PS Ch. 12 Notes.notebook

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Chapter 12 - Work and Energy

Section 1 - Work, Power, and Machines

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Imagine trying to lift a car without a jack

You might be exerting a lot of force, but not moving the

It would feel like you have done a lot of work...but youhave actually done NONE

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In science, is done only when a force causes a

change in the position or motion of an object

work = force x distance

So what are the units we measure work in?

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Work is measured in a unit called

1 J = 1 Nm = 1 kgm

Imagine a father playing with his daughter by lifting her

repeatedly in the air. How much work does he do with each

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Imagine walking up a flight of stairs vs. running up a

Which is more exhausting?

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The amount of time it takes to do work is animportant factor

measures work in relation to time

power = work/time

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So what are our units for power?

Joules/second = Watt

watt (W) is the amount of power required to do 1 J of

This is about how much power you would need to lift an

apple over your head in 1 second

746 watts = 1 horsepower

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It takes 100kJ of work to lift an elevator 18 m. If

this is done in 20 seconds, what is the average power

of the elevator during this process?

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Which is easier...lifting a car with a jack, or by yourself?

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Compare the amount of work required to lift a four-wheeler straight up onto the bed of a pickup, with

the amount of work required to push the same four-

The ramp allows you to apply a smaller

larger distance, but the work done is the same

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allow us to do the same amount of work by

the distance while

force or by

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a machine describes how

mechanical advantage =input force

input distance

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A machine with a mechanical advantage greater than 1

multiplies the input force

force, but increases distance and speed

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Calculate the mechanical advantage of a ramp that is

5.0 meters long and 1.5 meters high

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Section 2 - Simple Machines

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The Lever Family

Levers are divided into three classes:

1st, 2nd, and 3rd Class

All levers have a rigid that pivots around a

point called the fulcrum

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have a fulcrum located betweenthe input and output forces

Examples: Seesaw, hammer

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the arm and the input force is applied at the other

Examples: Wheelbarrow, two wheel cart (dolly)

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Third-Class Levers have a fulcrum at one end with

the output force on the other end.

They multiply distance rather than force.

Therefore have a mechanical advantage of less than 1

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Pulleys are modified levers because the pulley

itself is like the fulcrum of a lever

A single fixed pully has a mechanical advantage

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A single movable pulley has a mechanical advantage of 2

Therefore you can lift a 100 pounds with only exerting

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Multiple pulleys can be used to get even higher

mechanical advantages

When multiple pulleys are put together in a singleunit it is called a

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wheel and axle is a lever or pulley connected to a shaft

A small input force can be applied and the force is

multiplied to become a large output force on the shaft

Steering wheel, screwdrivers, cranks

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The Inclined Plane Family

In inclined plane is basically a ramp

The input force is parallel to the ramp

object off the ground

Output Force

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Remember, incline planes work by spreading

the work out over a larger distance, which

decreases the force required!

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is a modified inclined plane

A wedge is two incline planes placed back to back

The wedge turns a single downward force into twoforces directed out to the sides

Examples: Axe, nail

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is an inclined plane wrapped around a cylinder

The steeper the threads, the more force required to

Examples: Screws, jar lids, spiral staircase

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combines two or more simple

machines

A scissor: Uses two first class levers joined at a

single fulcrum AND a wedge that cuts into the paper

Bicycle: Wheel and axle, levers

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Section 3 - What is Energy?

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Remember, work is only done when an object

experiences a change in its position or motion

However, energy can be present in an object when

is the ability to do work

Energy and work are both expressed in units of

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A stretched rubber band (say on a slingshot) has

stored energy

Potential Energy

Often called "energy of position"

Think of an apple hanging form a tree...the energythat could potentially do work on the apple results

from its position above the ground

This is called gravitational potential energy

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The equation to calculate gravitational potential energy is:

Potential energy = mass x free fall acceleration x height

PE = mgh

This means that potential energy is dependent on mass and

height

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A 65 kg rock climber ascends a cliff. What is the

climber's gravitational potential energy at a point 35 m

above the base of the cliff?

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is the energy an object has because

of its motion

starts to fall it gains kinetic energy

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The equation for calculating kinetic energy is:

Kinetic energy = 1/2 x mass x speed squared

kinetic energy?

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What is the kinetic energy of a 44 kg cheetah running

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The sum of potential energy and kinetic energy is

mechanical energy

Are there any other forms of energy?

We eat it for energy right?

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Energy that does not affect the motion of an object is

nonmechanical energy

Atoms and molecules have kinetic energy because they are

moving on an atomic scale

Chemical reactions involve potential energy

- Chemical bonds store energy

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The sun gets its energy from nuclear reactions

- Nuclear fusion, nuclear fission

Electricity is a form of energy

lectromagnetic waves

carries energy across empty space