Forces and Newton’s Laws

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

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

Newton’s First Law: An object will remain at rest or in motion at constant velocity unless acted upon by a net force 1000 N F net = 0 N 1000 N700 N F net = 300 N to the right 300 N (an unbalanced force)

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: -A net force will cause a mass to accelerate -If force increases for the same mass, acceleration increases -If mass increases for the same force, acceleration decreases

F = m a F ma Unit of force: the “newton”, N N kgm/s 2 Can be summarized in an equation: Also called the Law of Acceleration

Example: A force of 25.0 N acts on an object of 2.0 kg. Calculate the acceleration. a = F m = 25.0 N 2.0 kg = 12.5 m/s 2

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

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

Which object has more inertia?battleship Which object has more momentum?speedboat An object must be in motion to have momentum

Another force:Gravity It affects the motion of objects, so it’s a force Acceleration of gravity: m/s 2 (in a vacuum)

All objects exhibit gravity; most are too small to notice Need planet-size objects for gravity to become apparent Cause objects to have weight

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 (notice that force is also measured in newtons) Mass never changes; weight can change, depending on location crystal quartz -- 1 kg piece

Falling Objects in an Atmosphere Objects dropped initially accelerate at 9.8 m/s 2 downward, due to gravity FgFg As they fall, they begin to encounter air resistance, which increases with speed FgFg FaFa FgFg FaFa Eventually the air resistance force will match the gravity force What is the net force?

FaFa FgFg F net = 0 so acceleration = 0 so velocity change = 0 Object falls with a constant velocity ( F a = F g ) This velocity is called Terminal velocity

Comparing Free Fall The elephant and the feather -Which will land first? mmedia/newtlaws/efff.cfm

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