Gravitation and Weightlessness – JSS1 Basic Science Lesson Note (Third Term)

Gravitation is the force of attraction that pulls objects towards the earth. The earth has a great mass, so its pull is strong and is always directed towards its centre. Weight measures this pull on a body. Weightlessness is the feeling of having no weight, although the body still has weight.

Behavioural objectives

By the end of the lesson, students should be able to:

1. define gravitation and gravitational attraction;
2. state the direction of the earth’s gravitational pull;
3. explain what weight is and how it differs from mass;
4. explain what weightlessness means;
5. state the effects of gravitation on objects.

What is gravitation?

Gravitation is the force of attraction between any two objects, and it keeps objects on the earth stable.

• Gravitational attraction: the pull that the earth exerts on any object near it.
• Example: a ball tossed up in the air falls back to the ground because the earth pulls it.
• Gravitational pull of the earth: strong, because the earth has a great mass.
• Direction: always towards the centre of the earth.

What affects the strength of gravitational attraction?

The strength of gravitational attraction depends on the masses of the objects and the distance between them.

• Mass: the greater the mass of an object, the greater its pull. The earth has a great mass, so its pull is strong.
• Distance: the force between two objects increases as they come closer to each other.
• Distance (reverse): the force becomes weaker as the objects move further apart.

In which direction does the earth’s gravitational pull act?

The earth’s gravitational pull always acts towards the centre of the earth.

• Objects dropped in Lagos, Kano or anywhere else fall straight down towards the ground.
• “Down” always means towards the centre of the earth.

What is weight?

Weight is the force with which the earth pulls a body towards its centre.

• The weight of a person measures the gravitational pull of the earth on that person.
• Weight is measured in newtons (N).
• Weight = mass × gravitational pull per kilogram (g).
• Near the earth’s surface, g is about 10 N/kg in school calculations.
• Worked example: a bag of mass 5 kg weighs about 5 × 10 = 50 N.

What is the difference between mass and weight?

Mass is the amount of matter in a body. Weight is the pull of gravity on that body.

What is weightlessness?

Weightlessness is the feeling of having no weight, although the body still has weight.

• It does not mean that the body has no weight.
• As a person moves away from the earth’s surface, the weight reduces gradually.
• Far from the earth, in outer space, a body weighs almost nothing because the earth’s pull is very weak there.
• Astronauts in a spacecraft float and feel weightless for this reason.

What are the effects of gravitation on objects?

Gravitation holds objects on the earth, gives them weight and keeps the moon and planets in their paths.

1. Stability of the earth: gravitation helps every object on the earth, including man, to remain stable.
2. Weight: the weight of an individual is a measure of the earth’s gravitational pull on that person.
3. Revolution of the moon around the earth: the moon revolves around the earth because the earth’s pull attracts it.
4. Balance of planets in the solar system: the sun and the planets pull on one another and balance one another in space.
5. Work done against gravity: lifting an object from the ground opposes the earth’s pull, so work is done against gravitational attraction.

What are everyday examples of gravity at work?

Gravity causes everything that falls to move towards the ground.

• Falling fruit: a ripe mango falls from the tree to the ground.
• Rain: raindrops fall from the clouds to the earth.
• Flowing water: water in a stream runs downhill.
• Thrown ball: a ball thrown upwards slows down, stops and falls back.

Evaluation

1. Define gravitational pull.
2. What is weightlessness?
3. Why does the earth have a great gravitational pull?
4. In which direction is the gravitational pull of the earth always directed?
5. What does the weight of a person measure?

Assignment

1. State any four effects of gravitation on objects.
2. Explain why work is done when an object is lifted from the ground.