Work & Energy

Chapter 13, Section 1 - Work, Power and Machines                                                                                                                    

Work - Using a force to move an object across a distance (in the same direction as the applied force).

Work = Force x Distance
  • If the object does not move, then work = 0.
  • Work involves transferring energy to the object to make it move.
  • Measured in Joules (J).
1 J   =        1 N*m        =             1 kg*m2/s2                    These units are interchangeable, so use the one that works best for the calculation you are doing.
Work = Force x Distance = Mass x Acceleration x Distance


Power - The rate at which work is done OR energy is transferred.

Power = Work / Time
  • Measured in watts (W).
1 W  =  1 J / 1 s
  • It takes more power to do the same amount of work in less time!

Machine - A devices that helps to do work.

Machines help do work by...
...changing the size of a force.
...changing the direction of a force.
...changing both the size and direction of a force.

Machines can multiply the force that you put into them by decreasing the distance over which the force is applied.  (Multiplying the force.)


Mechanical Advantage - How much a machines multiplies a force or distance.
  • Machines with MA > 1 multiply the input force.  
    • (Makes you stronger than you really are!)
  • Machines with MA < 1 multiply the output distance.  
    • (Makes you faster than you really are!)
Different forces can do the same amount of work - work is a product of force and distance!


Homework:
    Pg. 437 #1-8


Chapter 13, Section 2 - Simple Machines                                                                                                                                    

Simple Machine - One of the six basic types of machines.  (All other forms of machines are based on these.)
    Simple machines are divided into two families...

The Lever Family
    Defined by an arm that turns around a point (fulcrum).
  • Simple Lever
    • rigid arm rotates around fulcrum
    • 3 classes
      • depends on where the fulcrum, effort and resistance are.
      • 1st Class - multiplies force OR distance
      • 2nd Class - multiplies force
      • 3rd Class - multiplies distance
  • Pulley
    • rope (arm) around a grooved wheel (fulcrum)
    • 3 types
      • fixed - pulley is fixed, rope moves
      • moveable - one end of rope is fixed, pulley moves
      • block & tackle - combination of fixed & moveable pulleys
  • Wheel & Axle
    • wheel (arm) connected to a shaft/axle (fulcrum)

The Inclined Plane Family
    Defined by a ramp or slanted surface.
  • Inclined Plane
    • A ramp.
    • Inclined planes reduce the force needed to lift an object by spreading it over a larger distance.
    • Mechanical advantage = length of inclined plane / height of inclined plane
  • Wedge
    • Two inclined planes, back-to-back.
    • Changes one downward force into two sideways forces.
  • Screw
    • Inclined plane wrapped around a cylinder.
    • Spreads the force over a larger distance (the threads of the screw)

Compound Machines - A machine made of more than one simple machine.
ex. - scissors (2 levers), bicycle, car

Homework:
    Pg. 443 #1-7.


Chapter 13, Section 3 - What Is Energy                                                                                                                                      

Energy - The ability to do work.

Energy is transferred or transformed whenever work is done.

Measured in Joules (J).

Potential Energy (PE) - Energy an object has because of its position, shape or condition.

Elastic PE - PE that an object has when it is stretched or compressed.  (It will release this energy when it returns to its original shape.)

Gravitational PE - PE from the gravitational attraction of two objects separated by a vertical distance.
  • gravitational PE = mass x free-fall acceleration x height
    • grav PE = mgh
      • mass x free-fall acceleration = weight (a force)
      • height = distance (that the object will fall, not necessarily to the ground)
        • So, gravitational potential energy = force x distance  (Just like the formula for "work"!)
Kinetic Energy (KE) - Energy of an object due to its motion.
  • Depends on the object's mass and speed.
KE = 0.5 x mass x speed2

J     =              kg    x    m2/s2

  • Speed has a greater effect on KE than mass.
    • Because speed is squared, a small change in speed = a larger change in energy.
  • KE is related to temperature.
    • Atoms/molecules are always moving => they have kinetic energy.
    • Temperature is the measure of the average kinetic energy of the particles.

Mechanical Energy - The amount of work an object can do because of its potential and kinetic energy.  (PE + KE = mechanical energy)

Chemical Energy - A type of potential energy that involves making and breaking bonds between atoms.
A reaction that gives off energy decreases the potential energy in the substance.

Living things get energy from the Sun.
Photosynthesis => light energy to chemical energy (stored in sugars, lipids, etc.)
Sun gets energy from nuclear fusion (combining small atomic nuclei to make larger ones).
Nuclear fission (used in power plants) breaks large atomic nuclei into smaller ones.

Energy can be stored in fields.
Moving electrons create...
  • electric fields
  • magnetic fields
Electomagnetic waves (such as light) are made up of electric and magnetic fields.


Homework:
    Pg. 452 #1-3, 5-9.

Chapter 13, Section 4 - Conservation of Energy                                                                                                                          

Energy can be transformed, but the total energy stays the same.

Potential Energy <=> Kinetic Energy
On a rollercoaster, potential and kinetic energy get changes back and forth.

Some energy gets changes to other forms, such as heat (by friction) and sound (by air resistance).

Law of Conservation of Energy
Energy cannot be created or destroyed.
(But it can be transformed or transferred!)

Energy is always transferred as work or heat.
1st Law of Thermodynamics
For any system, the net change in energy = energy transferred as work or heat.

System - A set of connected parts that work together.
  • Open System - Matter and energy are exchanged with the system's environment.
  • Closed System - Energy is exchanged with the environment.
  • Isolated System - Nothing is exchanged with the environment.

Efficiency - The ratio of useful work output (from a machine) compared to work input.

Efficiency = Work Output / Work Input

No machine is 100% efficient - some work is converted to heat, sound, etc.
(No such thing as a perpetual motion machine.)
Machines always need energy input to work.

Homework: