 # Chapter 7 Energy, Part 1 Work Power Mechanical Energy Potential Energy

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Chapter 7 Energy, Part 1 Work Power Mechanical Energy Potential Energy
Work-Energy Theorem Conservation of Energy Efficiency Comparison of Kinetic Energy and Momentum Energy for Life Sources of Energy Next time

Work Something 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) h F

The Unit of Work is the Joule (or N-m)
Since work is a force moving over a distance ….. Work = Force x distance The Unit of Work is the Joule (or N-m) W = F x d Work IS done on an object when: Work IS NOT done on an object when there is no motion. Force Acts on it in the direction of motion Or when the force is applied perpendicular to the motion. Or has a component in the direction of motion

Question 1 A 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. Energy B. Engine speed C. Work D. Displacement

Question 1 Answer A 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. Energy B. Engine speed C. Work D. Displacement

The Unit of Power is the “Watt” (or Joules/sec)
Power is the amount of Work done per unit time Work done Power = Time interval It’s the RATE of doing Work or, the RATE of burning Energy The 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.

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.

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: h PE = Weight x height or It 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 = mgh In Joules

Kinetic Energy With 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

(doesn’t matter how LONG the ramp is)
Work-Energy Theorem The work done on an object is equal to the change in its kinetic energy. Work = ΔKE In 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 mv2 If 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.

Question 2

Chapter 7 Energy, Part 2 Work Power Mechanical Energy Potential Energy
Work-Energy Theorem Conservation of Energy Efficiency Comparison of Kinetic Energy and Momentum Energy for Life Sources of Energy Last time

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 = mgh The 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 + KE Using 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.

Conservation of Energy
All Potential Energy, no Kinetic Energy 3/4 Potential Energy, 1/4 Kinetic Energy Work done to get diver to the top of the tower! 1/2 Potential Energy, 1/2 Kinetic Energy 1/4 Potential Energy, 3/4 Kinetic Energy No Potential Energy, all Kinetic Energy

Question 1 What will be the kinetic energy of a pendulum bob when it undergoes a 10 kJ decrease in potential energy? A. 10 MJ B kW C. 10kJ D. None of the above

A System of pulleys that changes the direction and multiplies force
Machines … … multiply forces or change the direction of forces. Lever Lever Pulley Changes Direction of Force Block and Tackle A System of pulleys that changes the direction and multiplies force Work in = work out Pulley Multiplies Force Input distance large Output distance small Input force small Output force Large Can’t get more work out than you put in … conservation of energy !!!

Efficiency In 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 output Efficiency = Total energy input

Question 2 You’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%

Kinetic Energy vs. Momentum
Both are properties of motion. But they are different properties. Kinetic Energy Momentum Scalar Vector Proportional to v2 Proportional to v Can be converted to other energy forms Cannot be converted, can only be gained or lost

Sources of commercial energy
Petroleum and coal Solar energy – photovoltaic cells Wind generation Nuclear Power Geothermal Energy Hydrogen-based fuel technologies

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