HW #2-4-solutions

# HW #2-4-solutions - pasha(sep635 HW#2-4 Antoniewicz(56445 =...

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pasha (sep635) – HW #2-4 – Antoniewicz – (56445) 1 This print-out should have 20 questions. Multiple-choice questions may continue on the next column or page – fnd all choices beFore answering. 001 20.0 points AtennisballhasamassoF0 . 057 kg. A proFes- sional tennis player hits the ball hard enough to give it a speed oF 50 m / s. The ball hits a wall and bounces back with almost the same speed. As indicated in the Following fgure, high-speed photography shows that the ball is crushed 2 cm at the instant when its speed iss momentarily zero, beFore rebounding. 2cm ~ p i Making the very rough approximation that the large Force that the wall exerts on the ball is approximately constant during contact, determine the approximate magnitude oF this Force. Hint: Think about the approximate amount oF time it takes For the ball to come momentarily to rest. (±or comparison note that the gravitational Force on the ball is quite small, only about (0 . 057 kg)(9 . 8N / kg) 0 . 6N . AForceoF5Nisapproximatelythesameasa Force oF one pound.) Correct answer: 3562 . 5N. Explanation: ±irst we want to fnd the time interval as the ball is slowing down during the collision. ~ v avg = ~ v i + ~ v f 2 v avg ,x = v i,x + v f,x 2 = 50 m / s+0 2 =25m / s v avg ,x = Δ x Δ t Δ t = Δ x v avg ,x = 0 . 02 m 25 m / s =0 . 0008 s . Now apply the momentum principle. De- fne the system to be the tennis ball. The only signifcant Force on the ball during the colli- sion is due to the wall. Thus F net ,x = F wall ,x . | F net ,x | = ± ± ± ± Δ p x Δ t ± ± ± ± ± ± F wall ,x ± ± = ± ± ± ± p f,x - p i,x Δ t ± ± ± ± = ± ± ± ± 0 - (0 . 057 kg)(50 m / s) 0 . 0008 s ± ± ± ± = 3562 . 5N . 002 (part 1 of 9) 3.0 points A0 . 6kgb locko Ficei ss l id ingbyyouona very slippery ²oor at 2 . 6m / s. As it goes by, you give it a kick perpendicular to its path. Your Foot is in contact with the ice block For 0 . 003 s. The block eventually slides at an angle oF 22 From its original direction. The overhead view shows in the Following diagram is approximately to scale. The red arrow represents the average Force your toe applies brie²y to the block oF ice. II I

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pasha (sep635) – HW #2-4 – Antoniewicz – (56445) 2 1. III 2. I 3. II correct Explanation: Image II is correct because the kick is brief, and after the kick, ~ v is constant so the block will move with a constant velocity. 003 (part 2 of 9) 3.0 points Which components of the block’s momentum are changed by the impulse applied by your foot? (List all that apply, separated by com- mas. Remember that the diagram shows a top view, looking down on the xz plane.) Enter all choices that apply, separated by commas. A. The
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## This note was uploaded on 11/17/2011 for the course PHY 303K taught by Professor Turner during the Fall '08 term at University of Texas.

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HW #2-4-solutions - pasha(sep635 HW#2-4 Antoniewicz(56445 =...

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