CHAPTER 5

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WORK

The transfer of energy to cause or make an object move

WORK

If there is no movement, no work is done

Ex: Lifting a text book (work) vs pushing on a brick wall (no movement = no work)

WORK AND ENERGY

When work is done, a transfer of energy occurs

You become tired when you walk or carry things up a flight of stairs

JOULE

The unit used to express work (J)

HOW DO YOU CALCULATE WORK?

Work = Force X Distance

Or

W = F x D

TRY IT

You apply a force of 10 N to a shopping cart. You moved the cart 10 m. How much work did you do on the shopping cart?

POWER

The rate at which work is done or energy is transferred

WATT

The unit used to express power.

• Symbol for Watt (W)

• Usually written in italics

HOW DO YOU CALCULATE POWER?

Power = Work/Time

Or

P = W/T

TRY IT

It takes you 5 s to do 100 J of work on a shopping cart to move it down the sidewalk. What is your power output?

MACHINES

A device that makes doing work easier

Changes force (increases it)DOES NOT change the amount of work

WORK DONE BY MACHINES

Input force – force that is applied to the machine = Fin

Output force – the force applied by the machine = Fout

WORK INPUT

B/c of friction machines aren‘t 100% efficient

Work Input= force YOU exert on machine = Win

Input Distance = distance YOU are using

Ex: Rowing boatInput force - how hard you pull oarsInput distance - how much oar handles move

WORK OUTPUT

Output Force = force exerted by machine

Output distance = distance machine uses

Work Output = what machine does/accomplishes = Wout

Ex: Rowing boatOutput force - how hard oars push waterOutput distance - distance oar ends move in waterWork Output - moving through water

WORK INPUT AND OUTPUT

CONSERVING ENERGY

When energy is used by a machine, some of the energy is transferred as heat due to friction

Wout is never greater than Win

Wout is always smaller than Win

MECHANICAL ADVANTAGE

Advantage of using a particular machine

Mechanical Advantage Equation:

Mechanical Advantage = output force (in newtons)

input force (in newtons)

OR

MA = Fout / Fin

TRY IT

Calculate the mechanical advantage of a hammer if the input force is 125N and the output force is 2000N.

MECHANICAL EFFICIENCY

Measure of how much of the work put into a machine is changed into useful work output by the machine

Calculating Efficiency:

Efficiency(%) = output work (in joules) x 100%

input work (in joules

OR

efficiency = Wout / Win x 100%

Higher the number, the more efficient

TRY IT

Find the efficiency of a machine that does 800J of work if the input work is 2400J.

TYPES OF MACHINES (SIMPLE MACHINES)6 Simple Machines make other

machines1. Lever Family

1. Lever2. Pulley3. Wheel & axle

2. Inclined Plane Family1. Simple inclined plane2. Wedge3. Screw

LEVERS

Have a rigid arm that turns around a fulcrum

3 classes:1. 1st class2. 2nd class3. 3rd class

1ST CLASS LEVER

Fulcrum in middle of arm

Exs: scissors, pliers, hammer claw, seesaw

2ND CLASS LEVERFulcrum at

one end and force at other end

Load in middle

Exs: wheelbarrow, door, nutcracker

(MA > 1)

3RD CLASS LEVER

Fulcrum at one end, force in middle, and load on other end

Exs: tweezers, biceps

PULLEY

Fulcrum in middle of circle

Lever = rope

More pulleys – easier work

TYPES: A.) Fixed wheel attached in a fixed

positionMA = 1

B.) Movable attached to the object being

moved MA = 2

C.) Block and Tackle

Combination of fixed and movable pulleys

MA = depends on the number of rope segments

WHEEL & AXLE

2 different sized wheels

Axle is fulcrum, wheel is lever

Exs: steering wheel, screw driver

Gears – toothed W & A

MECH. ADV. OF A WHEEL AND AXLE

MA = Radius of Wheel/Radius of Axle

INCLINED PLANE

Spreads work over long distances

Easier to use a long ramp

Exs: stairs, ramps, escalators

MECH. ADV. OF INCLINE PLANES

MA = Length/Height (L/H)

WEDGE

2 inclined planes back to back

Holds together or separates objects

Exs: nails, axes

WEDGES

SCREW

Threads are spiraled incline plane

Exs: jar lid, spiral staircase

SCREW

COMPOUND MACHINE

More than 1 simple machine together

Ex: scissors - lever (handles) and wedge (blade)