Post on 16-Dec-2015
CHAPTER 5
WO
RK
AN
D M
AC
HI N
ES
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)