Download Work, Power and Energy, (study buddy)

Work, Power and Energy, (study buddy)
I. Work

What is Work? (definition/description/equation)
Symbol: W
Unit: Joule or Newton-meter
Work is done when a net Force is applied to an object, and it moves in the direction of the applied force
Work = Force * Distance
(must use fundamental units: meters, Newtons, etc)
Work: (1) applied force, (2) causes motion, (3) in the same direction
Energy is expended when work is done
Work is directly proportional to both Force and distance moved
 Examples of Work and non-work:
Note that Torque also = F•d. but the d’s are defined differently. In torque, the object may not be
moving.
II. Power

What is Power? (definition/description/equation)
Symbol: P
Unit: Watt = J/s
Power is the rate at which work is done
Power = Work/time

Examples of Power:
III. Energy
 What is Energy?
Energy is the ability to cause work to be done : makes matter move or change

Two Types of Energy:
o Kinetic Energy (definition/description/equation)
Symbol: Uk or KE
Unit:Joule
Energy of Motion
Kinetic Energy = ½ * m * v2 (note that KE cannot be negative)
Change in Kinetic Energy = Uk final – Uk initial
KE is directly proportional to the mass, but is directly proportional to the velocity squared
o Potential Energy (definition/equation)
Symbol: PE
Unit:Joule
Stored Energy. It has the potential to do work. The common potential energies used in physics are
functions of position. Is (+) when object can do work. Is (-) when work must be done on object
o Types of Potential Energy:
 Gravitational Potential Energy
GPE, Ug
Energy Stored due to height . Ug = m * g * h = mass * accel of gravity * height
 Elastic Potential Energy
EPE, Ue
Energy Stored due to stretching or compressing an elastic material, (usually a spring)
Ue = ½ * k * x2 = ½ * spring constant * distance spring was stretched or compressed2
 Hooke’s Law
Direct relationship between the Force applied to a spring, and the amount it stretches or compresses
F = k * x : The Force applied to a spring = spring constant * distance spring stretches or compresses.
This relationship is often used to find the spring constant, (as force and distance can usually be easily
measured)
 Other forms of Potential Energy:
There are other forms of potential energy such as thermal, nuclear or chemical.
IV. Law of Conservation of Energy
 Statement of the Law of Conservation of Energy:
Energy cannot be created nor destroyed in any physical or chemical process
However, energy can change forms, (from kinetic to potential, or vice versa)
Consider a shot-put falling onto a grassy field

A
B
C
D
Position A = ball being held at some height above the ground
Position B = ball halfway to the ground
Position C = right before the ball is stopped, but it has no height.
Position D = ball has embedded itself into the ground
If the total Energy at the beginning was Utotal , what can we say about the energy at each point
in terms of Kinetic and Potential?
Energy at point A: All energy is gravitational potential, (ball not moving yet)
Utotal = Ug
Energy at point B: Total energy is the sum of Gravitational + Kinetic
Utotal = Ug + Uk
Energy at point C: All energy has transformed into Kinetic
Utotal = Uk
Energy at point D: All energy has been dissipated into other forms, such as heat,
compression of the soil, etc.
Throughout this process, from A to D, the Utotal was constant.
Conservation of Mechanical Energy:
UTotal = Ukinetic + Upotential
(Ignores friction or other losses)
V. Work-Energy Theorem:
The Net Work done by or on an object is equal to that object’s change in Kinetic Energy:
W = ΔUk