“Work and Energy” Chapter 13. Work Work is the quantity that measures the effects of a force...

““Work and Energy”Work and Energy”

Chapter 13Chapter 13

WorkWork

WorkWork is the quantity that measures the is the quantity that measures the effects of a force acting over a distance.effects of a force acting over a distance.

WORK IS ONLY DONE WHEN SOMETHING WORK IS ONLY DONE WHEN SOMETHING PHYSICALLY CHANGES POSITION.PHYSICALLY CHANGES POSITION.

vs.

WorkWork

W = F x dW = F x d

(J) (N) (m)(J) (N) (m)The SI Unit for Work is Joules (J)The SI Unit for Work is Joules (J)

W

F x d

WorkWork

A car has run out of gas. Fortunately, there is a gas station A car has run out of gas. Fortunately, there is a gas station nearby. You mush exert a force of 715 N on the car in order nearby. You mush exert a force of 715 N on the car in order to move it. By the time you reach the station, you have done to move it. By the time you reach the station, you have done 27,200 J of work. How far have you pushed the car?27,200 J of work. How far have you pushed the car?

W =

F=

d =

715 N

?

27,200 J

d = 27200 J / 715 N

d = 38 m

d = W / F

W

F * d

PowerPower

PowerPower is defined as is defined as the rate at which work the rate at which work is done (how much is done (how much work is done in a work is done in a certain amount of certain amount of time).time).

PowerPower

Power = work / timePower = work / time

P = W / tP = W / t

(W) (J) (s)(W) (J) (s)

The SI unit for Power is Watts WThe SI unit for Power is Watts W

W

P x t

PowerPower

The world’s most powerful tugboats, which are built in The world’s most powerful tugboats, which are built in Finland, are capable of providing 8,170,000 W of power. Finland, are capable of providing 8,170,000 W of power. How much work does one of these tugboats do in 12.0 s?How much work does one of these tugboats do in 12.0 s?

P =

W=

t =

?

12 s

8,170,000 W8,170,000 W

W = 8,170,000 W x 12 s8,170,000 W x 12 s

d = 98,000,000 J

W = P * t

W

P * t

Simple MachinesSimple Machines

Simple Machines Simple Machines are the most basic are the most basic machines.machines.

There are two families of simple machines:There are two families of simple machines: LeverLever Inclined PlaneInclined Plane


The three simple machines in the lever The three simple machines in the lever family are:family are:

LeverLever PulleyPulley Wheel and AxleWheel and Axle


LeversLevers A lever is a board or bar that rests on a turning A lever is a board or bar that rests on a turning

point.point. This turning point is called the This turning point is called the fulcrumfulcrum..

An object that a lever moves is called the An object that a lever moves is called the resistance resistance or or loadload..

The closer the object is to the fulcrum, the easier it is The closer the object is to the fulcrum, the easier it is to move.to move.

Simple MachinesSimple Machines There are three classes of levers:There are three classes of levers:

F

R

E

F

R

EEE

R

F


PulleysPulleys A pulley is a modified lever.A pulley is a modified lever.

The point in the middle of the pulley is like the The point in the middle of the pulley is like the fulcrum of the lever, and the rest of the pulley acts fulcrum of the lever, and the rest of the pulley acts like the rigid arm of a lever.like the rigid arm of a lever.

Because the distance of the fulcrum is the same on Because the distance of the fulcrum is the same on both sides of a pulley, a single, fixed pulley has a both sides of a pulley, a single, fixed pulley has a mechanical advantage of 1.mechanical advantage of 1.

Using moving pulleys or more than one pulley at a Using moving pulleys or more than one pulley at a time can increase mechanical advantage.time can increase mechanical advantage.

Multiple pulleys put together are called Multiple pulleys put together are called block and block and tackletackle or or compoundcompound pulleys. pulleys.


Wheel and AxleWheel and Axle A wheel and axle is made of a lever or a pulley A wheel and axle is made of a lever or a pulley

(wheel) connected to a shaft (the axle).(wheel) connected to a shaft (the axle). Small input for large output.Small input for large output. Examples: steering wheel, screwdriver, crank.Examples: steering wheel, screwdriver, crank.


The three simple machines in the incline The three simple machines in the incline plane family are:plane family are:

Incline PlaneIncline Plane WedgeWedge ScrewScrew


Incline PlaneIncline Plane Incline planes or ramps redirect the force applied Incline planes or ramps redirect the force applied

on an object to lift it upwards.on an object to lift it upwards. An inclined plane turns a small input force into a An inclined plane turns a small input force into a

large output force by spreading the work over a large output force by spreading the work over a large distance.large distance.


Incline PlaneIncline Plane


WedgeWedge A wedge functions like two inclined planes back A wedge functions like two inclined planes back

to back.to back. A wedge turns a single downward force into two A wedge turns a single downward force into two

forces directed out to the sides.forces directed out to the sides. Examples: axes and nails.Examples: axes and nails.


WedgeWedge


ScrewScrew A screw is an inclined plane wrapped around the A screw is an inclined plane wrapped around the

cylinder.cylinder. Tightening a screw is like pushing an object up a Tightening a screw is like pushing an object up a

ramp; it requires a small force acting over a long ramp; it requires a small force acting over a long distance.distance.

Examples: cork screw, jar lids, and spiral stair Examples: cork screw, jar lids, and spiral stair cases.cases.


ScrewScrew

Compound MachinesCompound Machines

Compound MachinesCompound Machines Many devices you use every day are made of Many devices you use every day are made of

more than one simple machines.more than one simple machines. A machine that combines two or more simple A machine that combines two or more simple

machines is called a machines is called a compound machinecompound machine..

Mechanical AdvantageMechanical Advantage

Machines help us do work by redistributing Machines help us do work by redistributing the work that we put into them.the work that we put into them.

They do this by:They do this by: Changing the direction of the input force.Changing the direction of the input force. Change output force by changing the distance over Change output force by changing the distance over

which the force is applied (multiplying the force).which the force is applied (multiplying the force).


Mechanical Mechanical AdvantageAdvantage is defined is defined as the ratio between as the ratio between the output force and the output force and the input force. It is the input force. It is also defined as the also defined as the ratio between input ratio between input distance and output distance and output distance.distance.


MA= MA= resistance force (fresistance force (frr)) OR OR effort distance effort distance

(d(dee))

effort force (feffort force (fee)) resistance distance (dresistance distance (drr))

**Remember: The units for force is Newtons (N) and Remember: The units for force is Newtons (N) and the SI units for distance is meters (m). Mechanical the SI units for distance is meters (m). Mechanical advantage has NO units.advantage has NO units.


Things to keep in mind when calculating Things to keep in mind when calculating mechanical advantage:mechanical advantage: Ramps:Ramps:

de

dr


Things to keep in mind when calculating Things to keep in mind when calculating mechanical advantage:mechanical advantage: Levers:Levers:

dr

de

drde

fr

fe

frfe


Things to keep in mind Things to keep in mind when calculating when calculating mechanical advantage:mechanical advantage: Pulleys: Pulleys:

The effort force is what The effort force is what you pull with and the you pull with and the resistance force is the resistance force is the load you’re lifting.load you’re lifting.

Count the number of Count the number of ropes pulling ropes pulling upup to to determine the MA.determine the MA.

12

2fe

fe

fe

fr

fr

fr


A mover uses a pulley system with a mechanical A mover uses a pulley system with a mechanical advantage of 10.0 to lift a 1500 N piano 3.5 m. advantage of 10.0 to lift a 1500 N piano 3.5 m. What is the effort force required by the mover?What is the effort force required by the mover?

ffee = =

??

ffrr = =

1500 N1500 N

MA =MA =

1010

fr

fe x MA

fe = fr / MA

fe = 1500 N / 10

fe = 150 N


An axe used to split wood is driven into a piece of wood An axe used to split wood is driven into a piece of wood for an effort distance of 3.0 cm. If the mechanical for an effort distance of 3.0 cm. If the mechanical advantage of the axe is 0.85, how far apart (resistance advantage of the axe is 0.85, how far apart (resistance distance) is the wood split? distance) is the wood split?

ddrr==

??

ddee = =

3 cm3 cm

MA =MA =

0.850.85

de

dr x MA

dr = de / MA

dr = 3 cm / 0.85

dr = 3.5 cm

“Work and Energy” Chapter 13. Work Work is the quantity that measures the effects of a force...

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Transcript of “Work and Energy” Chapter 13. Work Work is the quantity that measures the effects of a force...