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#### Force

• Force is that which changes the shape, speed, or direction of a moving body.
• The Systeme International Unit (SI) unit of force is Newton (N).
###### Types of Force
• There are two types of force. They are:
• Contact Force – This is the force applied directly on an object. Examples are: push, pull, tension, friction, etc.
• Field Force – This is a non-contact force. Examples include: gravitational force, magnetic force, etc.

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##### GRAVITATION AND WEIGHTLESSNESS
• Gravity is the force that pulls objects down to the center of the earth.
• Isaac Newton discovered the principle of gravity.
• The earth has a strong gravitational pull because of its large size.
• The nearer the object is to the center of the earth, the greater the gravitational pull of the earth because the gravitational pull at any point on the earth’s surface is directed towards the center of the earth.
• The amount of force with which gravity pulls an object depends on:
• The mass of the object.
• The mass of the earth.
• The distance between the center of the earth and the object.
• Weightlessness is an apparent feeling of being weightless.
• It is the feeling an individual has when he/she moves away from the surface of the earth.
• A person in outer space has no weight because the gravitational pull of the earth is no longer acting.
• Therefore; astronauts, people who go on space travel, have to be given special training in artificially prepared conditions that stimulate weightlessness so they can function well in outer space.
###### Effects of the Earth’s Gravitational Pull on an Object
• Weight: Every person has weight. The weight of a person is due to the earth’s gravitational pull on that person.
• Stability on Earth: The earth is a sphere but no human being falls; this is because each human being is attracted to the earth by gravity.
• Balance of Planets in Our Solar System: The sun and all our planets in the solar system have gravitational pull on one another, therefore, these planets continue to move in their orbits. Without gravity, the planets will move away in different directions.
• Revolution of the Moon Round the Earth: The moon revolves round the earth because the earth’s gravitational pull attracts it. Without gravity, the moon will move away from the earth on a straight line.
• Work is Done: Whenever we lift an object from the earth, work is done; this is because we are opposing gravitational pull. Airplanes, spaceships, etc. spend a lot of energy to lift themselves from the ground.
• Vertical Motion: Ripe fruits fall to the ground, air crafts fly safely, and stones fall to the earth when thrown. We can also move about because our legs can move up and down. All these occur as a result of gravity.