Work, Energy & Machines Chapter 9 Physical Science Unit 3 Work, Energy & Machines Chapter 9 11/16/2018

Work 9.1 W = F (d) Work Work Equation A quantity that measures the effects of a force acting over a distance. Work is a result of motion in the direction of the force. There is no work without motion. Work Equation Work = Force x distance W = F (d) No movement results in d= 0, No work is done. 11/16/2018

Work is measured in Joules (J). Distance- Measuring work w = F(d) Work is measured in Joules (J). Distance- means distance in the direction of the force. If a force is vertical and motion is horizontal, No work is done. 11/16/2018

Find the work done by gravity when a 2.0 kg rock falls 1.5m. w = F (d) What is the formula for Force(F) F = m (g) or F = m (9.8 m/s2) w = (m · g) (d) w = (2.0kg · 9.8m/s2)(1.5m) w = 29 J 11/16/2018

Negative Work??? This guy is no dumb bell even though he is doing negative work! When is his work positive/negative? Can you think of forces that tend to do negative work? 11/16/2018

Good Question... Ex: How much work is needed to form a human pyramid as in the picture at right? 11/16/2018

Power Power: Measurement of the rate at which work is done. How much work is done per period of time. Running up stairs or walking. Both have the same amount of work done. 11/16/2018

Measuring Power Power = work/ time Units for Power 11/16/2018

Horsepower 1hp = 746 watts(W) Horsepower (hp) is a commonly used unit of power. 1hp = 746 watts(W) 11/16/2018

Simple Machines 9.2 The most basic machines are simple machines. Simple machines are divided into two families: Lever family Inclined plane family 11/16/2018

Can you name the six Simple Machines??? levers pulleys wheel and axles screws wedges inclined planes 11/16/2018

How Simple Machines Work??? The basic principle is (ideally): Win = Wout that is to say, f • D = F • d a small force applied over a large distance yields a large force over a small distance. Actually, the work out is always LESS than the work in…Why??? 11/16/2018

Machines & Mechanical Advantage Machines result in a multiplication of a force. Change direction of force Increase output force, but decrease output distance. They do NOT allow us to do the same job with less ENERGY, rather, with less FORCE—and we waste MORE ENERGY!!! Why??? 11/16/2018

Mechanical Advantage equations PHYSICAL SCIENCE HOME PAGE 11/16/2018

Ideal MA vs. Actual MA Some machines have an “Ideal Mech. Advantage (IMA), based on their geometry… Levers – effort arm/resistance arm Pulleys - # strings supporting the load Wheel and axle – radius wheel/radius axle Ramp – length/height 11/16/2018

Lever Family All levers contain a fulcrum, force is transferred from one part of the arm to another. Levers are divided into 3 classes depending on the location of the fulcrum, input force, & output force. 11/16/2018

1st class lever Fulcrum is in the middle of the arm. Input force acts at one end, other end applies the force. 11/16/2018

2nd class lever The fulcrum is at one end of the arm and the force is applied to the other end. 11/16/2018

3rd class lever The mechanical advantage of a third class lever is less than one. So why use it? Many 3rd class levers are found in the human body. 11/16/2018

So what kind of lever is it??? Just ask the question: “What is in the middle???” The fulcrum  class 1 The load  class 2 The effort  class 3 IMA = effort arm/resistance arm 11/16/2018

Levers... 11/16/2018

Wheel and Axle Made of a lever (pulley) connected to a shaft. When the wheel is turned the axle turns. Screwdrivers and cranks are common wheel and axle machines. IMA = radius wheel ÷ radius axle 11/16/2018

The Inclined Plane Family Inclined planes multiply and redirect force. Turns a small input force into a large output force by spreading it out over a greater distance. IMA = length up ramp/height 11/16/2018

Wedge A screw is an inclined plane Wrapped around a cylinder. Modified inclined plane. The ramp is being pushed rather than some other object. A screw is an inclined plane Wrapped around a cylinder. Jar lids operate the same way. 11/16/2018

Pulleys are Modified Levers The middle of the pulley is like the fulcrum. The rest of the pulley is the arm. Single fixed pulley Can you predict the force needed to lift a 100lb load??? Single movable pulley The output force is shared by the 2 sections of rope pulling upward. Input force on the right only supports half the weight. 11/16/2018

Adding multiple pulleys makes a block and tackle system Adding multiple pulleys makes a block and tackle system. It will give an even higher mechanical advantage. Can you determine the MA for each system below? 11/16/2018

Efficiency of Machines When using a machine, not all of the work you put into the machine transfers to the object you to do work on. Why??? Friction and air resistance are two forces that dissipate energy. Where does it go??? 11/16/2018

Calculating Machine Efficiency All of the useful work put into the machine does not result in useful work done by the machine. EFF = Wout ÷ Win Answers are usually expressed as a % and must be less than 1 (100%). Why? 11/16/2018

Energy 9.3 Mechanical Energy -Kinetic The ability to do work. An object has energy if it is able to produce change in itself or its surroundings. 2 Basic Forms of Energy Mechanical and non-mechanical Mechanical Energy -Kinetic -Potential 11/16/2018

Nonmechanical Energy Electromagnetic (Light) Nuclear Thermal (Heat flow) Chemical 11/16/2018

Kinetic Energy Energy due to motion. A brick falling at the same speed as a ping pong ball will do more damage. KE is dependent on mass. KE also depends on speed or velocity (v) 11/16/2018

Kinetic Energy -The unit for energy is the Joule(J) -Energy that appears in the form of motion. -Depends on the mass and speed of the object in motion. -The unit for energy is the Joule(J) 11/16/2018

Potential Energy -Energy that is a result of an objects position or condition. -All potential energy is Stored Energy. i.e. Pull back on a bow string and bend the bow. The object then possesses potential energy. 11/16/2018

-This is a form of gravitational potential energy. - A rock at the top of a ramp has more potential energy than when it is on the ground due to its position. -This is a form of gravitational potential energy. -Fuel is an example of chemical potential energy, due to its ability to burn. 11/16/2018

Gravitational Potential Energy Depends on mass and height. PE = m(g)h = W·h m = mass g = acceleration due to gravity h = height Units m = kg g = m/s2 h = m 11/16/2018

Conservation of Energy 9.4 In a roller coaster all of the energy for the entire ride comes from the conveyor belt that takes the cars up the first hill. Baseball Highest point = no kinetic energy Lowest point = high kinetic energy Where is the potential energy the highest? 11/16/2018

Law of Conservation of Energy Energy can change from one form to another. Energy can not be created or destroyed. The total energy in the universe is constant. Activity: Observing a Pendulum… 11/16/2018

The sum of the pendulums potential and kinetic energy is called its mechanical energy. ME = PE + KE ME = mechanical energy 11/16/2018

If energy is conserved, why does the pendulum eventually stop? What happened to the mechanical energy? When energy changes form mechanical to thermal it loses usefulness. It can not change back to mechanical by itself. Thermal. Sound 11/16/2018

Elastic potential energy: Some forms of matter are elastic. This means when stretched they will go back to the original shape: Elastic potential energy is calculated with the formula: E = ½k·x2 “x” is the distance of the stretch and “k” is the “elasticity” of the material Which has more elasticity, the steel of a car spring or a rubber band? 11/16/2018

Actually, steel is more elastic than rubber Actually, steel is more elastic than rubber! If we use elasticity as the ability to return to it’s shape after being deformed, NOT which is easier to stretch. Steel is able to have a greater force applied to it and still return to its original shape (e.g. greater k-constant) 11/16/2018

EPE Examples How much energy is stored in a spring that has elasticity k=225 N/m and is stretched 20.0 cm? What is the effective spring constant of a trampoline that is stretched 0.855 m by a 200 lb person jumping up and down? 11/16/2018

What is an elastic collision? An elastic collision is when objects collide or rebound without lasting deformation or generation of heat. Retain energy. 11/16/2018

What is an inelastic collision? Objects collide and become distorted or generate heat. Retain less of their energy. Clay, putty, and dough are inelastic. 11/16/2018