chapter 15 chapter 15 – work, power & chapter 15 – … 15 chapter 15 – work, power &...

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.


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.


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


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 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.


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.


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


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.


Compound Machine Def - A combination of 2 or

more simple machines

chapter 15 chapter 15 – work, power & chapter 15 – … 15 chapter 15 – work, power &...

Documents