Work, Power, & Work, Power, & Simple MachinesSimple Machines

SectionsSections14.1 – 14.414.1 – 14.4

What it means to do workWhat it means to do work WorkWork – force acting over a distance to – force acting over a distance to

move an objectmove an object 2 key points for work to be done2 key points for work to be done

An object has to moveAn object has to move The object must move in the direction of The object must move in the direction of

the applied forcethe applied force

WorkWork Work = Force x DistanceWork = Force x Distance W = F x DW = F x D UnitsUnits

Force = Newtons (kg m/sForce = Newtons (kg m/s22) - N) - N Distance = meters – mDistance = meters – m Work = joules – J (N m)Work = joules – J (N m)

JouleJoule – unit of work and energy; 1 – unit of work and energy; 1 newton-meternewton-meter

WorkWork Example:Example:

If a weight lifter lifts a 800 N barbell a If a weight lifter lifts a 800 N barbell a distance of 2.5 m what is the work done?distance of 2.5 m what is the work done?

W = F x DW = F x D 800N x 2.5m800N x 2.5m W = 2,000 JW = 2,000 J

PowerPower

Section 14.2Section 14.2

PowerPower PowerPower – the rate at which work is done, – the rate at which work is done,

or the amount of work per unit of timeor the amount of work per unit of time Power = work/time p = w/tPower = work/time p = w/t Power = Power = force x distanceforce x distance timetime

PowerPower Unit for power is called a Unit for power is called a watt (W)watt (W) Watt is a joule/secWatt is a joule/sec Ex: a 40 watt light bulb does work at a rate Ex: a 40 watt light bulb does work at a rate

of 40 J/sof 40 J/s Large quantities of power are measured in Large quantities of power are measured in

kilowatts (kW) kilowatts (kW) Horsepower (hp)Horsepower (hp) – used to rate electric – used to rate electric

motors and gas enginesmotors and gas engines 1 hp = 745.56 watts (746 watts)1 hp = 745.56 watts (746 watts)

Simple and Simple and Compound Compound MachinesMachines

SectionsSections14.3-14.414.3-14.4

Simple MachinesSimple Machines

Inclined planeInclined plane – a type of simple machine – a type of simple machine that has a flat, slanted surface with no that has a flat, slanted surface with no moving partsmoving parts The less slanted the surface, the longer the The less slanted the surface, the longer the

distance over which the effort force is applieddistance over which the effort force is applied IMA = IMA = effort distance effort distance = = length of slopelength of slope = = ll resistance dist. height of slope hresistance dist. height of slope h

Simple MachinesSimple Machines WedgeWedge – an inclined plane that moves – an inclined plane that moves

The greater the distance the wedge moves, The greater the distance the wedge moves, the greater the force is on an objectthe greater the force is on an object

Calculating IMA of a wedge is the same Calculating IMA of a wedge is the same as an inclined plane (except use width as an inclined plane (except use width instead of height)instead of height)

Simple MachinesSimple Machines ScrewScrew – an inclined plane wrapped – an inclined plane wrapped

around a cylinder to form a spiralaround a cylinder to form a spiral Tight threads = longer distance over which Tight threads = longer distance over which

effort is exerted = easier to turneffort is exerted = easier to turn Spaced threads = shorter distance over Spaced threads = shorter distance over

which force is exerted = harder to turnwhich force is exerted = harder to turn

Simple MachinesSimple Machines LeverLever – a rigid bar that is free to pivot or – a rigid bar that is free to pivot or

turn about a fixed pointturn about a fixed point FulcrumFulcrum – the fixed point – the fixed point Effort armEffort arm – the part of the lever where – the part of the lever where

the effort force is appliedthe effort force is applied Resistance armResistance arm – the part of the lever – the part of the lever

that exerts the resistance forcethat exerts the resistance force

LeversLevers 11stst class lever R E class lever R E

Ex: scissorsEx: scissors 22ndnd class lever R E class lever R E

Ex: wheelbarrowEx: wheelbarrow 33 rdrd class lever E R class lever E R

Ex: arm swingingEx: arm swinging a hammer a hammer

LeversLevers

IMA = IMA = length of effort arm___ length of effort arm___ = _ = _Le_Le_ length of resistance arm Lrlength of resistance arm Lr

Ex: A worker uses an iron bar to lift a Ex: A worker uses an iron bar to lift a manhole cover weighing 65 N. The effort manhole cover weighing 65 N. The effort arm of the lever is 60 cm long. What is arm of the lever is 60 cm long. What is the IMA of the bar?the IMA of the bar?

IMA = IMA = 60cm60cm = 6 = 6 10cm10cm

Simple MachinesSimple Machines PulleyPulley – rope, chain, or belt – rope, chain, or belt

wrapped around a grooved wrapped around a grooved wheelwheel A A fixed pulleyfixed pulley is attached to is attached to

something that doesn’t move something that doesn’t move (ceiling, wall, etc.)(ceiling, wall, etc.)

The IMA of a fixed pulley is 1The IMA of a fixed pulley is 1 A A movable pulleymovable pulley moves moves

The IMA of a movable pulley is 2 The IMA of a movable pulley is 2

PulleysPulleys Block and tackleBlock and tackle

IMA can be multiplied by counting every IMA can be multiplied by counting every rope supplying an upward force on the rope supplying an upward force on the resistanceresistance

Simple MachinesSimple Machines Wheel and axleWheel and axle – a simple machine – a simple machine

consisting of two wheels of different consisting of two wheels of different sizes that rotate togethersizes that rotate together

IMA = IMA = radius of wheelradius of wheel = = RwRw radius of axle Raradius of axle Ra

MachinesMachines Compound machineCompound machine – a combination of – a combination of

two or more simple machinestwo or more simple machines

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