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Unformatted text preview: karna (pk4534) – HW 5 – coker – (58245) 1 This print-out should have 19 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points A hockey puck is given an initial speed of 30 m / s on a frozen pond. The puck remains on the ice and slides 111 m before coming to rest. The acceleration of gravity is 9 . 8 m / s 2 . What is the coefficient of friction between the puck and the ice? Correct answer: 0 . 413679. Explanation: Basic Concept: Newton’s Second Law: F net = ma Solution: From kinematics, 0 = v 2 + 2 ax a = − v 2 2 x since the puck comes to a rest. Consider the free body diagram for the situation: μ N N mg Vertically, the puck is in equilibrium, so F net = N − mg = 0 ⇒ N = mg Horizontally, after the puck is given its initial velocity, the friction force f k = μ N = μmg is the only unbalanced force acting on it. Thus, f net = ma = 0 − f k = − μmg μ = − a g = v 2 2 gx = (30 m / s) 2 2(9 . 8 m / s 2 )(111 m) = 0 . 413679 002 10.0 points A block of mass 5 . 9 kg rests on a plane inclined at an angle of 26 ◦ . The static coefficient of friction between the block and the plane is . 64. What is the frictional force on the block? 1. 16 . 2218 N 2. 51 . 9683 N 3. 33 . 2597 N 4. 25 . 3466 N correct 5. 7 . 91307 N Explanation: The maximum static friction is f max = mgμ cos θ = (5 . 9 kg)(9 . 8 m / s 2 )(0 . 64) cos26 ◦ = 33 . 2597 N Since mg sin θ = (5 . 9 kg)(9 . 8 m / s 2 ) sin26 ◦ = 25 . 3466 N the friction is large enough to keep the block from moving, so the static friction is equal to mg sin θ . 003 (part 1 of 3) 10.0 points You are driving a car down a straight road at a constant 53 miles per hour. Consider the following forces: I) air drag pushing back on the car; II) gravity pulling down on the car; III) the ground pushing up on the car; IV) friction pushing the wheels (and the car) forward; and V) friction pushing the wheels (and the car) back. Which force(s) act on the car? 1. I, II and III karna (pk4534) – HW 5 – coker – (58245) 2 2. II, III and IV 3. I, II, III and IV correct 4. I and II 5. I, III and V Explanation: There are four forces acting on the car. The ground pushing up on the car balances the gravity pushing down in the vertical di- rection; the friction between the wheels and the ground pushing the car forward balances the air drag pushing back on the car. 004 (part 2 of 3) 10.0 points Is there a net or unbalanced force acting on the car? 1. No; the velocity is constant. correct 2. Yes; a net force in the horizontal direc- tion. 3. Cannot be determined 4. No; the speed is constant. 5. Yes; a net force in the vertical direction....
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This note was uploaded on 10/29/2009 for the course PHY 303K taught by Professor Turner during the Spring '08 term at University of Texas.
- Spring '08