Some scientists have suggested that spacecraft with sails of the kind described in Conceptual Example 4 could be propelled by lasers. Suppose such a sail is constructed of a highly reflective material thin enough so that one square meter of the sail has a mass of just 3.8 x 10-3 kg. The sail is to be propelled by an ultraviolet laser beam (wavelength = 154 nm) that will strike its surface perpendicularly. (a) Use the impulse- momentum theorem to determine the number of photons per second that must strike each square meter of the sail in order to cause an acceleration of 9.8 x 10-6 m/s?, which is one million times smaller than the gravitational acceleration at earth's surface. Assume that no other forces act on the sail, and that all the incident photons are reflected. (b) Determine the intensity (power per unit area) that the laser beam must have when it strikes the sail. Sail Incident photons Reflected photons (a) Number Units (b) Number Units C