General Science Chapter 13 Work & Energy Chapter 13 April 17 General Science Chapter 13

General Science Chapter 13 Work Everyday concept of work. Scientific definition: Work is the transfer of energy through motion. A force applied, in which the object moves, to an object in the direction that the object moves. April 17 General Science Chapter 13

General Science Chapter 13 Work In order for work to take place, a force must be exerted through a distance. In order for work to be done, there has to be motion, and the motion has to be in the direction of the applied force. If there is no motion, no work will be done April 17 General Science Chapter 13

General Science Chapter 13 Recall “work” lab Did you do more work lifting the books to shoulder level or over your head? Greater distance means more work Did you do more work lifting 2 books or 4 books? Greater force means more work April 17 General Science Chapter 13

General Science Chapter 13 Work Equation Work, like energy, is measured in joules. 1 J = 1 N ∙ m. April 17 General Science Chapter 13

General Science Chapter 13 Work and Energy Work is the transfer of energy through motion. When 1 J of work is done on an object, 1 J of energy has been transferred to the object. April 17 General Science Chapter 13

General Science Chapter 13 Example A student’s backpack weighs 10 N. She lifts it from the floor to a shelf 1.5 m high. How much work is done on the backpack? Force, F = 10 N Distance, d = 1.5 m Work = (Force)(Distance) Work = (10 N)(1.5 m) Work = 15 N ∙m = 15 J April 17 General Science Chapter 13

General Science Chapter 13 Example #2 A dancer lifts a 400-N ballerina overhead a distance of 1.4 m and holds her there for several seconds. How much work is done on the ballerina? Work = (400 N)(1.4 m) Work = 560 J Note that the time was not important for us to determine the work done. April 17 General Science Chapter 13

General Science Chapter 13 Example #3 A carpenter lifts a 45-kg beam 1.2 m high. How much work is done on the beam? Remember that weight equals mass times acceleration due to gravity. Weight = (45 kg)(9.8 m/s2) = 441 N Work = (441 N)(1.2 m) Work = 529.2 J April 17 General Science Chapter 13

General Science Chapter 13 Power The rate at which work is done. How much work is done in a given amount of time The ratio of work to time April 17 General Science Chapter 13

General Science Chapter 13 Watts Power is measured in Watts, named after James Watt, who helped develop the steam engine. 1 W = 1 J/s Very small unit, so we often use kW. 745.6 Watts = 1 horsepower April 17 General Science Chapter 13

General Science Chapter 13 Example A figure skater lifts his partner, who weighs 450 N, 1 meter in 3 seconds. How much power is required? April 17 General Science Chapter 13

General Science Chapter 13 You try A 25 000 N elevator rises 30.0 m in 60.0 s. How much power is required? Express your answer in kW. April 17 General Science Chapter 13

General Science Chapter 13 Making Connections 4.184 Joules = 1 calorie Joules are units for energy and work 1 Calorie = 1000 calories A Calorie is used for foods, so if a candy bar has 250 Calories it is the same as 250000 calories. April 17 General Science Chapter 13

General Science Chapter 13 Example #1 Tommy eats a candy bar that has 230 Calories. How many Joules is that? 1 Calorie = 1000 calories 1 calorie = 4.184 Joules so 1 Cal = 4184 Joules therefore 230 Calories = 962320 Joules April 17 General Science Chapter 13

General Science Chapter 13 Example #1 continued How much Power can be produced with the 230 Calorie candy bar in 1 hour? 230 Cal = 962320 Joules Use the equation below April 17 General Science Chapter 13

General Science Chapter 13 Example #1 continued April 17 General Science Chapter 13

General Science Chapter 13 Discussion #1 Define work and what are the SI units? What units are used to measure Power? Why is the unit kW used more often than W? What is the conversion factor for horsepower to Watts? April 17 General Science Chapter 13

General Science Chapter 13 Machine A device that makes work easier By using a machine you DO NOT DO LESS work. It just makes it seems easier to do the work. April 17 General Science Chapter 13

General Science Chapter 13 Work and machines Work is made easier by doing 1 of 3 things Changes the size of the input force Changes the direction of the force Changes both the size and the direction of the force Opening a paint can with a screwdriver Changes size – you can use less force Changes direction April 17 General Science Chapter 13

General Science Chapter 13 Forces and machines Effort force (Fe) – applied to the machine The force you exert Also called input force Resistance force (Fr) – applied by the machine to overcome gravity or friction The force the machine exerts Also called output force April 17 General Science Chapter 13

General Science Chapter 13 Work and machines Work input (Win) – work done on the machine Effort force times distance it moves Win = Fe X de Work output (Wout) – work done by the machine Resistance force times distance it moves Wout = Fr X dr April 17 General Science Chapter 13

General Science Chapter 13 Mechanical advantage The number of times a machine multiplies the effort force The ratio of output to input. (usually a force) April 17 General Science Chapter 13

General Science Chapter 13 Mechanical advantage Can be greater than 1 Opening paint can Can be equal to 1 Raising blinds Can be less than 1 Hitting a baseball April 17 General Science Chapter 13

General Science Chapter 13 Example A worker applies an effort force of 10 N to pry open a window that has a resistance of 500 N. What is the mechanical advantage of the crowbar? Fr = 500 N Fe = 10 N April 17 General Science Chapter 13

General Science Chapter 13 You try A jack is used to lift a 2000-N rock. The effort force is 200 N. Find the mechanical advantage. April 17 General Science Chapter 13

General Science Chapter 13 Discuss #2 A _____________ is a device that makes work easier. What are the 3 ways a machine can make work easier? What do we call the force applied to a machine? What do we call the force applied by a machine? April 17 General Science Chapter 13

General Science Chapter 13 Discussion #2 What is mechanical advantage? What does it mean when the MA value is equal to 1? How do we calculate MA? April 17 General Science Chapter 13

General Science Chapter 13 Simple machine A device that does work with only one movement There are six types Levers Inclined Plane Pulley Wedge Wheel & Axle Screw April 17 General Science Chapter 13

Ideal Mechanical Advantage The ratio of output to input (usually a force) disregarding friction and gravity. When the output work = the input work. Machine would be 100% efficient, which is impossible. 4/14/2017 General Science Chapter 13

General Science Chapter 13 Levers Examples Crowbars Seesaws Baseball bat April 17 General Science Chapter 13

General Science Chapter 13 Definitions A lever is a bar that is free to pivot, or turn, about a fixed point. A fulcrum is the fixed point of a lever. The effort arm is the part of the lever on which the force is applied. The resistance arm is the part of the lever that exerts the resistance. April 17 General Science Chapter 13

General Science Chapter 13 Lever Effort force Resistance force Resistance arm Effort arm fulcrum April 17 General Science Chapter 13

General Science Chapter 13 Mechanical advantage Review, we learned that We can also use for levers April 17 General Science Chapter 13

General Science Chapter 13 You try You use a crowbar 160 cm long as a lever to lift a large rock. The rock is 20 cm from the fulcrum. You push down on the other end of the crowbar. What is the length of the effort arm? The resistance arm? What is the IMA of the lever? April 17 General Science Chapter 13

General Science Chapter 13 First class levers The fulcrum is in the middle Seesaw crowbar Effort force Resistance arm Effort arm fulcrum April 17 General Science Chapter 13

General Science Chapter 13 Second class levers The resistance is in the middle wheelbarrow nutcracker Effort force Effort arm Resistance arm fulcrum April 17 General Science Chapter 13

General Science Chapter 13 Third class levers The effort is in the middle Baseball bat broom Effort force Effort arm Resistance arm fulcrum April 17 General Science Chapter 13

General Science Chapter 13 Discuss #3 What is a lever? What is a fulcrum? What is the effort arm? What is the resistance arm? April 17 General Science Chapter 13

General Science Chapter 13 Discussion #3 What are the 3 types of levers? What is an example of each type of lever? Which type usually has a IMA value < 1? April 17 General Science Chapter 13

General Science Chapter 13 Pulleys Pulley – grooved wheel with a rope or chain running along the groove Acts like a lever The two ends of the rope are the effort arm and the resistance arm The wheel acts like the fulcrum April 17 General Science Chapter 13

General Science Chapter 13 Fixed pulley Attached to something that doesn’t move Change the direction of a force IMA of 1 Le Lr Fr Fe April 17 General Science Chapter 13

General Science Chapter 13 Movable pulley Attached to the object being moved IMA greater than 1 Effort distance must be greater than resistance distance Le Lr Fr Fe April 17 General Science Chapter 13

General Science Chapter 13 Block and tackle System of fixed and movable pulleys Has IMA equal to the number of ropes that support the resistance weight Count every rope coming off the movable pulleys that supports or moves the resistance force. April 17 General Science Chapter 13

Examples of Block & Tackle April 17 General Science Chapter 13

Example of Block & Tackle #2 April 17 General Science Chapter 13

General Science Chapter 13 Wheel and axle Consists of two wheels of different sizes that rotate together The effort force is usually applied to the large wheel The small wheel, or axle, exerts the resistance force Examples: doorknob, water faucet, gears, pencil sharpener April 17 General Science Chapter 13

General Science Chapter 13 Wheel and axle Can be thought of as a lever attached to a shaft Radius of wheel is effort arm Radius of axle is resistance arm Center of axle is fulcrum April 17 General Science Chapter 13

General Science Chapter 13 Inclined plane A ramp Lifting something along an inclined plane means you cover more distance than lifting it straight up, but you get to use a smaller force April 17 General Science Chapter 13

General Science Chapter 13 Screw An inclined plane wrapped in a spiral around a cylindrical post. As you drive in a screw, the inclined plane slides through the wood. April 17 General Science Chapter 13

General Science Chapter 13 Wedge An inclined plane with one or two sloping sides Examples Chisels Knives Axe blades The material stays in place while the wedge moves through it. April 17 General Science Chapter 13

General Science Chapter 13 Wedge  Thickness, T  Side, S April 17 General Science Chapter 13

General Science Chapter 13 Variations All six kinds of simple machines are variations of two basic machines The lever The inclined plane April 17 General Science Chapter 13

General Science Chapter 13 Compound Machine A machine that is made up of 2 or more simple machines. Examples of compound machines Fishing rod, pencil sharpener, an axe April 17 General Science Chapter 13

General Science Chapter 13 Discuss #4 What kind of simple machine is a ramp? What is an inclined plane wrapped in a spiral around a cylindrical post? What kind of simple machine are chisels, knives, and axes? April 17 General Science Chapter 13

General Science Chapter 13 Discussion #4 What type of pulleys have a MA = 1? What is the difference between a fixed pulley and a movable pulley? What is a block and tackle? What two groups can simple machines be broken into? What is a compound machine? April 17 General Science Chapter 13

General Science Chapter 13 Energy Scientific definition: Energy is the ability to cause change. Ability to do work Any sample of matter has energy if it can produce a change in itself or in its surroundings. Energy comes in many forms and will be classified as either Kinetic or Potential. April 17 General Science Chapter 13

General Science Chapter 13 Energy continued Kinetic Forms Radiant (solar), thermal, electrical, wind, sound Potential Forms Gravitational, mechanical, chemical, and nuclear Energy is measured in joules (J). Named after a British scientist. April 17 General Science Chapter 13

General Science Chapter 13 Kinetic energy Energy in the form of motion Amount depends on the mass and velocity of the object. Greater mass at the same velocity OR greater velocity with the same mass will have greater kinetic energy KE = ½mv2 5 Types (STEWS or SHEWS) April 17 General Science Chapter 13

Solar Energy (Radiant) Electromagnetic energy that travels in transverse waves. Energy from the sun April 17 General Science Chapter 13

General Science Chapter 13 Thermal Energy Heat energy, the internal energy in a substance. Caused by the vibration and movement of atoms/molecules within substances. Geothermal energy is a good example of this type of energy. April 17 General Science Chapter 13

General Science Chapter 13 Electrical Energy Energy produced by the movement of electrons. Lightning and electricity are good examples of this form of energy. April 17 General Science Chapter 13

General Science Chapter 13 Wind Energy (Motion) Energy produced from the movement of objects from one place to another. Do not need to see this movement, we just know there is a change in position. Wind and some forms of hydropower are good examples of this form of energy. April 17 General Science Chapter 13

General Science Chapter 13 Sound Energy Movement of energy through substances using longitudinal or compressional waves. Obviously this is how we hear “things” A compressional wave is like the movement of an inch worm or an accordion. April 17 General Science Chapter 13

General Science Chapter 13 Potential energy Stored energy Depends on its position/condition/height, mass and gravity 4 Types (GECN) PE = mgh m = mass, g = gravity, h = height April 17 General Science Chapter 13

Gravitational Energy (Hydro) Potential energy of an object due to height above the earth’s surface. The higher the object is, the more potential energy it has. April 17 General Science Chapter 13

Elastic Potential Energy (Stored) Energy stored in a spring or rubber band or anything else that stretches. The farther it is stretched, the greater its potential energy. Energy based on the position April 17 General Science Chapter 13

General Science Chapter 13 Chemical Energy The energy stored in foods, fuels, and batteries. There must be a chemical reaction to get the energy out. April 17 General Science Chapter 13

General Science Chapter 13 Nuclear Energy Energy stored in the nucleus of an atom. Fusion and Fission are two examples April 17 General Science Chapter 13

General Science Chapter 13 Mechanical Energy The sum of potential and kinetic energy in a system is called mechanical energy. Think about a roller coaster or bungee jumping. April 17 General Science Chapter 13

Discuss #5 Energy Review Define energy What units are used to describe energy What are the 2 main forms of energy List 3 of the 4 subsets of stored energy List 3 of the 5 subsets of moving energy What is Mechanical energy? April 17 General Science Chapter 13

General Science Chapter 13 Discussion #5 Why is the first hill of a roller coaster ride the highest? Where would a roller coaster be moving fastest? April 17 General Science Chapter 13

General Science Chapter 13 Discussion #5b Why can you not travel in a circular loop when on a roller coaster? Is it possible for the second hill to be taller than the first hill? Explain why? When does a coaster have the most PE? When does a coaster have the most KE? April 17 General Science Chapter 13

Conservation of Energy Energy cannot be created or destroyed but it can change from one form to another. Example – Swing Why does it stop? April 17 General Science Chapter 13

Conservation of energy You can never get more work out than you put in If force increases, distance must decrease. April 17 General Science Chapter 13

General Science Chapter 13 Internal energy The total energy of all the particles that make up a sample of matter. Includes both kinetic and potential energy of the particles. The more mass a material has, the more internal energy it has. April 17 General Science Chapter 13

General Science Chapter 13 Internal energy Different materials have different internal energies even when they have the same mass and temperature. This is because the particles in the materials are arranged differently. April 17 General Science Chapter 13

General Science Chapter 13 Separate energies Internal energy of a material depends on the total energy of its particles. Mechanical energy (kinetic and potential) of the material itself has no effect on internal energy. April 17 General Science Chapter 13

1st Law of Thermodynamics The net change in energy equals the energy transferred as work and heat. Q = Heat W = Work ΔU = Internal energy April 17 General Science Chapter 13

General Science Chapter 13 Efficiency Measure of how much of the work put into a machine is changed to useful work put out by the machine. April 17 General Science Chapter 13

General Science Chapter 13 Efficiency Can it ever be more than 100%? How can we increase efficiency? April 17 General Science Chapter 13

General Science Chapter 13 example A worker pushes a 1500 N chair up an inclined plane that is 4.0 m long and 1.0 m high. The worker exerts a force of 500 N. What is the efficiency of the inclined plane? 4 m 1 m April 17 General Science Chapter 13

General Science Chapter 13 You try Using a fixed pulley, you pull the rope down 1.0 m with a force of 72 N. A 65-N object is raised 1.0 m. What is the efficiency of the pulley? April 17 General Science Chapter 13