What do the F and d stand for What are the units for F and d What do the W and t stand for What is the unit for t Work is a form of energy What is the unit for work and energy Work and Power ID: 196300
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Bell Work: Work Intro
What do the F and d stand for?What are the units for F and d? What do the W and t stand for?What is the unit for t?Work is a form of energy. What is the unit for work and energy?
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Work and PowerSlide3
Work
Work is a force causing something to move a distance.Work = force x distanceW=f x dUnits for work: Newton-meter (N-m) or Joule Direction of motion and force must be the same.Slide4
Work is NOT
Work does not involve time. Work is not motionless.Slide5
Examples
Example of work: weight lifter pushing barbells from shoulder to over headTwo men moving 20 boxes to another room.Example of NO work: weight lifter holding barbells over his headTwo men taking a break from moving boxesSlide6
Power
Power - how fast work is done.Power is the rate at which work is done, or amount of work done per unit of time.Power = work divided by time P = W/tSlide7
Power Units
Units for power: Newton-meter/second (N-m/s) or Joule/second (J/sec) or Watt (W)Kilowatt (kW) – used to measure large quantities of power like electrical power.1000 W in a kWHorsepower (hp) – also a unit of power. 1 hp = 750 wattsSlide8
Examples
Lifting barbell in 2 secondsTwo men moving 20 boxes to another room in 10 minutes.Slide9
Machines
A machine is a device that makes work easier.Examples: hammer, bicycle ramp, scissors, shovel, and doorknobSlide10
Machines
Machines can do a variety of jobs: pump water from a well, hoist a sail, plow a field, catch a fish.Machines make work easier by changing the size or direction of the applied force.Slide11
Two forces involved in using a machine
1. The force applied to a machine is the effort force (FE)Example: when you pull down on the handle of a crowbar, you are applying an effort force.Slide12
Two forces…
2. The force applied by the machine is the resistance force (FR)Example: Motor turns fanSlide13
Input
Work input (WI) – work done on a machine WI = FE x dEExample: work input of the crowbar (WI) is equal to the force you apply to the handle (FE) times the distance the handle moves (dE)Slide14
Output
Work Output –Work done by a machine . WO = FR x dRExample: The work output of the crowbar (WO) is equal to the force the crowbar exerts on the nail (FR ) times the distance the nail moves (dR).Slide15
What do machines do??
Machines make work easier, but do not multiply work.So, work output can never be greater than work input Slide16
Mechanical Advantage
The number of times a machine multiplies the effort force is called the mechanical advantage (MA)MA = FR/FEExample: if a crowbar allows you to exert only 20 newtons of force to raise a 200 newton object, its mechanical advantage is 10.Slide17
Efficiency
The comparison of work output to work input is called the efficiency of a machine.Efficiency = WO/WI x 100High efficiency means that much of the work input is changed to useful work output.Low efficiency means that much of the work input is lost and a great deal of useful work output does not result.Slide18
Simple Machines
I. Inclined Plane – a slanted surface used to raise an object. Example: a rampWedges and Screws – wedge is an inclined plane that moves. Have an edge that is thinner at one end. Examples: knife, ax, wedge A screw is also an inclined plane. It is an inclined plane wrapped around a cylinder to form a spiral.Slide19
Simple Machines
II. Levers – a bar that is free to pivot a fixed point when an effort force is applied. The fixed point of the pivot is called the fulcrum.Slide20
Levers
****Three classes of levers:Fulcrum is between the effort force and the resistance force. Example: crowbar, pliers, scissors and seesawsResistance force is between fulcrum and effort force. Examples: wheelbarrows, doorsEffort force is greater than the resistance force and the fulcrumSlide21
Simple machines
III. Pulleys – a chain, belt, or rope wrapped around a wheel. A pulley can change either the direction or the amount of an effort force.IV. Wheel and Axle – a lever that rotates in a circle. Made of 2 wheels of different sizes. Examples: bicycles, Ferris wheels, water wheels and gears.