt)
dL/dt = (x/L)(dx/dt)
First let's solve for x at the moment in time described in the problem using the Pythagorean theorem:
L² = x² + 50²
Given values:
L = 100ft
Plug in and solve for x:
100² = x² + 50²
x = 86.6ft
Now let's find dL/dt. Given values:
x = 86.6ft, L = 100ft, dx/dt = 4ft/sec
Plug in and solve for dL/dt:
dL/dt = (86.6/100)(4)
dL/dt = 3.46ft/sec
