Work, Power, and Machines

9.1

WorkA quantity that measures the effects of a force acting over a distance

Work = force x distanceW = Fd

Work

Work is measured in:NmJoules (J)

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

Work ExampleWork = FdWork = (5200 N)(25m)Work = 130000 N m

= 130000 J

PowerA quantity that measures the rate at which work is done

Power = work/timeP = W/t

Power

Watts (W) is the SI unit for power

1 W = 1 J/s

Power ExampleWhile rowing in a race, John uses 19.8 N to travel 200.0 meters in 60.0 s. What is his power output in Watts?

Power ExampleWork = Fd Work = 19.8 N x 200.0 m= 3960 J

Power = W/tPower = 3960 J/60.0 sPower = 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

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

Output Force

Input Force

Input distance

Output DistanceMech. Adv =

Mech. Adv. =

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

Mech. Adv. ExampleInput = 6.0 mOutput = 1.5 mMech. Adv.=6.0m/1.5mMech. Adv. = 4.0

Simple Machines

9.2

Simple MachinesMost basic machines Made up of two familiesLeversInclined planes

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

The Lever FamilyLevers are divided into three classes

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

First Class LeversHave fulcrum in middle of arm.

The input/output forces act on opposite ends

Ex. Hammer, Pliers

First Class LeversOutput Force Input Force

Fulcrum

Second Class LeversFulcrum is at one end.Input force is applied to the other end.

Ex. Wheel barrow, hinged doors, nutcracker

Second Class Levers

Output Force

Input Force

Fulcrum

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 ForceFulcrum

PulleysAct like a modified member of the first-class lever family

Used to lift objects

Pulleys

Input forceOutput 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

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

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

9.3-9.4

Energy and WorkEnergy is the ability to do work

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

EnergyEnergyEnergy 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

PotentialPotential Energy PEEnergy PEPE of any stretched elastic material is called Elastic PE

ex. a rubber band, bungee cord, clock spring

Gravitational PEGravitational PEenergy that could potentially do work on an object do to the forces of gravity.

Gravitational PEGravitational PEdepends 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 ExamplePE = mghPE=(65kg)(9.8m/s2)(35m)PE = 2.2 x 104 JPE = 22000 J

Kinetic Energythe energy of a moving object due to its motion.

depends on an objects mass and speed.

Kinetic EnergyWhat influences energy more: speed or mass?

ex. Car crashesSpeed 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 ExampleKE = ½ mv2

KE= ½(44kg)(31m/s)2

KE=2.1 x 104 JKE = 21000 J

Mechanical EnergyMechanical 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 upKE particles cool down

Chemical Reactions

during reactions stored energy (called chemical energy)is released

So PE is converted to KE

Other FormsOther Forms

nuclear fusion nuclear fission ElectricityLight

Energy Transformations

9.4

Conservation of Energy

Energy is neither created nor destroyed

Energy is transferred

Energy Transformation

PE becomes KEcar going down a hill on a roller coaster

Energy Transformation

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

Physics of roller coasters http://www.funderstanding.com/k12/

coaster/