Chapter 7 Hm. 4 Solutions

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

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Solutions to Homework #4 Chapter 7 10. Picture the Problem : The mop head is being pushed downward into the floor. Strategy: Use equation 7-3 keeping in mind the angle between the force and the direction of motion. Solution: 1. (a) Use equation 7-3: ( 29 ( 29 o cos 50.0 N 0.50 m cos55 14 J W Fd θ = = = 2. (b) If the angle is increased to 65°, a smaller component of the force will be along the direction of motion and therefore the work done by the janitor will decrease. Insight: Only the component of the force along the direction of the motion does any work. The vertical component of the force increases the normal force. 22. Picture the Problem : The car slows down as it rolls horizontally a distance of 30.0 m through the sand. Strategy: The kinetic energy of the car is reduced by the amount of work done by friction. The work done is the force times the distance, so once we know the work done and the distance, we can find the force. Solution: 1. (a) The net work done on the car must have been negative since the kinetic energy decreased. 2. (b) The work done by friction equals the average force of friction times the distance the car traveled. Apply equations 7-1 and 7-7, including a minus sign to indicate the force and distance are in opposite directions: ( 29 ( 29 ( 29 ( 29 2 2 1 f i 2 2 2 1 1 f i 2 2 2 2 2 2 1 2 so that 1300 kg 15 18 m /s 2100 N 2.1 kN 30.0 m m v v W Fd K mv mv F d F - = - = ∆ = - = - - = - = = Insight: Kinetic friction always does negative work because the force is always opposite to the direction of motion. The actual force exerted on the car would be −2100 N if the distance traveled is taken to be +30.0 m. 28. Picture the Problem : The work done by the force is the area under the force versus position graph. Strategy:

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## This note was uploaded on 10/17/2011 for the course PHY 101 taught by Professor Ashkenkai during the Fall '08 term at FIU.

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

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