One Chapter 4

• Kinematics - The branch of physics dealing with the description of motion.
  • involves displacement, velocity, and acceleration.
• Dynamics - The branch of physics that deals with the causes of motion.
• Mechanics - Kinematics and dynamics together.

• The Greeks incorrectly thought that a force causes constant motion. To them, increased force causes faster motion, and without force, all objects tended to their natural state of motion which was at rest.
• Galileo reasoned:
  • A ball speeds up when rolling down a slope and slows down when rolling up a slope. Therefore, on a level surface it would neither slow down nor speed up.
  • Objects stop when moving horizontally due to friction.
  • A ball rolling down an inclined plane, when it roles up a second plane, always rises to the same height as it was released from.
  • If the slope of the second plane is decreased, the ball will role farther but rise to the same height.
  • If the second plane is removed (Therefore, the ball cannot rise to the same height.), the ball should then role forever.
  • It is just as natural for a body to move with constant motion as to be at rest. This idea contradicts Aristotle's ideas.

• Newton's First Law - If no net force acts on an object, the object will maintain in its state of rest, or constant speed in a straight line.
• Implications of the first law:
  • external forces change velocity but internal forces have no effect.
  • The vector sum of external forces must not be zero, for motion to change.
  • Objects at rest, remain at rest if F_net = 0.
  • Objects moving in a straight line, have no changes in motion if F_net = 0.

Inertia - The fundamental property of all matter to resist changes in motion.

• If you do not wear your seat belts, then if the car you are in comes to an unexpected stop, because you are not attached to the car, you will continue moving in a straight line at constant speed (The same speed as what the car was moving at).
• When shoveling the sidewalks, you pick up the snow and throw it with the shovel. The snow leaves the shovel because you stop moving the shovel, and because the snow is not attached to the shovel, it continues to move in a straight line.
• When removing snow from your boots, you stamp your feet. You raise your foot and then lower your boot to stamp. The snow moves with the boot while you lower your foot. When the boot comes to a stop, because the snow is not attached to the boot, it continues to move and falls off.
• Inertia is measured by mass. The greater the mass of an object, the greater is the object's inertia.
• Newton's first law can be thought of as being the Law of Inertia.

• In the equation, F is the net external unbalanced force (net force)
  • F = F₁ + F₂ + .......+ F_n
• Note that 1 N = 1 kgm/s².
• Inertial Mass - The mass obtained by applying a known force to an object and then measuring acceleration. These are substituted into F = ma to calculate the mass.
• Free Body Diagram - A diagram that shows all the forces acting on an object.
  • Free body diagrams are used to determine F_net.
  • Rules:
    • Draw a sketch of the object removed from its surroundings.
    • Represent each force on the object with a force vector.
    • Do not include forces of the object on the surroundings.
    • Vectorally add the vectors to get F_net.

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October 30, 2013