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13-Work and Energy

13-Work and Energy. 1-Work, Power, and Machines Work: The transfer of energy to an Object by the application of force that Causes the object to move in the Direction of the force. Work = force * distance w = F * d Unit of work : work is measured in “ Joule ”

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13-Work and Energy

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  1. 13-Work and Energy 1-Work, Power, and Machines Work: The transfer of energy to an Object by the application of force that Causes the object to move in the Direction of the force. Work = force * distance w = F * d Unit of work: work is measured in “Joule” Work zero when object is not moving on applying force. Work is zero when force is not applied even if an object is moving certain distance.

  2. Math skills- work • A crane uses an average force of 5200 N to lift a girder 25 m. How much work does the crane do on the girder? • Given: F = 5200 N, distance (d) = 25 m. • Write the required equation, W = F x d • Insert the known values and solve. w = 5200 N x 25m = 130,000 N.m W=F x dF = W/dd = W/F

  3. Math skills-Work • A mechanic uses a hydraulic lift to raise a 1,200 kg car 0.50 m off the ground. How much work does the lift do on the car? • Given: mass of car=1200 kg, convert into weight. Weight = mass x g (constant value) • Weight = m x g(9.8m/s2)=1200 kg x 9.8 m/s2 = 17760 kg.m/s2 • The value of weight is equal to the amount of force. Therefore, W = F x d W = 17760 N X 0.50m = 88830 N.m

  4. Power • Power: A quantity that measures the rate at which work is done or energy is transformed. In other words. • “ How much work is done in a given amount of time.” Equation: Power = work/time, p = w/t • Power is measured in unit - “watts” • Watt is a SI-unit. One watt equals one joule of work is done in one second. 1watt = 1 j/s • Another unit is : “Horse power”  One horse power is equal to 746 watts. As you might have guessed, one horse power was originally the average power output of a horse.

  5. Math skills: Power • p = W/t W = p . t t = W/p • Lisa walks up the stairs on her way to class. She weighs 565 N, and the stairs go up 3.25 m vertically. If Lisa climbs the stairs in 12.6 s, what is her power output? and If time is 10.6 s ? • Given: F = 565 N, d = 3.25 m, t =12.6 s and 10.6 s • Write required equation: p = W/t but W = F.d • Insert the known values and solve. p = F*d/t = 565 N*3.25 m/12.6 s = N.m/s p = F*d/t = 565 N*3.25 m/10.6 s = N.m/s

  6. Machine; A device with moving parts that work together to accomplish a task. Example: a bicycle All parts of bicycle work together to transform forces from muscles to speed and motion. Machines make work easier by increasing the input force,& changing the direction of force. Machines & Mechanical Advantage

  7. Mechanical Advantage • Mechanical Advantage: It is defined as the ratio between the output force and input force, it is also equal to the ratio between the input distance and the output distance if friction is ignored. Equation formula to calculate MA is, • MA =Output force/input force • MA = Input distance/output distance • MA = FO/Fiand MA = di/do • Input force: The force you apply on machine • Output force: The force that machines apply for you.

  8. Math skills: calculate-MA • Calculate the MA of a ramp that is 5.0 m long and 1.5 m high. MA = di/do di=MA * do Given: Input distance=5.0 m, do=di/MA Output distance=1.5 m MA = input distance/output distance = 5.0 m/1.5 m = 3.3 Mechanical advantage has no unit, because the similar units cancelled out. It is simply a numeric number. MA = Fo/Fi Fi = Fo/MA Fo = Ma * Fi

  9. Math skills: • Alex pulls on the handle of a claw hammer with a force of 15 N. If the hammer has a mechanical advantage of 5.2, how much force is exerted on the nail in the claw? • Given: input force Fi=15 N, MA = 5.2, Fo = ? • Required equation is, Fo= MA * Fi • Insert the values and solve. Output force = 5.2 * 15 N = 78 N • For more practice, visit go.hrw.comand enter Keyword HK8MP

  10. Home work • Page # 432 do practice problems 2,3 • Page # 434 do practice problems 1 • Solve all problems from given work sheet • Page 437 Section 1- review Answer 1 t0 4 and math skills, solve problems 5 to 8

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