Lecture 12

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# Lecture 12 - PowerPoint PPT Presentation

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

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

F12

F21

• Forces exist always in pairs.
• These forces are called action-reaction forces
• The action and reaction forces act on different objects

Examples

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

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

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

Weight: dependence on the environment

No gravity

Mass is not a weight!!!

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

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

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.

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

y

Weight: case a=0

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

y

Weight: case a>0

• The object become heaver

y

Weight: case a<0

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