* Forces and Newton’s Laws force: a push or a pull Forces affect the motion of objects

History Aristotle: Natural state of bodies is at rest Galileo: Natural state of bodies is at rest or in motion at constant velocity Newton: Extended Galileo’s ideas; summarized in three Laws of Motion

Newton’s First Law: An object will remain at rest or in motion at constant velocity unless acted upon by a net force (an unbalanced force) 1000 N 1000 N Fnet = 0 N 700 N 1000 N Fnet = 300 N to the right 300 N

Both balls have inertia: the tendency of an object to resist a change in its motion                                                       Inertia depends on mass; more mass, more inertia

Newton’s First Law is also called the Law of Inertia Examples: -When you are riding down the road in a car and you come to a sudden stop, you lurch forward in the car (your inertia keeps you moving forward) -When you start after being stopped , you lurch backward in the car, but not really -Really, you stay at rest (because of your inertia), and the car moves forward underneath you, until an unbalanced force (the seat) exerts a force on you

Newton’s Second Law: Law of Acceleration - A net force will cause a mass to accelerate - The acceleration is directly proportional to the force - The acceleration is inversely proportional to the mass - The acceleration is in the same direction as the force

Also called the Law of Acceleration Can be summarized in an equation: F = m a Unit of force: the “newton”, N

Example: A force of 25.0 N acts on an object of 2.0 kg. Calculate the acceleration. F a = m 25.0 N = = 12.5 m/s2 2.0 kg

Mass: the quantity of matter in an object; measured in kg Mass vs. Weight Mass: the quantity of matter in an object; measured in kg Weight: the force caused by the gravitational pull on an object; measured in newtons Mass never changes; weight can change, depending on location crystal quartz -- 1 kg piece

Newton’s Third Law: Every action force is accompanied by a reaction force equal in size and opposite in direction -Also called the Law of Action-Reaction Examples: -the recoil of a cannon after firing Action force: cannon on ball Reaction force: ball on cannon

-Walking, jumping Action force: you on Earth Reaction force: Earth on you -Rockets Action force: rocket on gas Reaction force: gas on rocket

*Forces affect the motion of objects Example of a force: friction We know it’s a force because it affects the motion of objects

Falling Objects in an Atmosphere Objects dropped initially accelerate at 9.8 m/s2 downward, due to gravity Fg Fa As they fall, they begin to encounter air resistance, which increases with speed Fg Eventually the air resistance force will match the gravity force Fa What is the net force? Fg

Fnet = 0 Fa ( Fa = Fg ) so acceleration = 0 so velocity change = 0 Object falls with a constant velocity Fg This velocity is called Terminal velocity

The greatest speed at which an object can fall in an atmosphere Terminal velocity: