# Work, Energy and Power

Definitions

Work can be defined as transfer of energy. In physics we say that work is done on an object when you transfer energy to that object. If one object transfers (gives) energy to a second object, then the first object does work on the second object.

Work is the application of a force over a distance. Lifting a weight from the ground and putting it on a shelf is a good example of work. The force is equal to the weight of the object, and the distance is equal to the height of the shelf (W= Fxd).

Work-Energy Principle --The change in the kinetic energy of an object is equal to the net work done on the object.

Energy can be defined as the capacity for doing work. The simplest case of mechanical work is when an object is standing still and we force it to move. The energy of a moving object is called kinetic energy. For an object of mass m, moving with velocity of magnitude v, this energy can be calculated from the formula E= 1/2 mv^2.

Types of Energy

There are two types of energy in many forms:

Kinetic Energy = Energy of Motion

Potential Energy = Stored Energy

Forms of Energy

Solar Radiation -- Infrared Heat, Radio Waves, Gamma Rays, Microwaves, Ultraviolet Light

Atomic/Nuclear Energy -energy released in nuclear reactions. When a neutron splits an atom's nucleus into smaller pieces it is called fission. When two nuclei are joined together under millions of degrees of heat it is called fusion

Electrical Energy --The generation or use of electric power over a period of time expressed in kilowatt-hours (kWh), megawatt-hours (NM) or gigawatt-hours (GWh).

Chemical Energy --Chemical energy is a form of potential energy related to the breaking and forming of chemical bonds. It is stored in food, fuels and batteries, and is released as other forms of energy during chemical reactions.

Mechanical Energy -- Energy of the moving parts of a machine. Also refers to movements in humans

Heat Energy -- a form of energy that is transferred by a difference in temperature

What is Power

Power is the work done in a unit of time. In other words, power is a measure of how quickly work can be done. The unit of power is the Watt = 1 Joule/ 1 second.

One common unit of energy is the kilowatt-hour (kWh). If we are using one kW of power, a kWh of energy will last one hour.

Calculating Work, Energy and Power

WORK = W=Fd

Because energy is the capacity to do work , we measure energy and work in the same units (N*m or joules).

POWER (P) is the rate of energy generation (or absorption) over time:P = E/t

Power's SI unit of measurement is the Watt, representing the generation or absorption of energy at the rate of 1 Joule/sec. Power's unit of measurement in the English system is the horsepower, which is equivalent to 735.7 Watts.

Try these exercise!

1) A force of 20 newtons pushing an object 5 meters in the direction of the force. How much work is done?

joules

2) If you do 100 joules of work in one second (using 100 joules of energy). How much power is used?
watts

3) 1 horsepower is equal to how many watts?
watts

Multiple Choice Questions

1. An example of Kinetic Energy would be:
a) a moving car
b) a stretched rubber band that was just released
c) a charge particle in an electric field
d) all of the above

2. An example of Potential Energy would be:
a) a moving car
b) a battery
c) a book resting on a table
d) both b and c

3. Which is not an example of Solar Radiation
a) microwaves
b) magnetism
c) gamma rays
d) visible light

4. An example of a system having both kinetic and potential energy would be:
a) a book resting on a table
b) a piece of sugar
c) an object in free fall
d) a stretched rubber band

5. Which of the following statements is not correct
a) energy is the capacity to do work
b) Work can be express as Force x Distance
c) power is the amount of work done in a unit of time
d) the unit of power is the joule

#### Force, Work and Energy

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About Energy: What is Energy, Types of Energy, Kinetic Energy, Potential Energy, Law of Conservation of Energy, Energy Efficiency, Sources of Energy, Assessment Test...

Work, Energy and Power: Definitions for work, energy and power. Types of energy, calculating work, and power.

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Mathematical Relationships in Science: See Lab 5, Acceleration.