CHAPTER 5 WORK AND MACHINES
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 = F in Output force – the force applied by the machine = F out
WORK INPUT B/c of friction machines aren‘t 100% efficient Work Input= force YOU exert on machine = W in Input Distance = distance YOU are using Ex: Rowing boat Input force - how hard you pull oars Input 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 boat Output force - how hard oars push water Output distance - distance oar ends move in water Work 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 W out is never greater than W in W out is always smaller than W in
MECHANICAL ADVANTAGE Advantage of using a particular machine Mechanical Advantage Equation: Mechanical Advantage = output force (in newtons) input force (in newtons) OR MA = F out / F in
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 = W out / W in 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 machines 1.Lever Family 1.Lever 2.Pulley 3.Wheel & axle 2.Inclined Plane Family 1.Simple inclined plane 2.Wedge 3.Screw
LEVERS Have a rigid arm that turns around a fulcrum 3 classes: 1.1st class 2.2nd class 3.3rd class
1ST CLASS LEVER Fulcrum in middle of arm Exs: scissors, pliers, hammer claw, seesaw
2ND CLASS LEVER Fulcrum 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 position MA = 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)