Forces

Chapter 12, Section 1 - Newton's First and Second Laws                                                                                                   

Newton's First Law of Motion - The Law of Inertia
  • An at rest remains at rest, and an object in motion remains in motion (without changing velocity) unless it experiences a net force.
Net Force - The combination of all the forces acting on an object.  If the total force is not equal to zero, there is a net force on the object.

Inertia - The tendency of an object to resist change in motion (unless a net force acts on it).
  • Mass is related to inertia.
    • It takes a small force to push a marble.
    • It takes a larger force to push a bowling ball.
  • Inertia can be demonstrated by the time it takes to stop a moving car or bicycle (or to get it started moving).
    • When a car stops suddenly, your body's inertia causes you to continue moving forward.  Seat belts protect you by keeping you in your seat.
      • The seatbelt provides an unbalanced backward force to slow you down with the car.
Newton's Second Law of Motion
  • The net force acting on an object is equal to the object's mass times its acceleration.
    • The size of the net force determines how much an object speeds up, slows down, or changes direction.
    • If the amount of force on two objects is the same...
      • the object with the smaller mass will have greater acceleration.
      • the object with the larger mass will have less acceleration.
    • If two objects have the same mass...
      • the object that has less force on it will accelerate less.
      • the object that has more force on it will accelerate more.
Newton (N) - The SI (metric) unit of force.
  • 1 N = 1 kg x 1 m/s2         (force = mass x acceleration)

In the English System, the unit of force is the pound (lb).
  • 1 lb = 4.45 N
  • 1 N = 0.225 lb

Homework:
    Pg. 402 #1-7


Chapter 12, Section 2 - Gravity                                                                                                                                           

Mass - The amount of matter in an object. 
Weight - The force of gravity pulling on an object's mass.

Free-Fall Acceleration (g) - Acceleration due to gravity.  The constant acceleration that all objects near a massive object (like a planet) experience.
  • When you fall, your body accelerates at a constant rate.  In other words: the longer you fall, the faster you fall.
  • On Earth, g = 9.8 m/s2.
  • Since force = mass x acceleration, and g is acceleration...
    • weight = mass x free fall acceleration
Weight is measured in Newtons (N).

Law of Universal Gravitation
  • All objects in the universe attract each other by the force of gravity.
  • As the mass of one or both objects increases, the force of gravity between them increases.
  • As the distance between the two objects increases, the force of gravity between them decreases.
    • Distance is always measured from the centers of the objects!
Free Fall - An object's motion when the only force acting on it is gravity.
  • If there is no air resistance, all falling objects will accelerate at the same rate.  Mass doesn't matter.
  • If there is air resistance,...
    • Air resistance acts in the opposite direction as gravity.  (Like friction opposes motion.)
    • Air resistance gets stronger as the object falls faster.
    • If the air resistance equals the weight of the object, the object stops accelerating.  (It moves at a constant speed.)
    • Terminal Velocity - The constant speed of a falling object when its air resistance is equal to its weight.
  • Objects in orbit are in free fall.
Projectile Motion - The curved path of an object that has been thrown/launched/projected near the surface of the Earth.
Projectile motion has two components...
  • Horizontal Component - The projectile continues to move forward at a constant velocity.
  • Vertical Component - The projectile accelerates towards the ground.
Orbiting objects fall in a curve that matches the curve of the Earth's surface.



Chapter 12, Section 3 - Newton's Third Law                                                                                                                  

Newton's Third Law of Motion - The Law of Action and Reaction
  • For every action force, there is an equal and opposite reaction force.
    • When one object exerts a force on a second object, the second object exerts a force equal in size and opposite in direction on the first object.
Forces always act in pairs.
  • Action force and Reaction force.
Force pairs do not act on the same object.

Equal forces don't always have equal effects.
  • When you jump, you push down on the Earth with a force.
    • You have less mass, so you get greater acceleration.
    • The Earth has more mass, so it gets less acceleration.
Momentum - A property of all moving objects that is equal to an object's mass times its velocity.
  • Measured in kg*m/s.
  • Has direction.
  • As mass increases, momentum increases.
  • As velocity increases, momentum increases.
Force is related to change in momentum.
  • As the time increases for a change in momentum, the force needed to change the momentum decreases.
    • Airbags allow you to slow down over a longer time than hitting the dashboard would, so you hit with less force.
Law of Conservation of Momentum
  • The total amount of momentum in an isolated system is conserved.
    • The total momentum is the same before and after a collision.


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