Chapter 13: Work and Machines

Aim: How does height affect an object’s gravitational potential

energy?

Gravitational Potential Energy•Is any time an object is raised above the ground

•The greater an object’s height and weight the more gravitational potential energy

•You can calculate it by multiplying the object’s weight by its height

•Unit is joules when the height is meters and weight is newtons

•The higher an object is, the more speed it will pick up on its way down

Aim: How does energy change?

•Kinetic energy depends on both an objects speed and mass

•Greater the mass, greater the speed, greater the kinetic energy

•Potential energy changes to kinetic energy

•When a roller coaster is coming over a hill it has potential energy

•When the roller coaster begins moving down the hill its energy is changing to kinetic energy

•It reaches its highest speed at the bottom of the hill because the potential energy has completely changed to kinetic energy

•Food stores potential energy stored in chemicals

•Our bodies change the food energy into other forms, such as kinetic energy, electrical energy and heat energy

Aim: How do batteries produce electricity?

Batteries•Change chemical energy into electricity•A chemical reaction inside the cell(battery) causes a flow of electrons

•This flow is electricity1.Dry-cell batteries•Called a primary battery b/c it can only be used once

•Dead dry-cell batteries should be thrown away after being used

•This will prevent toxins from leaking out

2. Secondary Battery• Can be recharged• In order to recharge the battery needs to

be hooked up to an outside source of electricty

• During recharging chemicals are turned back into their original form, enabling the battery to be used again

• Lead-acid storage batteries found in cars• Nickel-cadmium battery found in laptops,

camcorders, portable tools and space shuttles

Aim: What is work?

Work•Is applying a force to an object to move it through a distance

•W= distance x force•Units is joules (d-meters, f-newtons)•Just holding a box is not work•Picking up a box you are doing work•When work is done energy is added to it•When throwing a ball your work is equal to the kinetic energy gained by the ball

•Work can be changed into heat•Friction between the table and the

moving books turns mechanical energy, used to do work, into heat energy

Aim: How do we calculate work?

Aim: What is the law of conservation of energy?

Law of Conservation of Energy•Total amount of energy in the universe remains constant

•Energy can change its form•Energy can never be created nor destroyed

Aim: What are simple machines?

Simple Machines•A machine is a device that makes it easier for us to do work

•Simple machines are the most basic kinds

•Have none or a few moving parts•Changes the direction of a force we apply to something

•Can also increase the strength of an applied force

•Ex: pulling a nail out with a hammer

•Effort force is the force that you apply to a simple machine

•Force the machine applies to an object in response to the effort force is called the output force

•The force against which the machine acts is called the resistance force

•6 kinds of simple machines:•Lever, pulley, wheel and axle, inclined plane, screw and the wedge

Levers•All levers have a rigid bar that rests on a fulcrum, which is the pivot point

•Effort arm-side that applies an effort force

•Resistance arm-side that produces an output force

•3 classes of levers:1.First-class levers•Fulcrum lies between the effort force and the output force

•Change the direction of the effort force

•Ex: hammer pulling a nail out•Ex: seesaw2.Second Class Lever•Output force is between the effort force and the fulcrum

•Do not change the direction of the effort force

•Output force is greater than the effort force b/c the effort arm is longer

•Exs: wheelbarrow, nut cracker, bottle opener, paper cutter

3. Third Class Lever• Effort force is between the fulcrum

and the output force• Do not change direction of effort

force• Produce an output force that is less

than the effort force• Distance of the force is multiplied• Exs: fishing pole, tweezers, human

forearm and a broom

Aim: How do levers multiply force?

•Moving the position of the fulcrum changes the amount of the output force

•First-class levers can produce an output force that is greater than the effort force when the fulcrum is placed closer to the output force than the effort force

•Effort arm must be longer than the resistance arm b/c input work must equal output work

Aim: What is mechanical advantage?

Mechanical Advantage•Is the number that tells you how much a simple machine should multiply your effort

•Lever’s mechanical advantage is found by dividing the distance the effort arm moves by the distance the resistance arm moves

•When the output force is greater than the input force, the mechanical advantage is greater than one

•Using a broom has a mechanical advantage less than one

•The other way to find the mechanical advantage is dividing the length of the effort arm by the length of the resistance arm

Aim: How do pulleys work?

Pulleys•A rope is threaded through a wheel and axle

•Pulling on the rope can lift an object which is tied to the other end of the rope

•Can be either fixed or movable1.Fixed pulley•The wheel is attached to a fixed support•It does not multiply the effort force•It changes the direction of the effort force

2. Movable Pulley• Pulley is attached to a movable object

and moves with it• Multiplies the effort force by 2• Has a mechanical advantage of 2• Does not change the direction of the

effort force

Pulley System•Is made up of fixed and movable pulleys

•The pulleys act together•Has the same mechanical advantage as a single movable pulley

•It does change the direction of the effort force

Aim: What is the mechanical advantage of a pulley?

•By increasing the number of wheels and ropes, we can increase the mechanical advantage

•While mechanical advantage increase, the effort force required decreases

•2 ways to find a pulley’s mechanical advantage

1.By dividing the distance the effort rope moves by the distance the object moves

2. By counting the number of strands of rope that feel a downward pull from the load

Aim: How does a wheel and axle work?

Wheel & Axle•Fulcrum lies between the effort arm and the resistance arm

•The wheel applies the effort force•Small axle produces the output force•The mechanical advantage can be found by dividing the length of the effort arm by the length of the resistance arm

•Effort arm is the radius of the wheel

•Resistance arm is the radius of the axle

•Has a large mechanical advantage

Aim: What is an inclined plane?

Inclined Plane•Is a straight, slanted surface, like a ramp

•Have no moving parts•Makes it easier to do work because they multiply the effort force

•By using an inclined plane to move a heavy object to a different height, you need less force

•When using an inclined plane the force has to be exerted over a longer distance

•Lifting heavy objects can hurt your back

•To find the mechanical advantage divide the output force by input force

•The output force is the weight of the object

•The work put into a machine must equal the work produced by the machine, therefore the effort force must act over a greater distance than the output force

Aim: What is the mechanical advantage of a ramp?

Ramp’s Mechanical Advantage•The steeper the inclined plane, the more

force is needed to move an object up the incline

•Both girls are doing the same amount of work, even though the effort force is different

•You can find the MA of a ramp by dividing the output force by the input force or by dividing the length of the inclined plane by its height

•The longer the length of the ramp, the less the effort

Aim: How does a screw work?

Screws•They multiply effort force•They have a high mechanical advantage•Are created by wrapping an inclined plane around a central bar, which we called threads

•The head of the screw is the part we turn•The distance from thread to thread is called the pitch

•To find the mechanical advantage we divide the distance around the head by the pitch

•Screws with a larger pitch have a lower mechanical advantage

•Screws with large heads and very close pitch have a very high mechanical advantage

•Friction keeps screws in place•Without friction, the screw would unscrew and the object would fall

Aim: What is a wedge?

Wedges•A wedge is a single inclined plane or 2 inclined planes joined back to back

•A wedge must be moved by an effort force

•Wedges that are thin have a high mechanical advantage

•Ex: knife blades, ax heads, and chisels•These work best when they are sharpened

•A wedge changes the effort force and increases it’s strength

•The effort force is downward•The output force is horizontal

Aim: What is a compound machine?

Compound machines•A compound machine is a combination of 2 or more simple machines

•Ex: scissors (2 first-class levers and wedges)

•Ex: screwdriver and screw (screwdriver is a wheel and axle, screw is an inclined plane)

Aim: What is efficiency?

Efficiency•No machine runs without friction•When there is friction some of the input work is changed into heat energy, therefore not all of the input work becomes output work

•To find efficiency write a ratio: the work done by a machine over the work put into a machine and multiply by 100 to get a percentage

•The closer the efficiency is to 100% , the less energy the machine wastes

•The more friction there is, the lower the efficiency of a machine

Aim: How do machines help us?

•Simple machines make tasks easier•Machines, whether compound or simple, allow us to do things that we could not do with our bare hands