Work and Machines

•Work – energy transferred when a force makes an object move

•2 conditions must apply for there to be work:▫The object must move▫Movement must be in same direction as

applied force No work is done to something you carry

How do you calculate work?

•Work (Joules) = force (N) x distance (m)•Power is the amount of work done in one

second.•Power (Watts) = work (J)/time (sec)

▫What is a Kilowatt?•Also…Power = Energy/time

▫Work and Energy are the same thing!!!

Work problems• Chase lifts a 100 kg (220 lbs.) barbell 2 meters.

How much work did he do?

 

• Caitlin pushes and pushes on a loaded shopping cart for 2 hours with 100 N of force. The shopping cart does not move. How much work did Caitlin do?

Power problems• Danielle exerts 40 N of force to move an object

2 m in 4 seconds. What was her power?

 

• Charles bench presses 100 kg (220 lbs) 0.5 m for 20 reps in 20 seconds. What was the power of this impressive man of steel?

Machines

•Devices that make work easier.• But how???

▫They increase the force applied to an object▫They increase the distance over which the

force is applied

Ideal Machines (or Perfect Machines)

•Machines involved 2 types of work:▫Work input – what YOU put into the

machine▫Work output – what you get out of the

machine•Win = Wout

▫This is NOT possible…why? Friction!!!!

Mechanical advantage

•The ratio of output force to input force•MA = output force (N)/input force (N)

▫MA = Fout/Fin

•Ideal Mechanical Advantage (IMA) is the MA without friction

Efficiency

•(Work output/Work input) X 100•How can efficiency be increased?

Simple Machines

• Have few or no moving parts

• Make work easier• Can be combined to

create complex machines• Six simple machines:

Lever, Inclined Plane, Wheel and Axle, Screw, Wedge, Pulley

Lever• A rigid board or rod

combined with a fulcrum and effort

• By varying position of load and fulcrum, load can be lifted or moved with less force

• Trade off: must move lever large distance to move load small distance

• There are 3 types of levers

1st Class Lever

•The fulcrum is located between the effort and the load

•Direction of force always changes

•Examples are scissors, pliers, and crowbars

2nd Class Lever

•The resistance is located between the fulcrum and the effort

•Direction of force does not change

•Examples include bottle openers and wheelbarrows

3rd Class Lever•The effort is

located between the fulcrum and the resistance

•Direction of force does not change, but a gain in speed always happens

•Examples include ice tongs, tweezers and shovels

Mechanical Advantage: Lever

• The mechanical advantage of a lever is the distance from the effort to the fulcrum divided by the distance from the fulcrum to the load

• For our example, MA = 10/5 = 2

• Distance from effort to fulcrum: 10 feet

• Distance from load to fulcrum: 5 feet

MA =Distance, effort - fulcrumDistance, load - fulcrum

Inclined Planes• A slope or ramp that

goes from a lower to higher level

• Makes work easier by taking less force to lift something a certain distance

• Trade off: the distance the load must be moved would be greater than simply lifting it straight up

Mechanical Advantage: Inclined Plane• The mechanical

advantage of an inclined plane is the length of the slope divided by the height of the plane, if effort is applied parallel to the slope

• So for our plane MA = 15 feet/3

feet = 5

• Let’s say S = 15 feet, H = 3 feet

MA =Length of SlopeHeight of Plane

Wheel and Axle•A larger circular wheel

affixed to a smaller rigid rod at its center

•Used to translate force across horizontal distances (wheels on a wagon) or to make rotations easier (a doorknob)

•Trade off: the wheel must be rotated through a greater distance than the axle

Mechanical Advantage: Wheel and Axle

• The mechanical advantage of a wheel and axle system is the radius of the wheel divided by the radius of the axle

• So for our wheel and axle MA = 10”/2” = 5

2"

10"

MA =Radius of WheelRadius of Axle

Screw

•An inclined plane wrapped around a rod or cylinder

•Used to lift materials or bind things together

Mechanical Advantage: Screw• The Mechanical

advantage of a screw is the circumference of the screwdriver divided by the pitch of the screw

• The pitch of the screw is the number of threads per inch

• So for our screwdriver

MA = 3.14”/0.1” = 31.4

Diam.=1"

10 threadsper inch

Circumference = ∏ x 1” = 3.14”

Pitch = 1/10” = 0.1”

MA = Circumference of ScrewdriverPitch of Screw

Wedge

•An inclined plane on its side

•Used to cut or force material apart

•Often used to split lumber, hold cars in place, or hold materials together (nails)

Mechanical Advantage: Wedge• Much like the

inclined plane, the mechanical advantage of a wedge is the length of the slope divided by the width of the widest end

• So for our wedge, MA = 6”/2” = 3• They are one of the

least efficient simple machines

2"

6"

MA = Length of SlopeThickness of Widest End

Pulley• A rope or chain free to turn

around a suspended wheel• By pulling down on the rope,

a load can be lifted with less force

• Trade off: no real trade off here; the secret is that the pulley lets you work with gravity so you add the force of your own weight to the rope

• IMA = input arm length/ output arm length

The trick is WORK

•Simple machines change the amount of force needed, but they do not change the amount of work done

•What is work?▫Work equals force times distance▫W = F x d

•By increasing the distance, you can decrease the force and still do the same amount of work