Work & Power Physics 2009

Work In Physics, Work is done when a force moves a body through a distance. WORK = Force x Displacement

Work Units Since, Force  Newton (N) displacement  meter (m) DERIVED unit for work  Newton meter (Nm). The FUNDAMENTAL unit for work is the Joule. Joule (J) = N  m 1 JOULE = the ability to exert a force of 1 NEWTON over a distance of 1 METER.

Work Atlas bears the weight of the world on his shoulders. How much work is done? NONE…… the world did not move…The force must move the object.

Work or No Work Lifting your bookbag  Carrying your bookbag  A waitress carrying a tray around? No work because the supporting force is perpendicular to the motion. Work is done on an object only if the object moves and only if the force and displacement are in the same direction. DIRECTION IS THE KEY TO WORK…. nics/energy/work/work2.gif work no work No work occurs when:

Work Problems 1.A force of 825 N is required to push a car across a lot. Two students push the car 35 m. How much work is done? F= 825 N d = 35 m W = Fd Cutnell & Johnson, Wiley Publishing, Physics 5 th Ed.

Work Problems What work is done by a forklift that raises a 583-kg box 1.2m? m = 583 kg d = 1.2m

Work To move a refrigerator into the back of a moving van. Would you prefer to lift it straight up or walk it up a ramp? Which method required less work? Equal amounts of work are done. The ramp reduces the amount of effort it takes to do work, but DOES NOT CHANGE the amount of work that is done.

Work To calculate the work done in moving the refrigerator into the truck, which displacement do we use? –The displacement from the start of the ramp to the end, or the height of the ramp? Since a force is being applied to move the box upward, only movement in the direction of the force counts, so we use the height of the ramp.

Work Problems An airplane passenger carries a 215N suitcase up a flight of stairs a displacement of 4.2m vertically and 4.6m horizontally. How much work have they done?

Work Only force that is in the same direction of the motion counts. If you pull a wagon at an angle, only the horizontal part of motion counts – only the horizontal part of the pull does any work! Cutnell & Johnson, Wiley Publishing, Physics 5 th Ed.

Work If the angle between force and displacement is θ then F x = F cos θ Then the work done is W = F x x d= F cos θ d Cutnell & Johnson, Wiley Publishing, Physics 5 th Ed. W=Fd (cos θ)

Work W = Fd cos θ If θ = 90 o  W = Fd cos 90 o So, W = 0 Work is a scalar quantity. No direction is associated with work even though it depends on two vector quantities. Copywrited by Holt, Rinehart, & Winston

Copywrited by Holt, Rinehart, & Winston Work Direction Positive and Negative Work When positive work is done on an object, its speed increases; when negative work is done, its speed decreases. Cutnell & Johnson, Wiley Publishing, Physics 5 th Ed.

Work Problems A 10-N frictional force slows a moving block to a stop after a displacement of 5.0 m to the right. What is the net work done on the block? A 10-N force is applied to push a block across a frictional surface at constant speed for a displacement of 5.0 m to the right. What is the net work done on the block?

Work Problems A physics student pulls a box of books as shown. The 455 N force is applied along the rope itself. How much work is done by the student on the books if the box is dragged 24.5 m.

Questionable Work Which ramp requires more work to move the car up a set height? a.30° b.45 ° c.60 ° d.work is equal

Work Done by a Variable Force If the force changes with the displacement, W=Fd is no longer valid –Calculus is needed to calculate this directly However, If force is plotted vs displacement the work can be graphically found as the area under the curve. Constant Force Changing Force

Work Done by a Variable Force Varying Force of a Spring Recall the spring force is F s = kx So, the work is Calculate the work

Work Overview

Review Work is dependent on  force and displacement. The amount of work that is done in moving a 50N box a distance of 30 m is the ______ whether it takes 1 minute, 1 hour, or 1 day. Power is different. It takes ______ power to move that 50 N box 30 meters in 1 minute than it takes to move it the same distance in 1 hour. When you’re talking about power… TIME MATTERS! same more

Power Power is the rate at which work is done. Since work is equal to force x displacement: 

Power Units Derived Units: Fundamental Units: 1 kilowatt = 1 kW = 1000 watts 1 kWh (kilowatt hour) = 3.60 x 10 6 J 1 horsepower (hp) = W

Power Derivation P=Fv

Power Problems A small motor does 4000J of work in 20 seconds. What is the power of the motor in watts? W = 4000 J T = 20 s P = ?

Power Problems An electric motor lifts an elevator that weighs 1.2 x 10 4 N a distance of 9.00 meters in 15.0 s. What is the power of the motor in kilowatts? F = 1.2 x 10 4 N d = 9.00 m t = 15.0 s P = ? (kW)

Power Problems An electric motor develops 65 kW of power as it lifts a loaded elevator 17.5 meters in 35.0 s. How much force does the motor exert? P = 65 kW = 65,000 W d = 17.5 m t = 35.0 s F = ?