Chapter 15 Chapter 15 – Work, Power & Chapter 15 – … 15 Chapter 15 – Work, Power & Simple...
Transcript of Chapter 15 Chapter 15 – Work, Power & Chapter 15 – … 15 Chapter 15 – Work, Power & Simple...
Chapter 15 – Work, Power &Simple Machines
Chapter 15 Chapter 15 –– Work, Power & Work, Power &Simple MachinesSimple Machines
Essential Questions:I. What is Work? (In Physics Terms!)II. What is Power? (In Physics Terms!)III. How do machines make workeasier and how efficient are they?IV. What are the 5 types of simplemachines?V. What are compound machines?
15-1 What is Work? Work
Def. – Work is done when a forceacts on an object along theparallel direction the objectmoves
In order for work to be done, aforce must be exerted over adistance.Ex – you can push on a wall for
hours, you’ll be real tired, butyou haven’t done any work – inthe scientific sense, anyway…
15-1 Work Work
The amount of work done in moving anobject is equal to the force applied tothe object along the direction theobject moves times the distancethrough which the object moves
Distancex Force Work =
Units Force is measured in Newtons, Distance
is measured in meters. So, the unit isNewton X meters. A Newton•meter isknown as a Joule (J)
15-1 Work A 700 N person climbs a 50 m cliff. How
much work did she perform?
GIVEN:W = F * dF = 700 Nd = 50 m
WORK:W = F * dW = (700 N) (50 m)W = 35,000 J
15-1 Work An object weighing 200 N is lifted 0.5 m.
How much work was required?
GIVEN:W = F * dF = 200 Nd = 0.5 m
WORK:W = F * dW = (200 N) (0.5 m)W = 100 J
15-1 Work A dog does 50 N-m (Joules) of work
dragging a 0.05 N bone. How far did thebone move?
GIVEN:W = F * dW = 50 JF = 0.05 N
WORK:W = F * dd = W Fd = (50 J) (0.05 N)d = 1,000 m
15-1 Work Mrs. O’Gorman’s superhuman strength
allows her to lift a pickup truck 2.0 m abovethe ground. How much force was required if25.0 Joules (J) of work was done?
GIVEN:W = F * dW = 25.0 Jd = 2.0 m
WORK:W = F * dF = W
dF = 25.0 J
2.0 mF = 12.5 N
15-2 Power Power
Def: The rate at which work is done, orthe amount of work per unit time.
Power tells you how fast work is beingdone – so it is a rate – similar to the wayspeed, velocity and acceleration arerates. Power is work per unit time.
Any measurement per unit time is arate!!
Formula:
Time Work Power =
15-2 PowerPower
rate at which work is done measured in watts (W)
tWP =
P: power (W)W: work (J)t: time (s)
15-2 PowerFormula:Since work’s formula is force X Distance,
the formula for Power may ALSO bewritten as:
Time Distancex Force Power =
15-2 PowerUnits
Work is measured inJoules (J), So, the unit forPower is a Joule persecond (J/s).
The short way to write aJ/s is a Watt (W).
15-2 PowerWhen do we use Watts in our
Daily Lives?They are used to express
electrical power.Electric appliances and
lightbulbs are rated in Watts.Ex: A 100 Watt light bulb does
twice the work in one second asa 50 Watt lightbulb.
15-2 Power A small motor does 4000 J of work in
20 sec. What is the power of themotor in Watts?
GIVEN:W = 4000 JT = 20 secP = ?
WORK:P = W ÷ tP = 4000 J ÷ 20 sP = 200 J ÷ sSo P = 200 W
15-2 Power
GIVEN:P = 2400 WW = 120,000 JT = ?
WORK:t = W ÷ Pt = 120,000 J ÷ 2400 Wt = 50 sec
An engine moves a remote control carby performing 120,000 J of work. Thepower rating of the car is 2400 W.How long does it take to move the car?
15-2 Power
GIVEN:P = ?F = 450 Nd = 1.5 mt = 3.0 sec
WORK:
A figure skater lift his partner whoweighs 450 N, 1.5 m in 3.0 sec. Howmuch power is required?
PF x d
tP = F x d tP = 450 N x 1.5 m
3.0 secP = 675 J (N•m)
3.0 sec P = 225 W
15-2 Power A sumo wrestler lifts his competitor, who
weighs 300 N, 2.0 m using 300 Watts ofpower. How long did it take him toaccomplish this show of strength?
GIVEN:F = 300 Nd = 2.0 mP = 300 Wt = ?
WORK:P = W ÷ tW = F x dW = (300 N)(2.0 m) = 600 Jt = 600 J ÷ 300 Wt = 2.0 s
PW
t
15-3 Work Input & Work Output
Machine – def. – Any device thatchanges the size of a force, orits direction, is called a machine.
Machines can be anything froma pair of tweezers to a bus.
15-3 Work Input & Work Output
There are always 2 types ofwork involved when using amachineWork Input - The work that
goes into it.Work Output - The work that
comes out of it.The work output can NEVER be
greater than the work input!!!
So, if machines do not increasethe work we put into them, howdo they help us?
Machines make work easierbecause they change either thesize or the direction of the forceput into the machine.
15-3 Work Input & Work Output
Let’s analyze this…Machines can not increase the
amount of work, so work eitherstays the same or decreases.
The formula for work is:Work = force x distance
15-3 Work Input & Work Output
Again, the formula for work is:Work = force x distance
So, mathematically speaking, toend up with the same or lesswork: If the machine increases the
force then the distance mustdecrease.
If the machine increases thedistance, then the force mustdecrease.
15-3 Work Input & Work Output
15-3 EfficiencyWhy is it that machines can’t have
more work output than input?Where does all the work disappearto?
A machine loses some of theinput work to the force of frictionthat is created when the machineis used.
Part of the input work is used toovercome the force of friction.
There is no machine that peoplehave made that is 100% efficient
15-3 EfficiencyIf machines make our work
easier, how much easier do theymake it?
The ratio of how much workoutput there is to the amount ofwork input is called a machine’sefficiency.
Efficiency is usually expressedas a percentage (%).
15-3 EfficiencyEfficiency
measure of how completelywork input is converted to workoutput
100%WWEfficiency
in
out ×=
It is always less than 100% dueto the opposing force of friction.
15-3 Efficiency A worker exerts a force of 500 N to push
a 1500 N sofa 4.0 m along a ramp thatis 1.0 m high. What is the ramp’sefficiency?
GIVEN:Fi = 500 Ndi = 4.0 mFo = 1500 Ndo = 1.0 m
WORK:Win = (500N)(4.0m) = 2000 J
Wout = (1500N)(1.0m) = 1500 J
E = 1500 J _ 100 2000 JE = 75%
1.0m
1500N
4.0m500N
100%in
out
WWE ×=
15-3 Mechanical AdvantageMechanical Advantage is
another way of expressing howefficient a machine is.
Mechanical advantage is theratio of resistance force to theeffort force OR the ratio of theeffort distance to the resistancedistance.
Equations for MA
effort of force
resistance of forcee AdvantagMechanical =
resistance of distance
effort of distance AdvantageMechanical =
Mechanical Advantage A worker exerts a force of 500 N to push
a 1500 N sofa 4.0 m along a ramp thatis 1.0 m high. What is the mechanicaladvantage of the ramp?
GIVEN:Fe = 500 NFr = 1500 N
WORK:MA = F resistance
F effort
MA = 1500N 500 NMA = 3
1.0m
1500N
4.0m500N
effort
res
FFMA =
Mechanical Advantage A person is pedaling a bike with an axle
radius of 3 inches. They use a pedalwith a radius of 8 inches. What is themechanical advantage of the pedal ?
GIVEN:De = 8 inDr = 3 in
WORK:MA = D effort
D resistance
MA = 8 in 3 inMA = 2.7
resistance
effort
DDMA =
15-4 Simple & Compound Machines
Simple Machines There are six types
of simple machines.They are the:1 - Inclined plane2 - Wedge3 - Screw4 - Lever5 - Pulley6 - Wheel and axle
15-4 Simple & Compound Machines 1 - Inclined Plane
Def - A slantedsurface used toraise an object.
The forceneeded to liftthe objectdecreasesbecause thedistancethrough whichthe objectmovesincreases.
15-4 Simple & Compound Machines
2 - Wedge -Inclined PlaneType #1 Def – an
inclined planethat moves inorder to pushthings apart.Examples
are forks,axes, knifes.
15-4 Simple & Compound Machines 3 - Screw - Inclined Plane Type #2 -
Def - An inclined plane wrappedaround a central bar or cylinder, toform a spiral. Ex – screw –duh!!!
15-4 Simple & Compound Machines 4 - Lever
Def - A rigid barthat is free topivot, or movearound a fixedpoint called afulcrum.Ex – see saw
There are threemain types(classes) oflevers.
15-4 Simple & Compound Machines
3 classes of levers: First-class levers have
the fulcrum placedbetween the load andthe effort, as in theseesaw, crowbar, andbalance scale.Ex - a see-saw or
scissors
15-4 Simple & Compound Machines
3 classes oflevers: Second-class
levers havethe loadbetween theeffort and thefulcrum.Ex - a
wheelbarrow
15-4 Simple & Compound Machines 3 classes of levers:
Third-class levers have the effortplaced between the load and thefulcrum. The effort always travelsa shorter distance and must begreater than the load.Ex - a hammer or tweezer
15-4 Simple & Compound Machines 5 - Pulley Def - A rope,
chain or beltwrapped around agrooved wheel.
It can change thedirection of forceor the amount offorce needed tomove an object.
15-4 Simple & Compound Machines To calculate how
muchmechanicaladvantage apulley systemcreates… Countthe number ofropes that areattached to theMOVEABLEpulley – that # isyour mechanicaladvantage!!!
15-4 Simple & Compound Machines 6 - Wheel &
Axle Def - Made of
2 circularobjects ofdifferent sizesattachedtogether torotate aroundthe sameaxis.
15-4 Simple & Compound Machines
Compound Machine Def - A combination of 2 or
more simple machines