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/s^{2} (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 #17
Chapter 12, Section 2  Gravity Mass  The amount of matter in an object. Weight  The force of gravity pulling on an object's mass.
FreeFall 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/s^{2}.
 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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