Transcript Document
Chapter 11: Forces Already Covered: Newton’s 1st and 2nd laws of motion: 1. An object at rest remains at rest, and an object in motion remains in motion with the same velocity, unless acted upon by an unbalanced force 2. The acceleration of an object increases with increased force and decreases with increased mass F=mxa 2.3 Forces act in pairs Netwon’s 3rd Law: 3. Every time one object exerts a force on another object, the second object exerts a force that is equal in size and opposite in direction back on the first object Newton’s third law relates action and reaction forces Forces always act in pairs Jellyfish movement: squeeze water out of the umbrellalike body: applies a force (of the water) in one direction (downward), and moves in the opposite direction (upward) “Every time one object exerts a force on another object, the second object exerts a force that is equal in size and opposite in direction back on the first object” (For every action there is an equal and opposite reaction.) Action and Reaction Pairs Force exerted on an object/force the object exerts back = action/reaction force pair Jellyfish? Book’s ex: Space shuttle launch Stub your toe Press down on a table (no motion) Action and Reaction Forces Versus Balanced Forces Equal AND opposite, similar to balanced forces Balanced forces act on a single object: two friends pulling on a back pack, which doesn’t move Action and Reaction Forces act on different objects: drag the back pack across the floor Examples? The baseball forces the bat to the left; the bat forces the ball to the right. Together, these two forces exerted upon two different objects form the actionreaction force pair. Note that in the description of the two forces, the nouns in the sentence describing the forces simply switch places. Action: Baseball pushes glove leftwards. Reaction: _________________________ Action: Bowling ball pushes pin leftwards. Reaction: _________________________ Action: Enclosed air particles push balloon wall outwards. Reaction: _________________________ Newton’s three laws describe and predict motion Can explain the motion of almost any object (animals included) Work together (not independent of one another) Can use the laws to make predictions about motion Spacecraft: can predict where Mars will be at the time a spacecraft reaches it, and can control the force on the spacecraft to arrive at the right place/time Newton’s three laws of motion Acceleration is Independent of Mass Inertia Trick Until Galileo Galilei, "common sense" told people that heavier objects fall faster. Galileo showed the power of experiment over logic. Inertia is the tendency of an object to remain at rest if it's already at rest, or to keep moving if it's already moving. ---------------------------------------------Tablecloth trick: Too little force, too little time to overcome "inertia" of tableware. Inertia is Mass Inertia is also called mass. Mass is measured in kilograms. One kilogram is the amount of mass in a 2.2 pound weight Three Examples of Inertia Newton's Laws of Motion One newton is the approximate weight of a cube of butter. Isaac Newton (1642-1727) Newton's First Law Motion tends to continue unchanged. The elephant at rest tends to remain at rest. Newton's Second Law: Acceleration = Force / mass a=F/ m 2a=2F/m a=2F/2m Adding and Subtracting Forces Equilibrium Means "Zero Acceleration" Forces in balance: Equilibrium Equilibrium Forces are in balance, so block is in equilibrium, moving at constant speed (forces acting on the same object) Acceleration Due to Gravity Gravitational force (weight) is proportional to mass. Double the mass and the gravitational force will be doubled also. Ratio of weight to mass is Gravitational force is always the same: g proportional to mass. Ratio F / m is always the same: g Weight The weight of an object is the Example: force of the earth's pull and is A cube of butter has a mass of about 0.1 kg. given by the equation Weight = m g weight = mg = 0.1 kg x 10 m/s2 =1N Terminal Speed Air resistance increases as the speed increases. Eventually, the force R of air resistance becomes equal to the force exerted by the earth, and the object reaches equilibrium. Newton's Third Law of Motion: Action-Reaction Whenever one object exerts a force on another object, the other object exerts an equal but opposite force Note: each of the two forces in the pair acts on a different object. Hammer pushes on stake. Stake pushes on hammer. The hammer acts, the stake re-acts. More Action-Reaction Pair Examples If action-reaction forces are equal but opposite, why don't they cancel?