Chapter 7 Energy, Part 1 Work Power Mechanical Energy Potential Energy
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1 Chapter 7 Energy, Part 1 Work Power Mechanical Energy Potential Energy Work-Energy TheoremConservation of EnergyEfficiencyComparison of Kinetic Energy and MomentumEnergy for LifeSources of EnergyNext time
2 WorkSomething done by a force when the force acts on the object, moving it through a distance.For work to be done, a force must be applied to an object and the object must move in the direction of the force.Examples of doing work.You lift a heavy box from the floor to a table 3 ft higher.(You do work on the box)A book falls off a table and free falls to the ground.(Gravity does work on the book)A rocket accelerates through space.(The exhaust gases do work on the rocket)hF
3 The Unit of Work is the Joule (or N-m) Since work is a force moving over a distance …..Work = Force x distanceThe Unit of Work is the Joule (or N-m)W = F x dWork IS done on an object when:Work IS NOT done on an object when there is no motion.ForceActs on it in the direction of motionOr when the force is applied perpendicular to the motion.Or has a component in the direction of motion
4 Question 1A force sets an object in motion. When the force is multiplied by the time of its application, we call the quantity impulse, which changes the momentum of that object. What do we call the quantity force x distance?A. EnergyB. Engine speedC. WorkD. Displacement
5 Question 1 AnswerA force sets an object in motion. When the force is multiplied by the time of its application, we call the quantity impulse, which changes the momentum of that object. What do we call the quantity force x distance?A. EnergyB. Engine speedC. WorkD. Displacement
6 The Unit of Power is the “Watt” (or Joules/sec) Power is the amount of Work done per unit timeWork donePower =Time intervalIt’s the RATE of doing Workor, the RATE of burning EnergyThe Unit of Power is the “Watt” (or Joules/sec)A 100 kW engine will do 100 thousand Joules of Work every second!100 kW = 134 Horsepower (the English units for Power)A 60 W light bulb burns 60 Joules of energy every second.
7 Mechanical Energy Energy is the capacity for doing work. Some things store energy in their structure or or because of their physical nature.Examples: Gasoline, energy in food, energy stored in the atom, etc.Sometimes energy is stored by an object due to an it’s position or because it’s in motion .. This is “Mechanical Energy”.This kind of energy can be stored in objects by doing work on them.There are two most common forms of Mechanical Energy.Potential Energy – Energy stored by an object due to its position.Kinetic Energy – Energy stored by an object because it is moving.
8 Potential Energy Energy stored in a system because of its position. h When an object is lifted against gravity, it’s potential energy due to gravity is:hPE = Weight x heightorIt doesn’t matter what path the ball takes to get from the ground to height “h” … the gain in potential energy is the same!PE = mghIn Joules
9 Kinetic EnergyWith kinetic energy, an object or system has the ability to do work due to its motion.The faster something is moving and the heavier it is, the more work it can do, so …An object’s kinetic energy depends on its mass AND its speed.Again, in Joules
10 (doesn’t matter how LONG the ramp is) Work-Energy TheoremThe work done on an object is equal to the change in its kinetic energy.Work = ΔKEIn this example, the work done by gravity on the block (mgh) is equal to the change in kinetic energy of the block (½ mv2)so, mgh = ½ mv2If you know the height of the block, you can predict it’s speed at the bottom of the ramp!(doesn’t matter how LONG the ramp is)Gives the same answer as above.
13 Chapter 7 Energy, Part 2 Work Power Mechanical Energy Potential Energy Work-Energy TheoremConservation of EnergyEfficiencyComparison of Kinetic Energy and MomentumEnergy for LifeSources of EnergyLast time
14 Conservation of Energy Energy can take on several forms (kinetic, potential, heat, light, etc)Energy can’t be created or destroyed; it may be transformed from one form to another, but the total amount of energy can never change.A common example is the pendulum:The formula to calculate the potential energy is:PE = mghThe mass of the ball = 10kg The height, h = 0.2m The acceleration due to gravity, g = 9.8 m/s^2 Substitute the values into the formula and you get:PE = 19.6J (J = Joules, unit of energy) The position of the blue ball is where the Potential Energy (PE) = 19.6J while the Kinetic Energy (KE) = 0.The position of the purple ball is where the Kinetic Energy is at its maximum while the Potential Energy (PE) = 0.The position of the pink ball is where the Potential Energy (PE) is once again at its maximum and the Kinetic Energy (KE) = 0.Total Mechanical Energy = PE + KEUsing our common sense we know that it's impossible for the pendulum to swing higher than the height h without giving it a push yourself. If there was no friction, the pendulum would swing back and forth forever because of the law of conservation of energy.
15 Conservation of Energy All Potential Energy, no Kinetic Energy3/4 Potential Energy, 1/4 Kinetic EnergyWork done to get diver to the top of the tower!1/2 Potential Energy, 1/2 Kinetic Energy1/4 Potential Energy, 3/4 Kinetic EnergyNo Potential Energy, all Kinetic Energy
16 Question 1What will be the kinetic energy of a pendulum bob when it undergoes a 10 kJ decrease in potential energy?A. 10 MJB kWC. 10kJD. None of the above
17 A System of pulleys that changes the direction and multiplies force Machines …… multiply forces or change the direction of forces.LeverLeverPulley Changes Direction of ForceBlock and TackleA System of pulleys that changes the direction and multiplies forceWork in = work outPulleyMultiplies ForceInput distance largeOutput distance smallInput force smallOutput force LargeCan’t get more work out than you put in … conservation of energy !!!
18 EfficiencyIn a machine, there are always losses of energy, to heat, to friction, etc.Efficiency measures how well a machine limits losses.In a lever, if you do 100 J of work and the machine puts out only 98 J, you’ve lost 2 J to thermal heating of the machine, and the lever’s efficiency is 98%.Useful energy outputEfficiency =Total energy input
19 Question 2You’re using a block and tackle to lift a heavy load. You do 50 J of work on the rope of the machine, but the work done by the machine on the load is only 40 J. What is the efficiency of the block and tackle?A. 100%B. 80%C. 120%D. 60%
20 Kinetic Energy vs. Momentum Both are properties of motion.But they are different properties.Kinetic EnergyMomentumScalarVectorProportional to v2Proportional to vCan be converted to other energy formsCannot be converted, can only be gained or lost
21 Sources of commercial energy Petroleum and coalSolar energy – photovoltaic cellsWind generationNuclear PowerGeothermal EnergyHydrogen-based fuel technologies