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**Work, Power, and Machines**

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Work A quantity that measures the effects of a force acting over a distance Work = force x distance W = Fd

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Work Work is measured in: Nm Joules (J)

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Work Example A crane uses an average force of 5200 N to lift a girder 25 m. How much work does the crane do?

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**Work Example Work = Fd Work = (5200 N)(25m) Work = 130000 N m**

= J

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**Power A quantity that measures the rate at which work is done**

Power = work/time P = W/t

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Power Watts (W) is the SI unit for power 1 W = 1 J/s

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Power Example While rowing in a race, John uses 19.8 N to travel meters in 60.0 s. What is his power output in Watts?

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**Power Example Work = Fd Power = W/t Power = 3960 J/60.0 s**

Work = 19.8 N x m= 3960 J Power = W/t Power = 3960 J/60.0 s Power = 66.0 W

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Machines Help us do work by redistributing the force that we put into them They do not change the amount of work

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**Change the direction of an input force (ex car jack)**

Machines Change the direction of an input force (ex car jack)

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Machines Increase an output force by changing the distance over which the force is applied (ex ramp) Multiplying forces

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Mechanical Advantage A quantity that measures how much a machine multiples force or distance.

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**Mechanical Advantage Input distance Mech. Adv = Output Distance**

Output Force Mech. Adv. = Input Force

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Mech. Adv. example Calculate the mechanical advantage of a ramp that is 6.0 m long and 1.5 m high.

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**Mech. Adv. Example Input = 6.0 m Output = 1.5 m Mech. Adv.=6.0m/1.5m**

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Simple Machines 9.2

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**Simple Machines Most basic machines Made up of two families Levers**

Inclined planes

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The Lever Family All levers have a rigid arm that turns around a point called the fulcrum.

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**The Lever Family Levers are divided into three classes**

Classes depend on the location of the fulcrum and the input/output forces.

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**First Class Levers Have fulcrum in middle of arm.**

The input/output forces act on opposite ends Ex. Hammer, Pliers

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First Class Levers Input Force Output Force Fulcrum

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**Second Class Levers Fulcrum is at one end.**

Input force is applied to the other end. Ex. Wheel barrow, hinged doors, nutcracker

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Second Class Levers Output Force Fulcrum Input Force

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Third Class Levers Multiply distance rather than force. Ex. Human forearm

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Third Class Levers The muscle contracts a short distance to move the hand a large distance

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Third Class Levers Output distance Input Force Fulcrum

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**Pulleys Act like a modified member of the first-class lever family**

Used to lift objects

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Pulleys Output Force Input force

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The Inclined Plane Incline planes multiply and redirect force by changing the distance Ex loading ramp

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The Inclined Plane Turns a small input force into a large output force by spreading the work out over a large distance

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**Functions like two inclined planes back to back**

A Wedge Functions like two inclined planes back to back

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A Wedge Turns a single downward force into two forces directed out to the sides Ex. An axe , nail

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Or Wedge Antilles from Star Wars

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Not to be mistaken with a wedgIEEEEE

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**Inclined plane wrapped around a cylinder**

A Screw Inclined plane wrapped around a cylinder

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A Screw Tightening a screw requires less input force over a greater distance Ex. Jar lids

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**Compound Machines A machine that combines two or more simple machines**

Ex. Scissors, bike gears, car jacks

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Energy

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**Energy and Work Energy is the ability to do work**

whenever work is done, energy is transformed or transferred to another system.

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**Energy Energy is measured in: Joules (J)**

Energy can only be observed when work is being done on an object

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Potential Energy PE the stored energy resulting from the relative positions of objects in a system

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Potential Energy PE PE of any stretched elastic material is called Elastic PE ex. a rubber band, bungee cord, clock spring

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Gravitational PE energy that could potentially do work on an object do to the forces of gravity.

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Gravitational PE depends both on the mass of the object and the distance between them (height)

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**Gravitational PE Equation**

grav. PE= mass x gravity x height PE = mgh or PE = wh

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PE Example A 65 kg rock climber ascends a cliff. What is the climber’s gravitational PE at a point 35 m above the base of the cliff?

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**PE Example PE = mgh PE=(65kg)(9.8m/s2)(35m) PE = 2.2 x 104 J**

PE = J

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**Kinetic Energy the energy of a moving object due to its motion.**

depends on an objects mass and speed.

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**Kinetic Energy What influences energy more: speed or mass?**

ex. Car crashes Speed does

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**Kinetic Energy Equation**

KE=1/2 x mass x speed squared KE = ½ mv2

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KE Example What is the kinetic energy of a 44 kg cheetah running at 31 m/s?

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KE Example KE = ½ mv2 KE= ½(44kg)(31m/s)2 KE=2.1 x 104 J KE = J

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Mechanical Energy the sum of the KE and the PE of large-scale objects in a system work being done

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Nonmechanical Energy Energy that lies at the level of atoms and does not affect motion on a large scale.

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**Atoms Atoms have KE, because they at constantly in motion.**

KE particles heat up KE particles cool down

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Chemical Reactions during reactions stored energy (called chemical energy)is released So PE is converted to KE

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Other Forms nuclear fusion nuclear fission Electricity Light

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**Energy Transformations**

9.4

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**Conservation of Energy**

Energy is neither created nor destroyed Energy is transferred

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**Energy Transformation**

PE becomes KE car going down a hill on a roller coaster

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**Energy Transformation**

KE can become PE car going up a hill KE starts converting to PE

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**Physics of roller coasters**

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