Lecture 4 Force and Motion Gravitation

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Lecture 4Force and MotionGravitation
Chapter 2.7 ? 2.15
Outline

• Laws of Motion
• Mass and Weight
• Gravitation

The law of gravity 1666, 3 laws of motion
1686
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First Law of Motion
A force is needed to start motion or change its
velocity A force is any influence that can
change the speed or direction of motion of an
object

• If no net force acts on it, then

an object remains at rest
an object in motion remains in motion
An object continues to be accelerated as long as
a net force acts upon it
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Mass
The reluctance of an object to change its state
of rest or of uniform motion in a straight line
is inertia
Mass is a property of matter that shows itself as
inertia
The SI unit of mass is kilogram (kg) 1 liter
(0.26 gallon) has a mass of 1 kg 1 liter 10cm x
10cm x 10cm
More about mass and inertia
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Second Law of Motion
The more mass an object has, the less its
acceleration for a given force.
F net force
m mass
a acceleration
F a --- m
Fma
The net force on an object equals to the product
of the mass and the acceleration of the
object. The direction of the force is the same as
that of the acceleration.
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The Newton
The second law of motion defines the unit of
force.
Fma, mkg, am/s2 ? Fkg m/s2 newton (N)
1 lb 0.453 kg x 9.8 m/s2 4.45 N ? 1 N 0.225
lb
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Mass and Weight
The weight of an object is the force it is
attracted by the earths gravitational pull.
Mass is a measure of the amount of matter
included in an object.
Fma
wmg
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Third Law of Motion
When an object exerts a force on a second object,
the second object exerts an equal force in the
opposite direction on the first object.
Action force is equal to reaction force
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Gravitation
Centripetal force is an inward force on an object
moving in a curved path
mv2 Fc ---- r is the radius of a
circle r
The centripetal force always points toward the
center of curvature of the objects path.
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Centripetal and Centrifugal Force
Centrifugal force
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Mass of the Earth
The Earths mass can be found from the law of
gravity.
Weight of an object F mg
G m M F ---------- R2
Gravitational force on an object
G 6.67 10-11 N m2/kg2
gR2 M ------ 6 1024 kg G
R 6378 km 6.38 106 m
g 9.8 m/s2
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Orbiting the Earth
In order to launch a satellite, the gravitational
force has to be overcome.
Centripetal force gravitational force
mv2 ------ mg ? v2 rg ? v ?rg r
Minimal orbital speed
Escape speed (ve) the speed required to leave
the gravitational influence of a body.
For the Earth v 28,400 km/h, ve40,000 km/h
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Summary

• The three Newtons laws of motion and the law of
  gravity represent the foundation of the classic
  or Newtonian mechanics.

• They hold everywhere in the Universe, when an
  objects speed is much less than the speed of
  light.

• Mass is a measure of an objects inertia, while
  weight is the gravitational force on an object.