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Forces

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Transcript

  • 1. Forces
  • 2. What is a force?
    Something that causes acceleration
  • 3. Types of Forces
    Contact Forces
    Action-at-a-Distance Forces
    Frictional Force
    Tension Force
    Normal Force
    Air Resistance Force
    Applied Force
    Spring Force
    Gravitational Force
    Electrical Force
    Magnetic Force
  • 4. Newton’s First Law
    Newton’s first law is often called the law of inertia.
    Every object continues in its state of rest, or of uniform velocity in a straight line, as long as no net force acts on it.
  • 5. If an object is at rest, or in a state of constant motion, it is said to be in a state of equilibrium.
    Two types
    Static Equilibrium
    The object is resting at one location
    Dynamic Equilibrium
    The object is moving, with a constant velocity
    Bathtub
  • 6. Common Examples of the 1st Law
    Blood rushes from your head to your feet while quickly stopping when riding on a descending elevator.
    The head of a hammer can be tightened onto the wooden handle by banging the bottom of the handle against a hard surface.
    A brick is painlessly broken over the hand of a physics teacher by slamming it with a hammer. (CAUTION: do not attempt this at home!)
    http://www.youtube.com/watch?v=uV8c7p9JDhw&feature=related
    To dislodge ketchup from the bottom of a ketchup bottle, it is often turned upside down and thrusted downward at high speeds and then abruptly halted.
    Headrests are placed in cars to prevent whiplash injuries during rear-end collisions.
    While riding a skateboard (or wagon or bicycle), you fly forward off the board when hitting a curb or rock or other object which abruptly halts the motion of the skateboard.
  • 7. Newton’s Second Law
    Force is a vector, so F=ma is true along each coordinate axis.
    The unit of force in the SI system is the newton (N).
    Note that the pound is a unit of force, not of mass, and can therefore be equated to newtons but not to kilograms.
  • 8. Any time a force is exerted on an object, that force is caused by another object.
    Newton’s third law: Whenever one object exerts a force on a second object, the second exerts an equal force in the opposite direction on the first.
  • 9. A key to the correct application of the third law is that the forces are exerted on different objects. Make sure you don’t use them as if they were acting on the same object.
  • 10. 4-5 Newton’s Third Law of Motion
    Rocket propulsion can also be explained using Newton’s third law: hot gases from combustion spew out of the tail of the rocket at high speeds. The reaction force is what propels the rocket.
    Note that the rocket does not need anything to “push” against.
  • 11. 4-6 Weight – the Force of Gravity; and the Normal Force
    Weight is the force exerted on an object by gravity. Close to the surface of the Earth, where the gravitational force is nearly constant, the weight is:
  • 12. 4-6 Weight – the Force of Gravity; and the Normal Force
    An object at rest must have no net force on it. If it is sitting on a table, the force of gravity is still there; what other force is there?
    The force exerted perpendicular to a surface is
    called the normal force. It is exactly as large as needed to balance the force from the object (if the required force gets too big, something breaks!)
  • 13. Drawing Free-Body Diagrams
    A book is at rest on a table top. Diagram the forces acting on the book. See answer.
    A girl is suspended motionless from the ceiling by two ropes. Diagram the forces acting on the combination of girl and bar. See answer.
    A flying squirrel is gliding (no wingflaps) from a tree to the ground at constant velocity. Consider air resistance. Diagram the forces acting on the squirrel. See answer.
    A rightward force is applied to a book in order to move it across a desk with a rightward acceleration. Consider frictional forces. Neglect air resistance. Diagram the forces acting on the book. See answer.
    A rightward force is applied to a book in order to move it across a desk at constant velocity. Consider frictional forces. Neglect air resistance. Diagram the forces acting on the book. See answer.A skydiver is descending with a constant velocity. Consider air resistance. Diagram the forces acting upon the skydiver. See answer.
    A football is moving upwards towards its peak after having been booted by the punter. Diagram the forces acting upon the football as it rises upward towards its peak. See answer.
    A car is coasting to the right and slowing down. Diagram the forces acting upon the car. See answer.