GRAVITY, FORCE AND WEIGHT
When an unbalanced force is applied to an object, the object will accelerate. The size of the acceleration
depends on the force applied and the mass of the object. There is a relationship between force, mass and
acceleration. It is force = mass × acceleration.
Mass and force
All objects have mass. It is a measure of how much material the object contains, and in the International Unit
System we use the kilogram, kg. For the force, we use the Newton, N.
Newton was the scientist who discovered the force of gravity and how force and movement are related.
Gravity and weight
One of the most important forces is gravitational force (or gravity). In general, gravity is a force that attracts
objects with mass towards each other. When one of the objects is the Earth, the force that makes the Earth is
called weight. Weight makes objects fall down with an acceleration of free-fall, which is 9.8 m/s2.
weight = mass × acceleration of free-fall.
Gravity has the following effects. It:

pulls objects on Earth towards the centre of the planet;

holds the Earth's atmosphere in place;

holds all the components of the solar system in orbit around the Sun; and

holds all the components of the galaxy together.
Weight is not the same as mass. The mass of an object is the same everywhere, but its weight can change.
The weight depends on the mass of the object and the strength of the gravitational field around it.
Gravitational field strength
The gravitational field strength depends on the size and the mass of the planet or satellite under
consideration. The gravitational field in the Earth is bigger than in the moon, therefore the weight of an
object on the moon is lower than on the Earth, approximately a sixth. But in both cases the mass is the
same.
Some exercises
1 a) A person has a mass of 60 kg. How much does he weigh on Earth?
b) How much would the same person weigh on the Moon, if the acceleration of free-fall is 1.6m/s2?
2. What is the unit of force?
The Newton (N)
The Newton (n)
The kilogram (kg)
3. The same force is applied to three balls. Which one will have the greatest acceleration?
The 4kg ball.
The 2kg ball.
The 1kg ball
4. A rocket has a mass of 10,000 kg. What is its acceleration if the force of its engines is 20,000 N?
200,000,000 m/s²
10,000 m/s²
2 m/s²
5. An astronaut has a mass of 100 kg. What is his weight on Earth?
10 N
110 N
980 N
6. An astronaut has a mass of 100 kg. What is his mass on the Moon?
62.6 kg
100 kg
160 kg
7. Answer the following questions:
a) Can you name any effects that forces have?
b) What units do we use to measure force?
When Sir Isaac Newton was a boy he was more interested in making mechanical devices than in studying. He
made a windmill which could grind wheat and corn, and he made a water clock and other things. His teachers
thought of him as a weak student because he didn’t do very well.
He wanted to go to college, but he didn't have the money to go. He enrolled at the lowest level, which was the
cheapest. In this position he had to serve the other students and do chores for them. He even ate their
leftovers, but he would do anything to have the opportunity to learn.
When the university shut down because of the plague ( an illness which people caught and from which they
died suddenly ), he went home and continued studying on his own. He had a notebook with 140 blank pages
and he began to fill them with notes about the things he had read and studied.
*
His natural curiosity led him to make some very important discoveries when he became an adult. During a
period of a year and half he made three great discoveries.
One day, when he was drinking tea in the garden, he saw an apple fall to the ground. He started thinking
about why it fell, and finally decided that the same force (called gravity) which caused the apple to fall also
kept the moon in the sky around the earth. This same force, gravity, also kept the planets in the sky around
the sun.
The apple incident led to his three basic laws of movement:

An object in movement keeps moving if there isn’t any force to stop it.

An object moves with acceleration when a force pulls it.

For every action ( a force that works over an object) , there is an equal and opposite reaction
( a force that works over the first object at the same time).
Isaac Newton is known as the greatest scientist who ever lived. He is buried in Westminster Abbey, in London.
Now answer these questions in full sentences:
1.
What did Isaac Newton like doing when he was younger?
2. Write down two things that Newton made when he was a child.
3.
Why did the university close with the result that Newton had to go back home?
4.- Why did Newton enrol at the lowest level at college?
5-
What things did Newton have to do because he was enrolled at the lowest entry at college?
6.- When the college closed, Newton went home and spent time writing in his notebook. What did he
write in his notebook?
7.-Why do you think Newton made so many discoveries as an adult?
8.- What did Newton think when he saw the apple falling down?
9.- How many basic laws of movement did Newton discover?
10.- Write down one of Newton’s laws.