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Lecture 12

Lecture 12. Newton’s first Law. If there are no external forces, the object does not change its velocity and direction. LAW of INERTIA!!! . Acceleration. Newton’s Second Law. Frame A. Newton’s Third Law. F 12. F 21. Forces exist always in pairs.

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Lecture 12

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  1. Lecture 12

  2. Newton’s first Law If there are no external forces, the object does not change its velocity and direction. LAW of INERTIA!!!

  3. Acceleration Newton’s Second Law Frame A

  4. Newton’s Third Law F12 F21 • Forces exist always in pairs. • These forces are called action-reaction forces • The action and reaction forces act on different objects

  5. Examples • When you push the wall with force F then the wall pushes you with the same force

  6. Action-reaction

  7. Summary: Newton’s third Law • Interacting objects exert equal forces upon each other • These are called action reaction forces • Every action receives an equal but oppositereaction

  8. The Gravitational Force - Weight SI unit: 1 N= 1 kg m/s2 • Weight and mass are NOT the same: • Mass is an intrinsic property of the object determined by the type and amount of atoms • The weight is the measure of the gravitational force and therefore dependence on the conditions how the force is measured

  9. Weight: dependence on the environment No gravity Mass is not a weight!!!

  10. Normal force • Normal force is the force acting to the object due to the deformation of the surface. • Normal force is always perpendicular to the surface

  11. N Friction • Friction is the resistance to the motion due to the interaction of the object with the environment • The direction of frictional force is always opposite to the direction of motion • Friction is always proportional to the normal force

  12. Tension • Tension is the force transmitted through a “rope” from one end to the other. • An idealcord has zero mass, does not stretch, and the tension is the same throughout the cord.

  13. Free Body Diagram • Idealize (simplify) the system • Identify all the forces acting on the object • Choose an appropriate coordinate system Real life Free body diagram y x

  14. Free Body Diagram y x

  15. y Weight: case a=0 • Chose the coordinate system • Apply Newton’s second equation • From Newton’s third equation

  16. y Weight: case a>0 • The object become heaver

  17. y Weight: case a<0 • The object become lighter • For a=g the object becomes weightless

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