Laws of motion


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Laws of motion

  1. 1.  First law: Every body remains in a state of constant velocity unless acted upon by an external unbalanced force.[ Second law: A body of mass m subject to a net force F undergoes an acceleration a that has the same direction as the force and a magnitude that is directly proportional to the force and inversely proportional to the mass, i.e., F = ma. Third law: The mutual forces of action and reaction between two bodies are equal, opposite and collinear. This law is sometimes referred to as the action-reaction law. The action and the reaction are simultaneous.
  2. 2. 1 st Law of Motion (Law of Inertia)An object at rest will stay at rest, and anobject in motion will stay in motion atconstant velocity, unless acted upon by anunbalanced force.
  3. 3. 1st lawOnceairborne, unlessacted on by anunbalanced force(gravity and air –fluid friction), itwould never stop!
  4. 4. 1 st law Unless acted upon by an unbalanced force, this golf ball would sit on the tee forever.
  5. 5. Why then, do we observe every dayobjects in motion slowing down andbecoming motionless seeminglywithout an outside force? It’s a force we sometimes cannot see – friction. Objects on earth, unlike the frictionless space the moon travels through, are under the influence of friction.
  6. 6. What is this unbalanced force that acts on an object in motion?There are four main types of friction: •Sliding friction: ice skating •Rolling friction: bowling •Fluid friction (air or liquid): air or water resistance •Static friction: initial friction when moving an object
  7. 7.  Slide a book across a table and watch it slide to a rest position. The book comes to a rest because of the presence of a force - that force being the force of friction - which brings the book to a rest position. In the absence of a force of friction, the book would continue in motion with the same speed and direction - forever! (Or at least to the end of the table top.)
  8. 8. Don’t let this be you. Wear seat belts.Because of inertia, objects (including you) resist changes intheir motion. When the car going 80 km/hour is stopped bythe brick wall, your body keeps moving at 80 m/hour.
  9. 9. 2 nd LawThe net force of an object is equal to the product of its mass and acceleration, or F=ma.
  10. 10. If mass remains constant, doubling the acceleration, doubles the force.If force remains constant, doubling the mass, halves the acceleration.
  11. 11. 3RD LAWFor everyaction, there is anequal and oppositereaction.
  12. 12. According toNewton, whenever objects Aand B interactwith eachother, theyexert forcesupon eachother. When yousit in yourchair, your
  13. 13. There are two forcesresulting from thisinteraction - a forceon the chair and aforce on your body.These two forces arecalled action andreaction forces.
  14. 14. Consider the propulsionof a fish through thewater. A fish uses itsfins to push waterbackwards. In turn, thewater reacts by pushingthe fishforwards, propelling thefish through the water. The size of the forceon the water equals thesize of the force on thefish; the direction ofthe force on the water
  15. 15. Flying gracefully throughthe air, birds depend onNewton’s third law ofmotion. As the birds pushdown on the air with theirwings, the air pushes theirwings up and gives themlift.
  16. 16. Consider the flying motion of birds. A bird flies by use of itswings. The wings of a bird push air downwards. In turn, theair reacts by pushing the bird upwards.The size of the force on the air equals the size of the forceon the bird; the direction of the force on the air (downwards)is opposite the direction of the force on the bird (upwards).Action-reaction force pairs make it possible for birds to fly.
  17. 17. Other examples of Newton’s Third Law The baseball forces the bat to the left (an action); the bat forces the ball to the right (the reaction).
  18. 18. Consider themotion of a car onthe way to school. Acar is equipped withwheels which spinbackwards. As thewheels spinbackwards, theygrip the road andpush the roadbackwards.
  19. 19. The reaction of a rocket isan application of the thirdlaw of motion. Variousfuels are burned in theengine, producing hotgases.The hot gases pushagainst the inside tube ofthe rocket and escape outthe bottom of the tube. Asthe gases movedownward, the rocketmoves in the opposite