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Unformatted text preview: homework 02 KERR, KELSEY Due: Jan 21 2008, 11:00 pm 1 Question 1, chap 2, sect 1. part 1 of 3 10 points Two cars approach each other; both cars are moving westward, one at 79 km / h, the other at 58 km / h. What is the magnitude of the velocity of the first car relative to (in the frame of reference of) the second car? Correct answer: 21 km / h (tolerance 1 %). Explanation: All the motion is directed westward, so we can consider west to be positive. The velocity of the first car relative to the second car is v r = v 1 v 2 = 79 km / h 58 km / h = 21 km / h Alternate Solution: Let east be positive: The velocity of the first car relative to the second car is v r = v 1 v 2 = 79 km / h ( 58 km / h) = 21 km / h which has a magnitude of 21 km / h. Question 2, chap 2, sect 1. part 2 of 3 10 points What is the direction of the resultant ve locity? 1. eastward 2. westward correct 3. Unable to determine. Explanation: All motion is directed westward, as is their relative velocity. Question 3, chap 2, sect 1. part 3 of 3 10 points After they pass, how will their relative ve locity change? 1. No change. correct 2. Less than before. 3. Unable to determine. 4. Greater than before. Explanation: There is a change in relative position, but not in relative velocity. Question 4, chap 2, sect 1. part 1 of 1 10 points A car is moving at a constant speed of 13 m / s when the driver presses down on the gas pedal and accelerates for 11 s with an acceleration of 1 . 4 m / s 2 . What is the average speed of the car during the period? Correct answer: 20 . 7 m / s (tolerance 1 %). Explanation: First, we use the equation v = v + at to obtain the final speed after time t . v = 13 m / s + (1 . 4 m / s 2 )(11 s) = 28 . 4 m / s Then we use the equation v = v + v 2 to obtain the average velocity: v = 13 m / s + 28 . 4 m / s 2 = 20 . 7 m / s Question 5, chap 2, sect 2. part 1 of 2 10 points A runner is jogging at a steady v r = 4 . 9 km / hr. When the runner is L = 3 . 1 km from the finish line a bird begins flying from the runner to the finish line at v b = 24 . 5 km / hr (5 times as fast as the runner). When the bird reaches the finish line, it turns around and flies back to the runner. Even though the bird is a dodo, we will assume homework 02 KERR, KELSEY Due: Jan 21 2008, 11:00 pm 2 that it occupies only one point in space, i.e. , a zero length bird. L v b v r finish line How far does the bird travel? Correct answer: 5 . 16667 km (tolerance 1 %). Explanation: Let, dodo birds fly, and d r be the distance the runner travels. d b be the distance the bird travels. v r be the speed of the runner. v b be the speed of the bird. L = d r be the original distance to the finish line....
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