6 h phy student notes Work and Energy Chapter 6 Cutnell (1).docx

6 h phy student notes Work and Energy Chapter 6 Cutnell (1).docx

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Work and Energy Chapter 6 6.1 Work Done by a Constant Force According to physics you and I do very little work. To do work you need to apply a force to move something . A) Work in the same direction as force Work- the product of the amount of force exerted on an object moving in the dimension or plane compared to the force. W=(F cos θ ) d F= constant force (Newtons) W= work (Joules) d or s = displacement(meter) ( your book denotes it as s) If force and distance moved are in the same direction θ = 0 therefore cos 0 =1. 1) It takes 422N of force for a student to push a car 62m. How much work was done? (Force is in the same direction as work therefore θ =0) W=(F cos θ )d W=422 N (cos 0)62m W= 26000 J 2) It takes 38N of force for a student to push a car 12m. How much work was done? W=F d W=38 N (12m ) W= 460 J 3) During a tug-of-war, team A pulls on Team B by applying a force of 800. N to the rope between them. How much work does team A do if they pull B toward them a distance of 4.00 m? W=(F cos θ ) d W = 800.N (cos 0) 4.00 m W = 3.20 x 10 3 J 4) During a tug-of-war, team A pulls on Team B by applying a force of 320. N to the rope between them. How much work does team A do if they pull them B toward them a distance of 6.20 m? W=F d W = 320N ( 6.20 m) W = 1980 J 5) What work is done by a forklift raising a 1.52 kg box 2.40 m at a constant speed? Fnet= F pulling up + F (-gravity) O= F up + F(- gravity) F up=F gravity Work up=Work gravity (Remember you need force, but you have mass instead) Force is up and direction the object moves is up so θ = 0 W=(F cos θ ) d F = m g W = 14.9N (cos 0)2.40 m F = 1.52 kg (9.807 m) W = 35.7 J F = 14.9 6) What work is done by a forklift raising a 2.83 kg box 4.62 m? (Remember you need force, but you have mass) W=F d F = m g W = 27.75N (4.62m) F = 2.83 kg (9.807 m) W = 128 J Only the amount of force that is in the same dimension or plane of movement is considered work.
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7) A student removes the 15.5-kg stereo amplifier from a shelf that is 1.22 m high. The amplifier is lowered at a constant speed to a height of 0.600 m. What is the work done by (a) the person and (b) the gravitational force that acts on the amplifier? (When the person lowers the amplifier they actually pull upward and the direction is downward) a) The person is lowering the amplifier so work is negative. W=F (cos θ ) d If θ is 0° when upward then it is 180 ° going downward W = 152N(cos 180) (.62m) W = - 94.2 J b) W=F(cos θ ) d W = 152N (cos 0 )(.62m) W = 94.2 J 8) A student removes the 19.0-kg stereo amplifier from a shelf that is 2.66 m high. The amplifier is lowered at a constant speed to a height of 0.400 m. What is the work done by (a) the person and (b) the gravitational force that acts on the amplifier? a) W=F(cos θ ) d W = 186 N (cos 180) (2.66m - .400m) W = - 420. J b) W = 420. J B) Work when not all force is in the same dimension as movement 9) A delivery clerk carries a 23.0 N package up 4 flights of stairs totaling a height of 12.0m. How much work is done? The person is exerting an upward force so only the distance in the upward direction is considered work.
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