From Twins to Relativistic Energy solutions

From Twins to Relativistic Energy solutions - PHYSICS 6...

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PHYSICS 6 SPRING 2006 HOMEWORK 2 SOLUTIONS 1. A hockey puck is hit and travels at a velocity of 90 Km/hr toward the goal, 60 m away. How long will it take to reach the goal? time=distance/speed=60m/(90x10 3 m/hr)=6.67x10 -4 hr =6.67x10 -4 hr x 3600 sec/hr = 2.4 sec 2. A spy satellite travels around the earth in a low orbit every 90 minutes. What is the speed of the satellite? Circumference of earth =24,000 miles = 38,000 Km 90 min=1.5 hr Speed = 24,000 mi /1.5 hr = 16,000 mph = 26,000 Km/hr or 4.4 miles/sec = 7.0 Km/sec Is it accelerating? Yes, since its direction is changing continually. 3. A ball is dropped straight down from the top of a building. The ball hits the ground in 5 sec. a. How tall is the tower? Distance fallen = (1/2)gt 2 = (1/2) (9.8 m/sec 2 ) (5 sec) 2 = 4.9 x 25 m = 120 m b. If the ball is thrown from the tower horizontally at 10 m/s, how long does it take to reach the ground? Same 5 sec. c. In the latter case, how far from the tower does the ball land? Distance horizontal is independent of vertical motion, so d horizontal =10 m/s x 5 s = 50 m 4. The same ball that is dropped straight down from the top of a tower (and
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