Work, Power & Energy
Work W F d -Work is the transfer of energy through motion Units: Newton•meter (Joule) 1 J=the amount of work done pushing with a force of 1 N over a distance of 1 meter W F d
Work = F x d The amount of work done depends on two things: 1.) the amount of force exerted and 2.)the distance over which the force is applied. two factors to keep in mind when deciding when work is being done: 1. something has to move and 2.the motion must be in the direction of the applied force.
Work Work is done on the books when they are being lifted, but no work is done on them when they are being held or carried horizontally.
Work A force has to be applied in the same direction that you want the object to move in order for work to be done Some situations involve work done by or against the force of gravity (g). To lift something you need to apply an upward forcethat force is equal to the object’s weight in Newtons (force due to gravity) (Fg=mg)
Calculate Work 1.) During the ascent phase of a rep of the bench press, the lifter exerts an average vertical force of 1000 N against a barbell while the barbell moves 0.8 m upward How much work did the lifter do to the barbell?
Calculate Work 2.) Lucas wants to bench press 4600 Kg worth of weight 0.5 km off the ground. How much work will he do?
Work performed climbing stairs Amber runs ups the steps as her daily cardio exercise. Her mass is 65 kg and each step is 8 inches high. Is she doing work as she runs up the stairs? Calculate the amount of work being done as she runs up the 150 step flight of stairs: (1 m= 39.37 in) W=Fd Is she doing work as she descends? Why?
Calculating Work Practice Alexander has a mass of 70kg. His apartment is on the 2nd floor, 5 meters up from ground level. How much work does he do against gravity each time he climbs the stairs to his apartment? You did 190 Joules of work lifting a 30 newton backpack. How high did you lift the backpack? 3. Which person did the most work: John who lifts a 5.78 N dog a distance of 8 meters OR Sarah who pushes a 10.54 N dog a distance of .35 inches?
Power *Maximum power output of a human is 200 watts, but can only keep up this power for a short amount of time.*A trained athlete can sustain 350 watts of power for about 1hr. Power is the rate at which work is done the rate at which energy flows Unit for power is joules per second (1J/s)= 1watt Another unit for power is horsepower the power of an engine described in terms of how many horses it could replace 1horsepower= 746 watts Michael and Jim are both lifting 100 N of weight. How much work is each boy exerting? How much power is each boy exerting? Why is this person exerting (using) more power to lift the same amount of weight?
Calculate & compare power During a powerlifting competition, two lifters each exert an average vertical force of 1000 N against the barbell while the barbell moves 0.8 m upward. Lifter A does this in 0.50 seconds while Lifter B does it in 0.75 seconds. Are they both doing the same amount of work? Why? Who is exerting more power? Why?
Calculating Power Practice A roller coaster is pulled up a hill by a chain attached to a motor. The roller coaster has a total mass of 10,000kg. If it takes 20 seconds to pull the roller coaster up a 50 meter hill, how powerful is the motor? What would the motor’s power be if it took 40 seconds to pull the same roller coaster up? What is the power of a 70kg person who climbs a 10 meter high hill in 45 seconds? Video
Work/Power Review Questions 1.) How much work is done by a crane that lifts 1,000 newtons of material a distance of 150 meters? 2.) How much work is done when a 1 kilogram mass is raised a vertical distance of 1 meter? 3.) A 49 newton rock is lifted 2 meters in 5 seconds. a. How much work is done? b. What power is used? 4.) A teacher pushed a 98 newton desk across a floor for a distance of 5 meters. He exerted a horizontal force of 20 newtons for four seconds. How much work was done? b. What was his power?
Work and Power Partner Practice 1.) You must exert a force of 4.5 N on a book to slide is across a table. If you do 2.7 J of work in the process, how far have you moved? 2.) A child pulls a sled up a snow covered hill. The child does 405 J of work on the sled. If the child walks 15 m up the hill, how large of a force must the child exert? 3.) Ho much work is done on a small car if a 3150 N force is exerted to move it 75.5 m to the side of the road? 4.) A 190,000 W engine can accelerate from rest to a stop speed in 9 seconds. How much work did the engine do? 5.) If a runner exerts 350 J of work to make 125 W of power, then how long did it take the runner to do the work?
Work and Power Solo Practice 1.) How much work is done when a 5 N force moves a block 4 m? 2.) Fred applies 350 N of force to move his stalled car 40 m, how much work did Fred do? 3.) How far will 350 J of work raise a block weighing 7 N? 4.) How much power does it take to lift 30 N 10 meters high in 5 seconds? 5.) How much work can be down by a 300 W motor in 60 seconds?