Download Gravity - E

GRADE 7
Name:
DEFINITION:
Gravity - The tendency of one object to attract or pull another object.
Gravitational force is a force that attracts objects toward each other.
Any object with mass exerts a gravitational force on any other object
with mass.
All matter has mass and takes up space. Planets and moons are made of matter and
thus have mass. Mass of the objects and distance between the objects determine the
pull of gravity those two objects have on each other.
Mass and gravity
Mass is measured in grams (g) and kilograms (kg). Mass is a measurement of how
much matter there is in an object.
The size of the planets relates to their mass.
Activity 1
Visit the following resource http://nssdc.gsfc.nasa.gov/planetary/factsheet/
Fill in the missing information in the table below.
Planet
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Diameter (km)
4 879
Mass (x 1024 kg)
4,87
5,97
6 792
1898
120 536
86,8
49 528
MASS
MASS: 1 x 1024 kg means that you must multiply 1 by 10, 24 times. The
answer will be 1 000 000 000 000 000 000 000 000kg.
© e-classroom 2014 www.e-classroom.co.za
Grade 7 Natural Science: Planet Earth and Beyond: Relationship of the Moon to the Earth: Gravity
Gravity
GRADE 7
The bigger the mass of the object, the greater the gravitational pull. If we want to
compare the gravitational pull of the planet on an object on its surface, we need to
take account of the mass and the distance from the surface to the centre of the planet.
Here’s an example.
What is Earth’s diameter and mass?
What is Venus’s diameter and mass?
As Venus and Earth are almost the same size in diameter, the distance from their
surface to their centre is about the same. If you look at the mass of Venus and
Earth, you will see that Earth has a greater mass than Venus. If you were to stand on
the surface of Venus and then on the surface of Earth, you would notice that the
gravitational pull of Earth would be greater than that of Venus, because Earth has a
greater mass.
Earth exerts a gravitational force on everything on its surface because nothing on
Earth is bigger than itself.
Distance and gravity
The closer two objects are to each other, the greater the gravitational pull they exert
on each other. If those two objects were to be moved away from each other, they
would exert less of a force on each other.
© e-classroom 2014 www.e-classroom.co.za
Grade 7 Natural Science: Planet Earth and Beyond: Relationship of the Moon to the Earth: Gravity
Earth’s mass is 5, 97 x 1024kg = 5 970 000 000 000 000 000 000 000kg.
GRADE 7
If there are two objects with the same mass, but the distance between them can
change, write down which scenario they will experience the greatest amount of
gravitational pull on each other. Write down <, > or =.
5kg
500km
5kg
5kg
1000km
5kg
Let’s apply what we’ve learnt about gravity, relating mass and distance to the solar
system.
We know each planet orbits around the Sun. We know the Sun is the biggest object
in our solar system. The Sun’s gravitational pull on each of the planets is what keeps
them in the orbit around it. It’s the same principle for the Moon and Earth. We know
that the Earth is bigger than the Moon and we know that the Moon orbits around the
Earth. The Earth’s gravitational pull on the Moon keeps it in its orbit.
We know that the distance of each planet from the Sun is different – this is because
they are all held in their own orbit based on the gravitational pull exerted on them by
the Sun. Mercury is the closest to the Sun but it has the least mass (it is the lightest).
Jupiter has the largest mass but is much further away from the Sun. The planet’s
mass and distance from the Sun affects the Sun’s gravitational pull on them.
Gravitational pull or force
Have a look at this diagram of how gravitational force holds a planet in its orbit:
© e-classroom 2014 www.e-classroom.co.za
Grade 7 Natural Science: Planet Earth and Beyond: Relationship of the Moon to the Earth: Gravity
Activity 2
Activity 3
You will demonstrate these forces by using the following:
A piece of string or rope (about 1m) long; a plastic shopping bag (with handles) and
a tennis ball or similar
1) You will need to do this in a large, open space.
2) Put the ball into the plastic bag. Tie the plastic bag to the string or rope.
3) Stand still and swing the ball-in-the-bag around your head, in a circular motion.
4) Observe what happens.
5) When the ball is going at a constant speed, let go of the rope and watch what happens. (Everyone should be careful that they do not get hit by the flying ball)
Answer these questions in your class workbook.
1) What do you represent?
2) What does the ball represent?
3) What does the string or rope represent?
4) What does the path of the moving ball represent?
5) What happened when you released the string?
6) Draw a picture to represent this demonstration.
We have used the Sun and Earth as an example, but the same applies regarding the
Moon and the Earth.
Draw a picture or diagram to represent the relationship of the Moon to the Earth
regarding Earth’s gravitational pull.
© e-classroom 2014 www.e-classroom.co.za
GRADE 7
Grade 7 Natural Science: Planet Earth and Beyond: Relationship of the Moon to the Earth: Gravity
In the diagram you can see that Arrow 1 shows the force of the Sun on the Earth. This
is the gravitational pull of the Sun on the Earth. Arrow 2 shows the “sideway” force
that acts on the Earth. With these two forces, the Earth stays in its orbit and moves
around the Sun. If force 1 was not there, the Earth would “fly off” in the direction of
arrow 2. If force 2 was not there, the Earth would be drawn closer to the Sun. Both
forces work together.
GRADE 7
Activity 1
Look at the following resource http://nssdc.gsfc.nasa.gov/planetary/factsheet/
Fill in the missing information in the table below.
Planet
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Diameter (km)
4 879
12 104
12 756
6 792
142 984
120 536
51 118
49 528
Mass (x 1024 kg)
0,330
4,87
5,97
0,642
1898
568
86,8
102
Here’s an example.
What is Earth’s diameter and mass?
Diameter = 12 756
Mass = 5, 97 x 1024
What is Venus’s diameter and mass?
Diameter = 12 104
Mass = 4, 87 x 1024
Activity 2
If there are two objects of the same mass, but the distance between them can
change, write down in which scenario they will experience the greatest amount of
gravitational pull on each other. Write down <, > or =.
5kg
500km
5kg
Activity 3
>
5kg
1000km
5kg
Answer these questions in your book.
1) What do you represent?
The Sun
2) What does the ball represent?
The Earth
© e-classroom 2014 www.e-classroom.co.za
Grade 7 Natural Science: Planet Earth and Beyond: Relationship of the Moon to the Earth: Gravity
Answer sheet
GRADE 7
The Gravitational Force of the Sun on the Earth
4) What does the path of the moving ball represent?
The Orbit of the Earth around the Sun
5) What happened when you released the string?
It flew off in a straight line and did not keep travelling in the circular motion around me.
6) Draw a picture to represent this demonstration.
We have used the Sun and Earth as an example, but the same applies regarding the
Moon and the Earth.
Draw a picture or diagram to represent the relationship of the Moon to the Earth
regarding Earth’s gravitational pull.
© e-classroom 2014 www.e-classroom.co.za
Grade 7 Natural Science: Planet Earth and Beyond: Relationship of the Moon to the Earth: Gravity
3) What does the string or rope represent?