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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!!!
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Newton’s Second Law
Acceleration
Frame A
𝑣𝑣21≠=𝑐𝑜𝑛𝑠𝑡
0
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Newton’s Third Law
𝐹12 = −𝐹21
F12 F21
• Forces exist always in pairs.
• These forces are called action-reaction forces
• The action and reaction forces act on different objects
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Examples
• When you push the wall with force F
then the wall pushes you with the
same force
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Action-reaction
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Summary: Newton’s third Law
•
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Interacting objects exert equal forces upon each other
These are called action reaction forces
•
Every action receives an equal but opposite reaction
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The Gravitational Force - Weight
𝐹′
SI unit: 1 N= 1 kg m/s2
𝐹 = 𝑚𝑎
𝐹 𝑔 = 𝑚𝑔
𝑔
𝑊 = 𝑚𝑔
•
•
•
𝐹𝑔
𝐹𝑔
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
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Weight: dependence on the environment
No gravity
Mass is not a weight!!!
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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
𝑁
𝑁
𝑚𝑔
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•
•
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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
N
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Tension
•
Tension is the force transmitted
through a “rope” from one end to
the other.
•
An ideal cord has zero mass, does
not stretch, and the tension is the
same throughout the cord.
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Free Body Diagram
𝐹 = 𝑚𝑎
• Idealize (simplify) the system
• Identify all the forces acting on the object
• Choose an appropriate coordinate system
Real life
𝑇 − 𝑚𝑔 = 0
𝑇 = 𝑚𝑔
Free body diagram
𝑇
y
x
𝑚𝑔
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Free Body Diagram
𝑁
𝑓
𝑚𝑔
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Weight: case a=0
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Chose the coordinate system
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Apply Newton’s second equation
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From Newton’s third equation
y
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Weight: case a>0
y
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The object become heaver
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Weight: case a<0
y
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The object become lighter
For a=g the object becomes weightless
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