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Lecture 12 - PowerPoint PPT Presentation


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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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Presentation Transcript
slide2

Newton’s first Law

If there are no external forces, the object does not change its velocity and direction.

LAW of INERTIA!!!

slide3

Acceleration

Newton’s Second Law

Frame A

slide4

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
slide5

Examples

  • When you push the wall with force F then the wall pushes you with the same force
slide7

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
slide8

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
slide9

Weight: dependence on the environment

No gravity

Mass is not a weight!!!

slide10

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
slide11

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
slide12

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.
slide13

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

slide15

y

Weight: case a=0

  • Chose the coordinate system
  • Apply Newton’s second equation
  • From Newton’s third equation
slide16

y

Weight: case a>0

  • The object become heaver
slide17

y

Weight: case a<0

  • The object become lighter
  • For a=g the object becomes weightless