Presentation on theme: "Newton’s Third Law of Motion- Action and Reaction"— Presentation transcript:
1 Newton’s Third Law of Motion- Action and Reaction Chapter 4Newton’s Third Law of Motion-Action and Reaction
2 4.1 Force is Part of Interaction Force is more than a push or pullForce is mutual action, or interaction, between two thingsForces occur in pairsExamples= car crash (2 cars), you walking, car tires, swimming (what are other examples?)Doesn’t have to be living to exert force!Do not possess force, exert it (do possess momentum, kinetic/potential energy)
3 4.2 Newton’s Third LawNewton’s Third Law- Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the firstAka = To every action there is always an opposed equal reaction (V for Vendetta, Count of Monte Cristo)Forces are called the action force and the reaction force
4 3rd Law continuedDoesn’t matter which is called reaction, just have to keep in mind that both are needed (You complete me) or neither would existBut what happens with falling (floor is not equal and opposite to me)?
5 4.3 Simple Rule to Identify A & R To identify action and reaction:Action- object A exerts force on object BReaction-object B exerts force on object ACan switch aroundAction is what is not always presentSee drawings on board
6 4.4 Action and Reaction on Different Masses Firing cannon, cannon recoils (kicks)Equal forces, different effects (examples)Cannon has more mass, cannonball has more accelerationHow would this work for gas expelled from rocket?Used to think rocket needed to push against air, so couldn’t go to moon. Rocket just needs to push against expelled gases (and minimal air drag)
7 4.5 Action and Reaction on Different Objects Why don’t forces cancel?Bodies are differentExample: kicking ball with one foot, two feet (see drawing on board)Reaction slows foot downOnly cancel if same body, but there still is action and reactionOther examples
8 4.6 Horse-Cart ProblemWhy pull cart if would be cancelled out by equal and opposite pulls (horse on cart and vice-versa)3 systemsCart (farmer)HorseHorse-cartHow does the horse move forward? What is needed?
9 More Horse-cart As a whole, can cancel horse vs. cart forces Think about pushing a cart against the groundSee drawing on boardP. 61How does this apply to Newton’s other laws?
10 4.7 Action = ReactionPull on rope = tension in rope, otherwise acceleration and …Tug of war- both pulls in same magnitudeSo, how does one win? Force against ground, not rope!What if one side wears socks, the other shoes? Who would win? Why?Helicopter blades force air down (action) and air makes blades go up (reaction), or have lift. Greater lift equals rising helicopter
11 More action, reactionBirds fly by pushing air downward, so air pushes upWhen birds create updraft (swirl of air) for other birds, get the VAirplanesWhy can’t we fly?Can’t fight way out of paper bag, even if champion boxer!Problems on board with pulleys
12 4.8 Summary1st Law = at rest, stay at rest; in motion, stay in motion, unless change occurs2nd law = law of acceleration, F = ma (a = g in vacuum) and have to take into consideration air drag if falling from air (terminal speed if constant)3rd law = law of action-reaction: you push something, it pushes back equally and in opposite direction