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HW3 - PHYS 4D Solution to HW 3 Problem Giancoli 31-34(II...

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PHYS 4D Solution to HW 3 January 20, 2011 Problem Giancoli 31-34 (II) Estimate the radiation pressure due to a 75-W bulb at a distance of 8 . 0 cm from the center of the bulb. Estimate the force exerted on your fingertip if you place it at this point. Solution: Using Eq. 31-21a, P = S c = P 4 πd 2 c = 75 W 4 π (8 . 0 × 10 - 2 m ) 2 3 . 00 × 10 8 m/s = 3 . 1 × 10 - 6 N/m 2 . The force is pressure times area. We approximate the area of a fingertip to be 1 . 0 cm 2 , F = PA = ( 3 . 1 × 10 - 6 N/m 2 ) ( 1 . 0 × 10 - 4 m 2 ) = 3 . 1 × 10 - 10 N. Problem Giancoli 31-35 (II) Laser light can be focused (at best) to a spot with a radius r equal to its wavelength λ . Suppose that a 1 . 0- W beam of green laser light ( λ = 5 × 10 - 7 m ) is used to form such a spot and that a cylindrical particle of about that size (let the radius and height equal r ) is illuminated by the laser as shown in Fig. 31-23. Estimate the acceleration of the particle, if its density equals that of water and it absorbs the radiation. [This order-of-magnitude calculation convinced researchers of the feasibility of ”optical tweezers,” p. 829.] Solution: SA = dU dt = 1 . 0 W.
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