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 = 130000 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 200.0 meters in 60.0 s. What is his power output in Watts?

Power Example „Work „ Work

= Fd

= 19.8 N x 200.0 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

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

Mech. Adv =

Input distance Output Distance

Mech. Adv. =

Output Force 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 „Mech. Adv. = 4.0

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 Output Force

Input 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

Fulcrum

Input Force

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

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 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 = 22000 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

2 mv

KE Example „What

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

KE Example „KE

=½

„KE=

2 mv

2 ½(44kg)(31m/s)

„KE=2.1

4 10

x J „KE = 21000 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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