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phys121_s12_hw2_sol_mp_2

phys121_s12_hw2_sol_mp_2 - Homework 2 PHYS 121 Spring 2012...

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1 (Due Date: 02/09/2012) Homework # 2, PHYS 121 Spring 2012 Mastering Physics: P2.12, P2.15, P2.29, P2.65, P2.69, P2.59, P2.70, 2.16 P2.12. Prepare: Assume that Richard only speeds on the 125 mi stretch of the interstate. We then need to compute the times that correspond to two different speeds for that given distance. Rearrange Equation 1.1 to produce distance time speed Solve: At the speed limit: 1 125 mi 60 min time 115.4 min 65 mi/h 1h At the faster speed: 2 125 mi 60 min time 107.1min 70 mi/h 1h By subtracting we see that Richard saves 8.3 min. Assess: Breaking the law to save 8.3 min is normally not wort h it; Richard’s parents can wait 8 min. Notice how the hours (as well as the miles) cancel in the equations. P2.15. Prepare : Assume x v is constant so the ratio x t is also constant. Solve: (a) 30 m 30 m = =1.5 s =15 m 3.0 s 1.5 s 3.0 s x x (b) 30 m 30 m = = 9.0 s = 90 m 3.0 s 9.0 s 3.0 s x x Assess: Setting up the ratio allows us to easily solve for the distance traveled in any given time. P2.16. Prepare: Assume x v is constant so the ratio x t is also constant. Solve: (a) 100 m 400 m 400 m = =18 s = 72 s 18 s 100 m t t (b) 100 m 1.0 mi 1.0 mi 1609 m = =18 s = 290 s = 4.8 min 18 s 100 m 1.0 mi t t    Assess: This pace does give about the right answer for the time required to run a mile for a good marathoner. P2.29. Prepare: We’ll do this in parts, first computing the acceleration after the congestion. Solve: 12.0 m/s 5.0 m/s 7.0 m/s = = = 8.0 s 8.0 s v a t
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2 (Due Date: 02/09/2012) Homework # 2, PHYS 121 Spring 2012 Now use the same acceleration to find the new velocity. 2 f i 7.0 = =12.0 m/s m/s (16 s) = 26 m/s 8.0 v v a t Assess: The answer is a reasonable 58 mph. P2.59. Prepare: Fleas are amazing jumpers; they can jump several times their body height something we cannot do. We assume constant acceleration so we can use the equations in Table 2.4. The last of the three relates the three variables we are concerned with in part (a) : speed, distance (which we know), and acceleration (which we want).
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