Work and Machines

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

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

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

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 Circumference = ∏ x 1” = 3.14” Pitch = 1/10” = 0.1”

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

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

Work and Machines