Docstoc

D2C9197ECDDC4956993C158E2C984FEE

Document Sample
D2C9197ECDDC4956993C158E2C984FEE Powered By Docstoc
					• There are 6 basic types of simple machines.
  Simple machines are the most basic tools
  used to decrease force while increasing
  distance. They are:
  –   The Lever
  –   The Pulley
  –   The Inclined Plane
  –   The Wedge
  –   The Screw
  –   The Wheel and Axle
• Levers are simple machines that involve a
  rigid arm and a fulcrum. The fulcrum is
  the point about which a lever pivots.
                      “Give me a place to
                      stand and a lever
                      long enough, and I
                      will move the world.”
                      - Archimedes
Class 1: Input (effort) is on
opposite side of fulcrum from
output (load).
Class 2: Effort (input) is farther
from fulcrum than load
(output).
Class 3: Effort (input) is closer
to fulcrum than load (output).
This lever is does not have
much mechanical advantage.
  1st Class:



2nd Class:



3rd Class:
• Pulleys are devices that change the direction
  of tension in a rope or wire.
                                 • Combinations
                                   of pulleys
                                   allow you to
                                   exert less
                                   force, but
                                   over a greater
                                   distance.
• Inclined planes are
  useful because they
  require less force to
  ascend. What sorts
  of situations might
  you expect to see
  inclined planes?
• What is the mechanical advantage to the
  inclined plane shown below?
• Wedges are essentially
  inclined planes.
• An axe blade is a great
  example of a wedge.
• A screw is a central
  axle with a helical
  “thread” running
  around it.
• Rotary force is
  converted into
  vertical force.
• Some car jacks work
  on the principle of a
  screw.
• A wheel is a circular
  device which rotates
  around a central pivot
  called an axle.
• When you turn a
  steering wheel, you
  exert relatively little
  force, but large
  amounts of force are
  felt closer to the axle.
  Why do you think
  truckers have such big
  steering wheels?
• Mechanical energy is generally classified in two
  forms:
   – Kinetic energy – energy associated with movement.
     Examples: running, car moving, a ball thrown with a
     certain speed.
   – Potential energy – energy associated with temporary
     storage of mechanical. That is, it has the “potential” to be
     converted to kinetic. Examples: a rubber band wants to
     move when stretched; a ball wants to fall off a tall building
     b/c it possesses gravitational potential energy.
• To measure energy, the SI unit we use is the
  Joule. This is the equivalent energy needed to
  exert a force of 1 N over a distance of 1 m.
• Other units of energy include the kWhr, eV,
  Calorie, BTU, and even the erg.
• Energy is NOT to be confused with Power!
  Power is the dissipation of energy over time.
  Without a time description, one cannot have
  power.
• Objects that have mass and move at a non-zero
  speed have kinetic energy. The amount of this
  energy is supplied by a simple equation:




                                  1 2
                              KE  mv
                                  2
• 1.) If you have an object with a mass of 3kg that
  moves at a constant speed of 5 m/s, what is the
  kinetic energy associated with this movement?
• 2.) If you triple an object’s speed, what happens to
  its kinetic energy?
• When you have a mass at any height above the Earth’s
  surface, what happens to the mass when it is released? Why?
• We like to describe the energy that is available to masses by
  putting them at given heights above Earth’s surface by
  gravitational potential energy.
• For example, when an object falls from a building, it begins
  with a height (and gravitational potential energy). As it falls,
  that potential energy is directly converted to kinetic. At the
  bottom of the fall, the object now has zero gravitational
  potential, because it has all converted to kinetic, and there is
  no more height to supply additional potential.
• We have a simple equation for gravitational
  potential energy near Earth’s surface:



                      PE  mgh
   This equation is only valid for situations in which objects are close to
   Earth’s surface. This is b/c when we move away from Earth, the
   gravitational force decreases exponentially.
• We can actually prove this equation based on our
  knowledge of work:

                   Say you pull an object from the reference level
                   shown such that the object moves at a constant
                   speed (equilibrium). The amount of work performed
                   by our pull force is:




           W  F d   mg h   mgh
• 1.) A diver jumps off a 10-m
  platform. Assuming the diver
  “weighs” 52 kg, what is his or her
  potential energy on the platform?
• 2.) What is his or her potential
  energy when he or she reaches a
  height of 7 m?
• 3.) 5m?
• 4.) The water?
• Anytime there is frictionless “freefall”
  occuring, potential is converted directly into
  kinetic. There is no energy lost: the first law of
  thermodynamics. One cannot create or
  destroy energy.
• That is, the sum of kinetic and potential must
  always add to a constant, mechanical energy
  sum.

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:11
posted:6/26/2011
language:English
pages:22