Chapter 5: Work and Machines

Chapter 5:Work and Machines Making work easier for us

Section 5.1: Work • What is work? • List some examples of work: • Work is the transfer of energy that occurs when a force makes an object move • A force is a push or a pull • If the force applied doesn’t cause an object to move, then no work has been done • If the object doesn’t move in the same direction as the applied force, then no work has been done

Is work being done on your books? • You pick up your books from the floor • You hold your books while standing against the wall • You walk down the hall while holding your books • Are your arms doing work? • Are your legs doing work? • How could you do more work on the books? When work is done on an object, energy is transferred • Energy is the ability to do work

W F d Hidden units! Calculating Work • Work has 2 criteria: • A force is exerted • There is movement over a distance • The equation: • 1 N·m is also called 1 Joule (J) • the Joule is a unit of work Where W is work (N·m) F is force (N) d is distance (m)

W F d • P. 128

When is work done? • When you are causing an object to move over a certain distance • Pitching a baseball: • The pitcher is doing work from the time she is holding the ball through the release of the ball

W P t Hidden units! Power • Power is the amount of work done in a certain amount of time • The equation: • 1 J/s is also called 1 Watt (W) • the Watt is a unit of power Where W is work (J) P is power (Watts) t is time (s)

W P t • P. 130

When energy is transferred but no work is done: Example: turning on a light bulb

Section 5.2: Using Machines • Machine: A device that makes doing work easier • Examples??? • Machines make work easier by • Increasing the force that can be applied to an object • Bottle opener • Applying a large force over a small distance • Car jack • Applying a small force over a long distance

2.Increasing the distance over which a force can be applied • Ramp • Lifting a couch onto a truck • Using a ramp to push the couch into the truck • The couch is moved a longer distance but less force is required 3. Changing the direction of the applied force • Opening the blinds • Car jack • Downward force causes upward motion • Axe • Downward force results in outward pressure to split the wood apart The same amount of work is done

The Work Done By Machines • Effort and Resistance Forces • Effort force: The force applied to a machine (Fe) • Resistance force: The force applied by the machine to overcome resistance (Fr) • Example: Pulling a nail out with a hammer • Types of work • Input work (Win): The work done by you on a machine • Output work (Wout): The work done by a machine

When using machines, the law of conservation of energy is obeyed • Energy is transferred from you to the machine and from the machine to an object • Energy is not destroyed, only transferred • A machine can’t create energy • Wout is never greater than Win • Wout is never the same as Win • Some of your energy is changed to heat or friction • This energy isn’t used for work

Remember: • So, Win = Fe· deand Wout = Fr· dr • Where Fe is the effort force you apply de is the distance over which you apply the force Fr is the resistance force the machine must overcome to do work dr is the distance over which the machine must apply the resistance force • For an ideal machine, Win = Wout

Mechanical Advantage • The number of times that a machine multiplies your effort force is the mechanical advantage of the machine Crowbar • The long bar multiplies your force (Fr is greater than Fe) • You have to move the handle a great distance • When the direction of the Fe only causes a changes direction, Fe = Fr and MA = 1

Efficiency • Efficiency: The measure of how much of the work put into the machine is changed into useful output work by the machine • Remember: Some energy is always lost as heat or friction • Win will always be greater than Wout • Friction can be reduced by using lubricants • Machines that use a small amount of energy to do work are called “energy efficient”

Simple Machines • A simple machine is a machine that does work with only one movement • Six types: • Lever • Pulley • Wheel and axle • Inclined plane • Screw • Wedge • All simple machines are forms of either the lever or the inclined plane

Levers • A lever is a bar that is free to pivot (turn) about a fixed point PARTS: • FULCRUM: the fixed point of a lever • EFFORT ARM (De): the part of the lever on which the effort force is applied • RESISTANCE ARM (Dr): the part of the lever that exerts the resistance force

Examples of Levers

Types of Levers • FIRST CLASS: Fulcrum in the middle, resistance arm and effort arms on opposite sides of it • These levers always change the direction of the force • See saw, bottle opener, screw driver & paint can • A longer effort arm is better • Mechanical Advantage: the number of times that a machine multiplies your effort force

Draw the following levers and determine the MA of each: • De = 40cm; Dr = 40cm • De = 40cm; Dr = 10cm • De = 10cm; Dr = 40cm

SECOND CLASS: fulcrum on one side, resistance in the middle and effort on the end opposite the fulcrum • These levers always multiply the effort force (nut cracker, wheelbarrow) • They always help you to do work • A long effort arm combined with a short resistance arm results in the highest MA • Draw the following levers and determine the MA of each: • De = 90cm; Dr = 20cm • De = 90cm; Dr = 40cm • De = 90cm; Dr = 80cm

THIRD CLASS: fulcrum on one side, effort in the middle and resistance on the end opposite the fulcrum • This lever multiplies the distance the effort force travels • Hockey stick, tennis racquet, baseball bat • A long effort arm combined with a longer resistance arm results in the highest MA

A Review of Levers

Pulleys • Pulley: a grooved wheel with a rope or chain running along the groove • Like a first class lever • has a rope instead of a bar • axle acts like a fulcrum • the 2 sides of the pulley are the effort and resistance arms • The equation for determining the MA of a pulley is

Types of Pulleys 1. Fixed: a pulley that is attached to something that doesn't move • Changes the direction of an effort force • Pull down on the effort arm, object is raised by the resistance arm • Resistance arm and effort arm are the same length, so MA = 1 • It doesn’t multiply the effort force

2. Movable: a pulley that is attached to the object being moved • Multiplies the effort force • Increases the distance that the resistance force moves • MA is greater than 1 • the effort distance is twice as large as the resistance distance

3. Block and Tackle: a combination of fixed and movable pulleys • Multiplies the effort force • Has a large MA because of the number of pulleys • If there are too many pulleys, the MA decreases due to friction • Ideally, the MA of a pulley or pulley system is equal to the number of ropes that support the resistance weight Examples of pulleys from K’nex lab:

Wheel and Axle • Wheel and axle: a simple machine consisting of 2 wheels of different sizes that rotate together • Examples: doorknob, handle of a faucet, handle of a pencil sharpener, bicycle tire • It is a modified form of the lever • Effort force applied to the larger wheel (wheel) • Resistance force exerted by the smaller wheel (axle) • Gears are modified wheels and axles • Large gear is the effort gear, smaller gear is the resistance gear Examples of levers from K’nex lab:

Inclined Planes • INCLINED PLANE: A sloping surface used to reduce the amount of force required to do work • The amount of work done on a box is the same whether you lift it straight up or slide it up a ramp • LIFT BOX: shorter distance, greater force • USE RAMP: force is smaller, distance is longer Examples of inclined planes from K’nex lab:

Screws and Wedges Inclined planes that move • SCREW: An inclined plane that is wrapped in a spiral around a cylindrical post • the plane slides through the wood • WEDGE: an inclined plane with one or two sloping sides • the wedge moves down through the material and the force is changed to a horizontal motion to force an object apart • Examples: chisels, knives and axe blades Examples of screws and wedges from K’nex lab:

Compound Machines • A compound machine is one which incorporates two or more simple machines • Can opener: wheel and axle and a lever • Car: rotating parts, moving parts, gears

Chapter 5: Work and Machines