Bell Work: Work Intro
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Transcript of Bell Work: Work Intro
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Bell Work: Work Intro1. What do the F and d stand for?2. What are the units for F and d?3. What do the W and t stand for?4. What is the unit for t?5. Work is a form of energy. What is the unit for
work and energy?
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Work and Power
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Work Work is a force causing something to move a
distance. Work = force x distance W=f x d Units for work: Newton-meter (N-m) or
Joule Direction of motion and force must be the
same.
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Work is NOT Work does not involve time. Work is not motionless.
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Examples Example of work:
weight lifter pushing barbells from shoulder to over head
Two men moving 20 boxes to another room.
Example of NO work: weight lifter holding barbells over his head Two men taking a break from moving boxes
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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/t
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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 kW Horsepower (hp) – also a unit of power. 1 hp
= 750 watts
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Examples Lifting barbell in 2 seconds Two men moving 20 boxes to another room in
10 minutes.
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Machines
A machine is a device that makes work easier.
Examples: hammer, bicycle ramp, scissors, shovel, and doorknob
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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.
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Two forces involved in using a machine1. 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.
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Two forces…2. The force applied by the machine is the
resistance force (FR) Example: Motor turns fan
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Input Work input (WI) – work done on a machine WI = FE x dE
Example: work input of the crowbar (WI) is equal to the force you apply to the handle (FE) times the distance the handle moves (dE)
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Output Work Output –Work done by a machine . WO = FR x dR
Example: 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).
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What do machines do?? Machines make work easier, but do not
multiply work. So, work output can never be greater than
work input
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Mechanical Advantage The number of times a machine multiplies
the effort force is called the mechanical advantage (MA)
MA = FR/FE
Example: if a crowbar allows you to exert only 20 newtons of force to raise a 200 newton object, its mechanical advantage is 10.
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Efficiency The comparison of work output to work
input is called the efficiency of a machine. Efficiency = WO/WI x 100 High 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.
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Simple MachinesI. Inclined Plane – a slanted surface used to
raise an object. Example: a ramp Wedges 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.
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Simple MachinesII. 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.
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Levers****Three classes of levers: Fulcrum is between the effort force and the
resistance force. Example: crowbar, pliers, scissors and seesaws
Resistance force is between fulcrum and effort force. Examples: wheelbarrows, doors
Effort force is greater than the resistance force and the fulcrum
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Simple machinesIII. 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.