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Understanding Newton's Laws of Motion: Inertia, Force, and Action-Reaction Principles

Newton's First Law, also known as the Law of Inertia, states that a body remains at rest or moves uniformly in a straight line unless acted upon by a net force. Objects do not accelerate without this net force. The Second Law introduces the relationship between force, mass, and acceleration, encapsulated in the formula F = ma. The Third Law explains that for every action, there is an equal and opposite reaction. Together, these laws form the foundation of classical mechanics, essential for understanding motion in physics.

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Understanding Newton's Laws of Motion: Inertia, Force, and Action-Reaction Principles

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  1. Newton’s First Law (1642-1727) • “The Law of Inertia” • A body remains at rest or moves in a straight line at a constant speed unless acted upon by a net force. • Objects do not accelerate unless a net force is applied. http://teachertech.rice.edu/Participants/louviere/Newton/newton.html; http://www.library.usyd.edu.au/libraries/rare/modernity/newton3.html

  2. Objects at rest remain at rest and objects in motion remain in motion, unless acted upon by an outside force. Newton’s 1st Law of Motion http://talesfromechocanyon.blogspot.com/2007_03_02_archive.html

  3. Newton’s 2nd law of Motion …mathematically Net Force = (mass)(accel) Fnet = ma

  4. m m m m m M M M m NEWTON'S 2nd LAW OF MOTION a F F a F a F a F a F a

  5. When the acceleration is g we have Free Fall m 2m F 2F

  6. Terminal Velocity Net Force Acceleration = g Velocity = 0 but motion is about to begin mg F Acceleration < g v increasing downward mg F Acceleration << g v still increasing downward just not as rapidly as before mg F Acceleration = 0 Terminal velocity mg

  7. Falling with Air Resistance & Terminal Velocity • When falling the force of air resistance becomes large enough to balance the force of gravity. • At this instant in time, there is no net force — the object stops accelerating (see D below); terminal velocity has been reached. www.physicsclassroom.com

  8. Newton’s Third Law • Action-Reaction • When one object exerts a force on another object, the second object exerts a force of equal strength in the opposite direction on the first object. Example of Newton’s 3rd Law: http://bp3.blogger.com/_vQA3QRUnk3M/RrgyfrRniPI/AAAAAAAAAMs/DYlhM7pDeI8/s1600-h/newton

  9. Newton’s Third Law of Motion For every action, there is always a reaction of equal (magnitude) and opposite (direction) reaction. “action” or “reaction” refers to force. Action/Reaction forces do NOT act on the SAME object!

  10. Reaction: road pushes on tire Action: tire pushes on road

  11. Reaction: gases push on rocket Action: rocket pushes on gases

  12. Action- Reaction Forces • Do Action-Reaction forces cancel each other? • No, they are acting on different objects. Forces can only be added together when they are acting on the same object. http://quest.nasa.gov/space/teachers/rockets/principles.html

  13. Newton’s 3rd Law of Motion Action Force: A guy is pushing a ball leftwards Reaction Force: the ball is pushing the guy rightwards

  14. Tug-a-war • If Fido and Rover play tug-a war, how do the “pulls” of the dogs compare? • If each dog pulls with 20 N of force, what is the tension force in the middle of the rope (between the dogs)? • Answer= 20 N • Why?

  15. Putting Newton's Laws of Motion Together = Synthesis • An unbalanced force must be exerted for a rocket to lift off from a launch pad or for a craft in space to change speed or direction(First Law) • The amount of thrust (force) produced by a rocket engine will be determined by the rate at which the mass of the rocket fuel burns and the speed of the gas escaping the rocket(Second Law) • The reaction, or motion, of the rocket is equal to and in the opposite direction of the action, or thrust, from the engine(Third Law) http://www.spacetoday.org/images/Rockets/ArianeRockets/Ariane5LaunchArianespace.jpg http://quest.nasa.gov/space/teachers/rockets/principles.html

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