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Lecture 4 Monday: 26 January 2004

Newton’s Laws of Motion. Lecture 4 Monday: 26 January 2004. Newton’s Laws of Motion. Newton’s Second Law in One Dimension. Commonly shortened to “F=ma”. Correctly, it is : Only forces which act on that object affect the acceleration of the object.

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Lecture 4 Monday: 26 January 2004

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  1. Newton’s Laws of Motion Lecture 4 Monday: 26 January 2004

  2. Newton’s Laws of Motion

  3. Newton’s Second Law in One Dimension • Commonly shortened to “F=ma”. Correctly, it is : • Only forces which act on that object affect the acceleration of the object. • Forces exert by the object on another object do not.

  4. Using Newton’s 2nd Law to Solve Problems • Identify all forces acting on the object • -Pushes or Pulls -Frictional forces -Tension in a string -Gravitational Force (or weight = mg where g is 9.8 m/s2)- “Normal forces” (one object touching another). • Draw a “Freebody Diagram”-draw the object, show all forces acting on that object as vectors pointing in the correct direction. Show the direction of the acceleration. • Chose a coordinate system. • Translate the freebody diagram into an algebraic expression based on Newton’s second law.

  5. T a W=Fg earthelevator. Consider an elevator moving downward and speeding up with an acceleration of 2 m/s2. The mass of the elevator is 100 kg. Ignore air resistance.What is the tension in the cable? • Identify Forces: Tension in cable, weight of the elevator • Draw freebody diagram • Chose coordinate system: Let up be the +y direction and down –y. Then : • Translate the FBD into an algebraic expression. T-W = m(-a) so • T-(100 kg)(9.8 m/s2) = (100 kg)(-2 m/s2) v Note: No negative sign

  6. Newton’s Third Law • Whenever one object (object A) exerts a force on another object (object B), the second object exerts a force back on the first object. • These forces are ALWAYS equal in magnitude (but they point in opposite directions). • Such forces are called “Newton’s third law force pairs”. • Not all forces that are equal and opposite are third law force pairs. • The forces are on differentbodies, so do not add to zero.

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