Transcript Force and Motion. Gravitation.

Lecture 4
Force and Motion
Gravitation
Chapter 2.7  2.15
Outline
• Laws of Motion
• Mass and Weight
• Gravitation
The law of gravity – 1666, 3 laws of motion – 1686
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
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
Second Law of Motion
The more mass an object has, the less its acceleration
for a given force.
F = net force
m = mass
F
a = --m
a = acceleration
F=ma
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.
The Newton
The second law of motion defines the unit of force.
F=ma, [m]=kg, [a]=m/s2  [F]=kg m/s2 = newton (N)
1 lb = 0.453 kg x 9.8 m/s2 = 4.45 N  1 N = 0.225 lb
Mass and Weight
The weight of an object is the force it is attracted by the
earth’s gravitational pull.
Mass is a measure of the amount of matter included in an
object.
F=ma
w=mg
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
Gravitation
Centripetal force is an inward force on an object moving
in a curved path
mv2
Fc = ---r
r is the radius of a circle
The centripetal force always points toward the center of
curvature of the object’s path.
Centripetal and Centrifugal Force
Centrifugal force
Mass of the Earth
The Earth’s mass can be found from the law of gravity.
Weight of an object F = mg
Gravitational force on an object
GmM
F = ---------R2
G = 6.67 10-11 N m2/kg2
R = 6378 km = 6.38
g = 9.8 m/s2
106
m
gR2
M = ------ = 6 1024 kg
G
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, ve=40,000 km/h
Summary
• The three Newton’s 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 object’s
speed is much less than the speed of light.
• Mass is a measure of an object’s inertia, while weight is
the gravitational force on an object.