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
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?
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 INPUTB/c of friction machines aren‘t 100%
efficientWork Input= force YOU exert on machine
= WinInput Distance = distance YOU are using
Ex: Rowing boatInput force - how hard you pull oarsInput distance - how much oar handles move
WORK OUTPUTOutput Force = force exerted by machine Output distance = distance machine usesWork 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
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 WinWout is always smaller than Win
MECHANICAL ADVANTAGEAdvantage 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 EFFICIENCYMeasure 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
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
2ND CLASS LEVERFulcrum at
one end and force at other end
Load in middleExs:
wheelbarrow, door, nutcracker
(MA > 1)
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
INCLINED PLANE
Spreads work over long distances
Easier to use a long ramp
Exs: stairs, ramps, escalators