1. Natural Sciences Grade 7
Term 4: Planet Earth and Beyond
Relationship of the Moon to the Earth

2. Topic 2 Relationship of the Moon to the Earth
Gravity
Natural Sciences - Grade 7

3. Natural Sciences - Grade 7
Centre of gravity
The centre of gravity can be described as the point in a body/system around which its mass/weight is evenly balanced/distributed and through which the force of gravity acts.
Mass [how much matter is in an object] is measured in kilograms.
An object’s mass stays the same, irrespective where it is [Moon/in space/ on Earth].
Weight is measured in Newtons and it is the measurement of the pull gravity on an object.
Bodies/objects that have a great mass, will exert a stronger gravitational pull than objects which have smaller masses.
The strength of an object’s gravitational pull, is totally dependent on mass of the object, the object that is being pulled, as well as the distance between the objects.
Discuss the force of gravity: aeroplane ; person standing with feet apart; person on a surf board and a ball balanced on a finger while spinning.
Natural Sciences - Grade 7

4. Natural Sciences - Grade 7
Centre of gravity
Look at the pictures and discuss each one:
Bodies/objects that have a great mass, will exert a stronger gravitational pull than objects which have smaller masses.
The strength of an object’s gravitational pull, is totally dependent on mass of the object, the object that is being pulled, as well as the distance between the objects.
Discuss the force of gravity: aeroplane ; person standing with feet apart; person on a surf board and a ball balanced on a finger while spinning.
Natural Sciences - Grade 7

5. Natural Sciences - Grade 7
The force of gravity
The force of gravity causes the ball to return to the surface.
The weight force of gravity [Fg] causes the apple to drop to the ground.
Why are astronauts floating in space?
Why are astronauts floating in space?
The learners will surely come up with interesting answers.
Do not tell the learners who were right and who were wrong.
Let them watch the video: Why are astronauts weightless in space?
is stronger than the pull of the moon’s gravity /
After the video the following can be pointed out:
- the astronauts’ feet are not fixed firmly on the ground
- it seems that they are just bouncing around in space
- reason : pull of Earth’s gravity
Natural Sciences - Grade 7

6. Gravity role in the solar system
The gravitational pull of the Sun, keeps the Earth orbiting the Sun.
The Earth’s and Sun’s gravity has an effect on the Moon’s motion.
The Moon’s gravity causes the:
- sea levels to rise and fall
- ocean tides to rise and fall
Earth’s atmosphere is kept in place through gravity.
Gravity pulls on the clouds when the Earth spins on its own axis and it can have an effect on the weather.
Tell the learners that they will do a practical activity to demonstrate the pull of gravity.
Natural Sciences - Grade 7