Chapter 11 Hm 7

# Chapter 11 Hm 7 - Solutions to Homework#7 Chapter 11 4...

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Chapter 11 4. Picture the Problem : The arm extends out either horizontally and the weight of the crab trap is exerted straight downward on the hand. Strategy: The torque equals the moment arm times the force according to equation 11-3. In this case the moment arm is the horizontal distance between the shoulder and the hand, and the force is the downward weight of the crab trap. Solution: Multiply the moment arm by the weight: Insight: If the man bent his elbow and brought his hand up next to his shoulder, the torque on the shoulder would be zero but the force on his hand would remain 35 N or 7.9 lb. 14. Picture the Problem : The fish exerts a torque on the fishing reel and it rotates with constant angular acceleration. Strategy: Use table 10-1 to determine the moment of inertia of the fishing reel assuming it is a uniform cylinder ( 2 1 2 MR ). Find the torque the fish exerts on the reel by using equation 11-1. Then apply Newton’s Second Law for rotation (equation 11-4) to find the angular acceleration and equations 10-2 and 10-10 to find the amount of line pulled from the reel. Solution: 1. (a) Use table 10-1 to find I : ( 29 ( 29 2 2 2 1 1 2 2 0.84 kg 0.055 m 0.00127 kg m I MR = = = × 2. Apply equation 11-1 directly to find τ : ( 29 ( 29 0.055 m 2.1 N 0.12 N m r F = = = × 3. Solve equation 11-14 for α : 2 2 0.12 N m 92 rad/s 0.0013 kg m I × = = = × 4. (b) Apply equations 10-2 and 10-10: ( 29 ( 29 ( 29 ( 29 2 2 2 1 1 2 2 0.055 m 92 rad/s 0.25 s 0.16 m s r r t θ = = = = Insight: This must be a small fish because it is not pulling very hard; 2.1 N is about 0.47 lb or 7.6 ounces of force. Or maybe the fish is tired? 20. Picture the Problem : The person lies on a lightweight plank that rests on two scales as shown in the diagram at right. Strategy: Write Newton’s Second Law in the vertical direction and Newton’s Second Law for rotation to obtain two equations with two unknowns, m and cm x . Solve each to find m and cm x . Using the left side of the plank as the origin, there are two torques to consider: 1. (a) Write Newton’s Second Law in the vertical direction to

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Chapter 11 Hm 7 - Solutions to Homework#7 Chapter 11 4...

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