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Work, Power, and Machines
9.1

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

Work Work is measured in: Nm Joules (J)

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

Work Example Work = Fd Work = (5200 N)(25m) Work = 130000 N  m
= J

Power A quantity that measures the rate at which work is done
Power = work/time P = W/t

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

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?

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

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

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

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

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

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

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

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

Simple Machines 9.2

Simple Machines Most basic machines Made up of two families Levers
Inclined planes

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

The Lever Family Levers are divided into three classes
Classes depend on the location of the fulcrum and the input/output forces.

First Class Levers Have fulcrum in middle of arm.
The input/output forces act on opposite ends Ex. Hammer, Pliers

First Class Levers Input Force Output Force Fulcrum

Second Class Levers Fulcrum is at one end.
Input force is applied to the other end. Ex. Wheel barrow, hinged doors, nutcracker

Second Class Levers Output Force Fulcrum Input Force

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

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

Third Class Levers Output distance Input Force Fulcrum

Pulleys Act like a modified member of the first-class lever family
Used to lift objects

Pulleys Output Force Input force

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

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

Functions like two inclined planes back to back
A Wedge Functions like two inclined planes back to back

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

Or Wedge Antilles from Star Wars

Not to be mistaken with a wedgIEEEEE

Inclined plane wrapped around a cylinder
A Screw Inclined plane wrapped around a cylinder

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

Compound Machines A machine that combines two or more simple machines
Ex. Scissors, bike gears, car jacks

Energy

Energy and Work Energy is the ability to do work
whenever work is done, energy is transformed or transferred to another system.

Energy Energy is measured in: Joules (J)
Energy can only be observed when work is being done on an object

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

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

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

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

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

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?

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

Kinetic Energy the energy of a moving object due to its motion.
depends on an objects mass and speed.

Kinetic Energy What influences energy more: speed or mass?
ex. Car crashes Speed does

Kinetic Energy Equation
KE=1/2 x mass x speed squared KE = ½ mv2

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

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

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

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

Atoms Atoms have KE, because they at constantly in motion.
KE  particles heat up KE  particles cool down

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

Other Forms nuclear fusion nuclear fission Electricity Light

Energy Transformations
9.4

Conservation of Energy
Energy is neither created nor destroyed Energy is transferred

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

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

Physics of roller coasters

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