Gravitational Forces of the earth or Gravity
A force is necessary to produce motion in a body.
A body dropped from a height falls toward the earth. It indicates that the earth exerts a force of attraction (called gravity) on the body.
In fact, the earth attracts all the object its centre.
The force with which the earth pulls the object toward it is called the gravitational force of the earth or gravity.
The gravitational force between the sun and the earth keeps the earth in uniform circular motion around the sun.
The gravitational force of the earth (or gravity) is responsible -
l For holding the atmosphere above the earth.
l For the rain falling to the earth.
l For flow of water in the rivers.
l For keeping us firmly on the earth.
l For the revolution of moon around the earth, etc.
The gravitational force between the earth and the moon causes tides in the sea. Think why tides are high during full moon and new moon day.
A few facts about Lunar Tides.
The gravitational force of the moon in one ten- million that of earth, but when you combine other force such as the earth’s centrifugal force created by its spin, you get tides.
The sun’s gravitational force on the earth is only 46% that of the moon, making the moon the single most important factor foe the creation of tides.
The sun’s gravity also produces. Tides but since the forces are similar, as compared to the moon, the effects are greatly decreased.
Tides are not caused by the direct pull of the moon’s gravity. The moon is pulling upward on the water while the earth is pulling downward. Slight advantage to the moon and thus we have tides.
Whenever the moon, earth and sun are aligned the gravitational pull of the sun adds to that of the moon and thus moon causing maximum tides.
Spring tides happen when the sun and moon are on the same side of the earth ( New Moon) or when the sun and moon are on opposite sides of the earth ( Full Moon)
When the moon is at first quarter or last quarter phase (meaning that it is located at right angles to the Earth-sun line ), the sun and moon interface with each other in producing tides bulges and tides are generally weaker’; these are called neap tides.
Offshore, in the deep ocean, the difference in tides is usually less than 1.6ft.
The surf grows when it approaches a beach, and the tides increases. In bays and estuaries, this effect is amplified. ( in the Bay of Fundy, tides have a range of 44.6ft.)
The highest tides in the world are the Bay of Fundy in Nova Scotia, Canada.
Because the earth rotates on its axis the moon complete one orbit in our sky every 25hours (Not to be confused with moon’s 27 day orbit around the earth), we get two tidal peaks as well as two tidal troughs. These events are separated by about 12 hours.
Since the moon moves around the earth it is not always in the same place at the same time each day. So, each day, the times for high and low tides change by 50 minutes.
The type of gravitational force that causes tides is known as Tractive force.
Newtons’s Universal Law of Gravitation
Every body in the universe attracts every other body with a force which is directly proportional to the product of their masses and is inversely proportional to the square of the distance between them.
The direction of force is along the line joining the center of the two bodies.
Let two bodies A and B of masses m1 and m2 are placed at a distance of “r” from each other. Let “f” be the force of attraction between the bodies.
Then,
The force of attraction is directly proportional the product of their masses
F α m1m2
The force of attraction is inversely proportional the square of the distance between them
Where G is a constant called as “ universal gravitational constant.
Properties of gravitational forces
1. Gravitational forces is the weakest force in nature
2. It is an attractive force. ( unlike electrostatic and magnetic forces; they are both attractive and repulsive )
3. It is a mutual force. ( first body attracts the second body and the second body attracts the first body with equal forces)
4. It is a centre force ( acts along the line joining the centers of the bodies)
5. It is a mass and distance dependent.
6. It obeys inverse square law.
7. It is a long range force ( it decreases with distance as per inverse square law and become zero only at infinite distance - like electrostatics and magnetic forces)
8. It does not depends on the medium between the interacting bodies. ( their is no gravitational shielding)
Universal Gravitational Constant (G):
G = F r2/ m2*m2
The value of Universal Gravitational constant G is 6.67*10-11 Nm2 kg-2
The value of G does not depends on the medium between two bodies.
The value of G is same throughout the universe and hence the name.
Universal Gravitational Constant G is different from acceleration due to gravity g. Therefore correct symbol must be used accordingly.
S.I. Unit of G is Nm2 kg-2
If two bodies of mass 1kg each are separated by 1m from each other then, G = F.
Therefore, universal gravitational constant G is numerically equal to the force of gravitation which exists between two bodies of unit masses kept at a unit distance from each other.
Gravitational force is the weakest force of all.
If two bodies of mass 1kg each are separated by 1m from each other, then, F = G = 6.67*10-11 N
Though the various objects on this earth attracts one another constantly, they do not causes any visible motion because the gravitational force of attraction between them is very, very small.
Imagine a situation on the earth if the object continuously move toward each other due to gravitational force between them.
When both the object are very big, having very large masses, then the gravitational force of attraction between them becomes extremely large.
The gravitational force due to earth on the object of mass 1kg placed on the ground is 9.8 N.
The gravitational force between the earth and the moon even at a large distance is very large and is 2*1020 N.
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