A budding magician holds a 2.24-mW laser pointer, wondering whether he could use it to keep an object floating in the air with the radiation pressure. This might be an idea for a new trick! Assuming the laser pointer has a circular beam 3.00 mm in diameter and the magician rigs up a totally reflecting sail on which to shine the laser, what is the maximum weight the magician could suspend with this technique?

 W = 1.49 10⁻¹¹ kg

Explanation:

For this problem, let's use Newton's equation of equilibrium

           F - W = 0

            F = W              (1)

Strength can be found from the definition of pressure

        P = F / A

        F = P A

The radiation pressure for a reflective surface is

            P = 2 I / c)

We substitute in equation 1

         2 I / c  A = W

The intensity is defined by the ratio of the power between the area

          I = P / A

          P = I A

We substitute

          2 P / c = W

           W = 2  2.24 10-3 / 3 108

           W = 1.49 10⁻¹¹ kg

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