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Work and Energy Chapter 5 By Mr Leavings
What is Work In science Work has a specific meaning. If you push a box with a force of one newton for a distance of one meter, you have done exactly one joule of work. Units of Work: joules. One joule = One newton-meter
What is Work W = F x d
What is Work Work is force times distance moved in the direction of the force Force C Force B Force A: Does 1 joule Force B: Does < 1 joule Force C: No Work Force A 1 Meter
Work When we apply Force in machines we are doing Work. For example , when a Block and Tackle machine lifts weight, force is applied to do this. As a result of the force, the weight moves a distance.
Work done by Machines For a machine to do work it must have an input force. The work output of a simple machine CAN NEVER exceed the work input.
Work done by Machines Nature does not give something for nothing. The work input of the block and tackle is the same as the work output. You get Mechanical Advantage by trading force for distance
Work and Efficiency Ina very Efficient machine, all (or most) of the input work becomes output work. In the block and tackle example before the 10 joules of input work was transformed to 10 joules of output work. This machine was 100% efficient.
Work and Efficiency Is 100% efficiency possible? In a very efficient machine, all (or most) of the work input becomes output. In REAL machines, work output is always less than work input. Other forces such as friction use up some of the input work. Work Output Efficiency = ______________ Work Input
Work and Efficiency 4 Joules 4 Joules Input work = Output work Then it is 100% efficient 4 Joules 3 Joules Input Work does not equal output work 75 % Efficiency 1 Joule
Work and Efficiency 65% of the energy in gasoline is converted to heat. As far as moving the car goes, this heat energy is “lost”. The energy doesn’t vanish, it just does not appear as useful output work.
Work and Efficiency The most efficient machine designed? The bicycle is the most efficient machine ever invented for turning the work of humans muscles into motion. Its efficiency is more than 95%!
Work and Power What is power? You and a friend are pulling boxes across a floor. You drag one box with a force of 100 Newtons for 10 meters, and it takes you 10 seconds. Your friend then drags a similar box for the same distance but it takes him 60 seconds. Have you done the same amount of work?
Work and Power Power: The RATE at which work is done.
Energy The different Kinds of Energy: 6 kinds that we will talk about 1. Kinetic Energy (more on that Later) 2. Potential Energy (more on that Later) 3. Radiant Energy 4. Electrical Energy 5. Chemical Energy 6. Nuclear Energy
Energy • Mechanical energyis the energy possessed by an object due to its motion or its position. • Potential energy and kinetic energy are both forms of mechanical energy.
Energy • Radiant energy is energy that is carried by electromagnetic waves. • Light is one form of radiant energy.
Energy • Electrical energycomes from electric charge, which is one of the fundamental properties of all matter.
Energy • Chemical energyis a form of energy stored in molecules. • Batteries are storage devices for chemical energy.
Energy • Nuclear energyis a form of energy stored in the nuclei of atoms. • In the Sun, nuclear energyis transformed to heat that eventually escapes the sun as radiant energy.
Potential Energy Potential Energy comes from the position of an object relative to earth. Consider a marble that is lifted off the table. Since the Earth’s gravity pulls the marble down we must apply a force to lift it up. Remember the concept of WORK, we are applying a force over a distance, which is stored as the potential energy of the marble.
Potential Energy From Newton’s second law we know that the weight (Force due to weight) is equal to the mass of the marble (m, in kilograms) times the acceleration of gravity (g, equal to 9.8 m/sec2). We also know that the work is equal to force times distance. Since the force is the weight of the marble (mg) and the distance is how far up we lift the marble (h), the work done equals weight times height.
Potential Energy mass of object (kg) PE (joules) height object raised (m) EP = mgh gravity (9.8 m/sec2)
Potential Energy • Systems or objects with potential energyare able to exert forces (exchange energy) as they change. • Potential energy is energy due to position.
Kinetic Energy Objects also store energy in motion. A moving mass can exert forces, as you would quickly observe if someone ran into you in the hall. Kinetic energy increases as the square of the speed. This means the faster you are going the harder it is to stop!
Kinetic Energy KE (joules) mass of object (kg) EK = ½ mv2 velocity (m/sec) Going 60 mph a car has four times as much kinetic energy as it does at 30mph! At 90 mph you have 9 times the energy!
Solving Problems • A 2 kg rock is at the edge of a cliff 20 meters above a lake. • It becomes loose and falls toward the water below. • Calculate its potential and kinetic energy when it is at the top and when it is halfway down. • Its speed is 14 m/s at the halfway point.
Solving Problems • Looking for: • …initial EK, EP and EK, EP half way down. • Given: • mass = 2.0 kg; h = 20 m • v = 14 m/s (half way) • Relationships: • EP=mgh • EK= ½ mv2 • Assume rock starts from rest.
Conservation of Energy Law of Conservation of Energy: Nature NEVER creates or destroys energy; energy only gets converted from one form to another.