Work and Energy What is work? In your study of motion, you have learned that forces can cause motion. But in some cases, a force that is applied is balanced by another opposite force, and there is no net motion as a result. Look at the following illustrations, and identify the forces and motion in each one. (See illustrations on following slide.) 1. In one drawing, no motion is likely to occur. Which drawing is it? 2. Describe the forces that are acting in this diagram. If the person exerts slightly more force, what happens to the opposite force? Does it increase to match the new force of the person, stay the same, or decrease? Chapter 12 Work • Work: when a force causes change in the position or direction of an object – – – – – – The object will move in the direction of the force Work = force x distance W=Fxd Measured in Nm also known as Joules 1 Nm = 1 J = 1 kg m2/s2 You may apply a lot of force to try and move the object, but if the object does not move, then you have not done any work –in the physics sense. (Although it may feel like you have done work, unless it moves, you haven’t done work) The object must move or change direction to have had work done to it. Is holding a book above your head doing work? Ex 1: A crane uses an average force of 5200 N to lift a girder 25m. How much work does the crane do on the girder? Ex 2: While rowing in a race, John uses his arms to exert a force of 165 N per stroke while pulling the oar 0.800 m. How much work does he do in 30 strokes? Power • Running up a flight of stairs does not require more work than walking up slowly does, but it is more exhausting. • The amount of time it takes to do work is an important factor when considering work and machines. • The quantity that measures work in relation to time is POWER • Power = work P=W time t Power = work P=W time t How are power and work related, if time is constant? How are power and time related, if work is constant? Power is measured in Watts (W) (or hp- horsepower) A watt is the amount of power required to do 1 J of work in 1 second. Problems: W=Fxd P=W t 1. It takes 100 kJ of work to lift an elevator 18 m. If this is done in 20 s, what is the average power of the elevator during the process? 2. Anna walks up the stairs on her way to class. She weighs 565 N and the stairs go up 3.25 m vertically. Calculate the power output if she climbs the stairs in 12.6 seconds. 3. What if Anna climbs the stairs in 10.5 seconds, what would be her power output? MACHINE Any device that makes work easier Chapter 12 You may not think of a door as a simple machine, but it is one. It functions like a lever. Like other levers, when you exert a force on it (an input force), a force is exerted along the entire door (the output force). 1. For all levers, one point along the lever stays still while the rest of the lever moves. This point is called the fulcrum. Where is the fulcrum of a door? 2. You can push at any point along the width of a door and it will open. Which position requires the least force: pushing the door near the hinges, in the middle, or near the side farthest from the hinges? (Hint: Which of these feels easiest to do?) 3. If you are trying to prop the door open, but your only doorstop is not very heavy, is it likely to work best near the hinges, in the middle, or near the side farthest from the hinges? Machines and Mechanical Advantage Which is easier, lifting a car yourself or using a jack? Machines can be used to take advantage of the fact that force and distance are inversely proportional. So increasing one will decrease another The longer the distance, the less force needed to do the same work Machines do not increase the quantity of work that one can do Chapter 12 Force and Work Why is it easier to push a box up a ramp to a truck, rather than lift it up to a truck? Because you are increasing the distance, thus lowering the force needed to do the same work Machines Machines help us to do work by redistributing the work that we put into them. Machines can change the direction of an input force Machines can increase or decrease the force by changing the distance Some machines amplify force and some amplify distance and thus speed. A baseball bat is a machine that increases speed by increasing the distance In other words Make work easier by redistributing the work Change direction Increase or decrease force by changing the distance over which it is applied Work in = Work out Fin din = Foutdout If force decreases, distance increases so the work remains equal Mechanical Advantage M. A. is a ratio that measures how much a machine multiplies force or distance M.A. = Input distance = Output force Output distance Input force All of the calculations that you will learn for the MA of different machines are variations of this equation! EXAMPLE OF WHAT A MACHINE DOES FOR US Using a screwdriver to open a can of paint… Changes direction of force Changes size of force Simple Machines The most basic machines are called simple machines There are six simple machines Other machines are just combination of the six simple machines Two families of simple machines The lever family and the inclined plane family A simple machine does the work with only one movement Advantages? Changes the force you exert: In size In direction In both size and direction 6 TYPES of SIMPLE MACHINES (in 2 categories) LEVER WHEEL AND AXLE PULLEY INCLINED PLANE SCREW WEDGE The lever family • Simple lever, pulley, wheel and axle are the three types of simple machines in the lever family • Simple lever: Like a hammer pulling out a nail – – All levers have a rigid arm that turns around a point called a fulcrum (the pivot point) Force is transferred from one part of the arm to another LEVER A lever is a simple machine. It is a board or bar that rests on a turning point. This turning point is called the fulcrum. An object that a lever moves is called the load. The closer the object is to the fulcrum, the easier it is to move. Three parts of a lever FULCRUM…the pivot point OUTPUT …resistance, the load, transmitted out of the machine INPUT …effort, the force you put into the machine Types of Levers Look for which of the three parts of the lever is in between the other two. 1st Class Fulcrum is in between 2nd Class Output is in between 3rd Class Input is in between Mechanical Advantage = input length/output length MA=Lin/Lout (measure from fulcrum to input and from fulcrum to output) First Class Lever 1st Class lever: the fulcrum is center, input at one end and output at the other. They either multiply force or increase distance (hammer). The fulcrum is between the effort and the load Fulcrum is in between input and output See-saw, rowboat oar MA can be greater or less than 1 Second Class Lever 2nd Class Lever: The fulcrum is at one end and input force is at the other so as to multiply force. The output force is in the middle (wheelbarrow). The load is located between the fulcrum and the effort. The fulcrum is at one end and the load is in the middle..the effort is at the opposite end. Output in between fulcrum and input Wheelbarrow, bottle opener, nutcracker MA is always greater than 1 Third Class Lever 3rd Class Lever Input is in between output and fulcrum Hockey stick, tweezers The fulcrum is at one end and the input force is in the middle. The output force is at one end. They always increase distance. (the human arm) The effort is between the fulcrum and the load. The fulcrum is at one end and the load is at the other….the effort is in the middle MA is always less than 1 Used to increase distance The Lever Family Pulleys: are modified levers The point in the middle of the pulley is like the fulcrum….it is the pivot point Pulleys are like 1st class levers because the “pivot” point is in the center….between the input and the out…between the effort and the load/strength. Pulleys can be added together to amplify the advantage PULLEY This simple machine is made up of a wheel and a rope. The rope fits on the groove of the wheel. One part of the rope is attached to the load. When you pull on one side of the pulley, the wheel turns and the load will move. Pulleys let you move loads up, down, or sideways. Pulleys are good for moving objects to hard to reach places. It also makes the work of moving heavy loads a lot easier. MA= # pulleys if pulling down MA= #pulleys + 1 if pulling up EXAMPLES OF PULLEYS Flag Poles Clothes Lines Sailboat Blinds Crane The Lever Family Wheel and Axle: lever or pulley attached to a shaft Like a steering wheel When the wheel is turned, the axel also turns. When a small force is applied to turn the wheel, the force is multiplied to become a large output force applied to the steering column, which turns the front wheels of the car. Screwdriver and cranks WHEEL AND AXLE The wheel and axle is another simple machine. The axle is a rod that goes through the wheel. This lets the wheel turn. It is easy to move things from place to place with wheels and axles. Gears are a variation in which wheels move together because they are connected by teeth or a chain. MA of wheels=dw/da=rw/ra MA of gears=de/dr (the effort is often the larger gear) EXAMPLES OF WHEEL AND AXLE Cars Roller Skates Wagons Door Knobs Gears in Watches, Clocks, and Bicycles The Inclined Plane Family • An inclined plane, a ramp • A wedge: modified inclined plane • A screw: an inclined plane wrapped around a cylinder Inclined Plane Family INCLINED PLANE A simple machine that is a flat surface that is higher on one end. You can use this machine to move an object to a lower or higher place. Inclined planes make the work of moving things easier. You would need less energy and force to move objects with an inclined plane. MA=Lslope/hslope EXAMPLES OF INCLINED PLANES Ramp Slanted Road Path up a Hill Slide Materials move along an inclined plane Ramp • An inclined plane : ramp – – – Changes magnitude and direction of force Pushing a box up a ramp requires less force than lifting it directly. The work is spread over a greater distance Wedge Modified inclined plane – Turns downwards force into two forces directed out to the sides, like a nail WEDGE A wedge is a simple machine used to push two objects apart. A wedge is made up of two inclined planes. These planes meet and form a sharp edge. This edge can split things apart. A wedge moves through material to transfer a downward force sideways. MA: the thinner the wedge, the greater the MA EXAMPLES OF WEDGES Knives Axes Forks Nails A Screw • An inclined plane wrapped around a cylinder – – Jar lids and spiral stair cases are examples Gentle slopes of the threads of a screw make it easier because it requires less force. SCREW A screw is a simple machine that is made from another simple machine. It is actually an inclined plane that winds around itself. A screw has ridges and is not smooth like a nail. Some screws are used to lower and raise things. They are also used to hold objects together. MA: the smaller the pitch, the greater the MA Pitch is the distance between threads EXAMPLES OF SCREWS Jar Lids Light Bulbs Stools Clamps Jacks Wrenches Key Rings Spiral Staircase Types of Simple Machines Review What type of simple machines are these? • • • • • • • • • • Scissors: Hammer: Boat Oar: Can opener: Flag Pole: Bottle opener: Door knob: Axe: Jar: Tweezers: Compound Machines Many of the devices that we use are a combination of more than one simple machine Car Jack: uses a lever and a screw Bicycle: uses a variety of simple machines A compound machine Any device that uses more than one simple machine Example: Bicycle Wheels and axles Levers Screws A compound machine Any device that uses more than one simple machine Example: Bicycle Wheels and axles Levers Screws In the chapter on matter, you learned that energy is conserved. Instead of being created or destroyed, it is just changed from one form to another. The energy of the sunlight that reaches Earth is the ultimate source of most of the energy around us. Look at the illustration below, and identify the types of energy involved. 1. How did energy from sunlight provide the energy the girl needed to swing the bat? (Hint: What do you need to have energy?) 2. When the girl hits the ball, she exerts a force on it. Does she do work on the ball in the scientific sense of the term? Explain why. 3. After the girl hits the ball, the ball moves very fast and has energy. When the ball hits the fielder’s glove, it stops moving. Given that energy can never be destroyed but merely changes form, what happened to the energy the ball once had? (Hint: If you are the fielder, what do you hear and feel as you catch the ball?) You give yourself and your sled gravitational potential energy as you pull your sled to the top of a snowy hill. You get on board your sled and slide to the bottom of the hill, speeding up as you go. 1. When does the sled have the most potential energy? When does it have the least potential energy? 2. Where does the sled have the most kinetic energy? the least kinetic energy? 3. What happens to the relative amounts of potential and kinetic energy as the sled slides down the hill? What happens to the total energy? 4. After the sled reaches the bottom of the hill, it coasts across level ground and eventually stops. What happened to the energy the sled had? Energy and Work When you stretch a sling shot, you do work and you transfer energy to the elastic band. The elastic band then does work on the rock by transferring energy. Energy can not be created or destroyed Energy can be transferred Energy can be defined as the ability to do work, so both use Joules as the unit Energy can be present but undetected. It may only get noticed when it is actually transferred. Potential energy (PE): energy of position or energy that is stored and unused Elastic PE: energy stored in stretch or compressed elastic material Gravitational PE: any two objects separated by a distance (like an apple falling from a treethe greater the height, the greater the PE) Kinetic energy (KE): energy of motion or energy that is used KE depends on the mass and speed of the object The faster an object is going the more KE it has KE = ½ mv2 The units are joules (J) KE depends on speed more than mass which is why it is squared. This is why a car crash at high speeds is so much more dangerous than at lower speeds despite the mass being the same. Increasing temperature will increase movement and thus increase Kinetic Energy Also, the more Kinetic Energy you have the higher the temperature. KE ↑ b/c Temp ↑ Problems 1. Calculate the Kinetic energy of a 1500 kg car moving at 29 m/s. 2. Calculate the kinetic energy of 2000 kg car moving at 13 km/hr. 3. A 35 kg child has 190 J of kinetic energy after sledding down a hill. What is the child’s speed in meters per second at the bottom of the hill? Other Types of Energy Mechanical: the amount of work an object could do based off of the object’s potential and kinetic energy. Chemical Energy: the energy from chemical reaction. Electrical Energy: results from a flow of charged particles through conductive materials Moving electrons can cause light or magnetic fields Flow of Energy • People get energy from living things – We eat sugars and fats and carbs to get energy • Living things get energy from the Sun – This energy travels through electromagnetic radiation known as ultraviolet and visible light – Photosynthesis: when plants use energy from the sunlight and convert it to chemical energy, which is stored as sugars • The Sun gets energy from nuclear reactions. – Nuclear reactions are a form of potential energy – Fusion: when two nuclei are combined or fused to form a heavier nucleus – Fission: when a heavy nucleus is split into two lighter nuclei Energy can be transferred • But not created or destroyed! • If total energy cannot be changed then when KE goes up, PE goes down. • KE and PE are inversely related • TE = KE + PE • This is how a tennis ball can bounce – If you drop a tennis ball it will bounce up to the height it was dropped (in a perfect world) – If you throw the ball downward, the KE will be transferred into elastic PE as it compresses and then back to KE as it bounces back up to you. – Mechanical energy can turn into sound energy or heat energy, thus a bouncing ball will not return to its original height. – Friction and air resistance can also be a source of transferred energy Transfer of energy can result in loss of work: Because of friction and other factors, only some of the work done by a machine is applied to the task at hand. Some may be “lost” or transferred to some other form of energy. There is a difference between the total work and “useful” work Can something be in perpetual motion? Why?