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A street light is mounted at the top of a 15-ft-tall pole. A man 6 feet tall walks away from the pole with a speed of 4 ft/s along a straight path. How fast (in ft/s) is the tip of his shadow moving when he is 35 feet from the pole?

Question

A street light is mounted at the top of a 15-ft-tall pole. A man 6 feet tall walks away from the pole with a speed of 4 ft/s along a straight path. How fast (in ft/s) is the tip of his shadow moving when he is 35 feet from the pole?

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Solution

This is a related rates problem in calculus. We can solve it using the Pythagorean theorem and implicit differentiation.

  1. First, let's set up the problem. We have a right triangle formed by the man, the pole, and the tip of the man's shadow. Let's denote the distance from the man to the pole as x, and the length of the shadow as s. The height of the pole is 15 ft and the height of the man is 6 ft.

  2. According to the Pythagorean theorem, we have the following relationship:

    (15)^2 = (6 + s)^2 + x^2

  3. Differentiate both sides with respect to time t:

    0 = 2(6 + s)ds/dt + 2x dx/dt

  4. We know that dx/dt = 4 ft/s (the speed of the man), and we want to find ds/dt (the speed of the tip of the shadow). So we can solve the above equation for ds/dt:

    ds/dt = -x dx/dt / (6 + s)

  5. Now we need to find the values of x and s when the man is 35 ft from the pole. We can use the Pythagorean theorem again:

    (15)^2 = (6 + s)^2 + (35)^2

    Solving this equation gives s = 9 ft.

  6. Finally, substitute x = 35 ft, dx/dt = 4 ft/s, and s = 9 ft into the equation for ds/dt:

    ds/dt = -35 * 4 / (6 + 9) = -140 / 15 = -28/3 ft/s

So, the tip of the man's shadow is moving at a speed of 28/3 ft/s when he is 35 feet from the pole.

This problem has been solved

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