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Force & Newton's First Law
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Force & Newton's First Law


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  • electromagnetic, gravitational & atomic forces
  • Lack of motion or lack of CHANGE in motion does not mean forces are not being applied.
  • Acceleration = change in speed (faster or slower) or direction.
  • drop a paper flat vs. vertical: less air molecule resistance when vertical, so it falls faster (less friction) Is it easier to slide across a hard, polished floor or carpet?
  • drop a paper flat vs. vertical: less air molecule resistance when vertical, so it falls faster (less friction) Is it easier to slide across a hard, polished floor or carpet? * Adhesion can sometimes increase friction, esp. in fluids
  • Transcript

    • 1. Forces & Motion Momentum
    • 2. Newton's First Law
      • Newton's First Law : The velocity of an object will not change unless the object is acted on by an outside force .
        • An object is at rest will stay at rest until a force causes it to move.
        • An object in motion will continue in at its current velocity until a force changes its speed or direction.
      • In other words, objects resist changes in their motion (they tend to keep doing what they're doing). They have INERTIA .
    • 3. Inertia
      • Inertia : the resistance to changes in movement.
      • Mass is a measure of inertia.
        • The more mass an object has, the more resistant to changes in movement (inertia) it will have.
        • Momentum is similar, but it takes mass AND velocity into account when considering how resistant to change in motion an object is.
    • 4. Inertia
    • 5. What is a Force?
      • Force : a push or pull. Measured in Newtons.
        • Contact Forces
          • a force is applied as two objects touch
            • child throws snowball
            • horse pulls a wagon
            • hammer hits a nail
        • Action-at-a-Distance Forces
          • a force is applied even though objects don't touch
            • magnet picks up paper clip
            • Moon pulls Earth's oceans
            • electrons stay in orbit around an atom's nucleus
    • 6. Combining Forces
      • There are usually several different forces acting on an object all at once.
        • The combination of all these forces is called the net force .
    • 7. Balanced Forces
      • Two or more forces are balanced if they cancel each other out when combined.
        • If net forces are balanced, then no change in motion occurs.
    • 8. Unbalanced Forces
      • Multiple forces that do not cancel out are unbalanced and cause acceleration .
        • 1 N (1 Newton) is the amount of force needed to give a 1-kg object the acceleration of 1 m/s 2 .
    • 9.  
    • 10.
      • If Newton's First Law is true, then why does a ball eventually stop moving after you kick it?
        • ...because there are other forces acting on the object that make it slow down and stop.
          • Gravity (we'll talk about this next week)
          • Normal Force (we'll talk about this later, too)
          • Friction
    • 11. Friction
      • Friction is a force resulting from the contact of two surfaces that opposes motion.
        • Caused by surface interactions
          • Smooth surface = less friction
            • shiny/polished materials
          • Rough surface = more friction
            • fabric/carpet, rubber tires, sand paper
    • 12.
      • Fluids usually reduce friction
        • water, oil, air
        • atoms of fluids can slide past each other
        • the "thinner" the fluid, the less friction there is
    • 13. Fixed vs. Moving Friction
      • Ever notice that it takes more effort to GET something moving than to keep it moving?
      • Static Friction acts on unmoving objects.
        • Due to surface roughness and adhesion (weak attraction between molecules of the surface of each object)
      • Kinetic Friction acts on moving objects.
        • Usually weaker than static friction
      Friction ALWAYS acts in the opposite direction of motion.
    • 14. Rolling Friction
        • Wheels reduce the surface area that touches between the two objects.
        • Instead of surfaces sliding, wheels roll, allowing a continuously new area to bear the object's weight.
        • Much weaker than static or kinetic friction.
      Rolling Resistance is the frictional force between a rolling object and the surface it's moving over.
    • 15. Fluid Friction
      • Fluid Friction is a force between a solid object and a fluid (liquid or gas).
        • Examples:
          • rocket flying through the air (air resistance)
            • air molecules collide with the rocket, slowing it down
          • swimmer crossing an Olympic pool (drag)
            • water molecules slide past swimmer's skin
          • water in a garden hose (viscosity)
            • water molecules slide past each other and slide along the inside surface of the hose
            • viscosity is a measure of how "thick" or resistant to flowing movement the liquid is
    • 16. Measuring Friction
      • Coefficient of Friction
        • Tells how much frictional force must be overcome to result in motion (or how much resistance there is to movement).