Gravitational Forces → Main topic information

Introduction
- What you will learn
- Main topic information
Activities
Quiz
- Quiz
  5 questions05 min
Conclusion
- What I learned

Gravitational Forces

Main topic information

Vocabulary/new concepts:

Weight – a physical quantity defining the force of gravity with which a body of a given mass is attracted to the centre of another of greater mass (gravitational attraction). Weight is a force and is therefore measured in Newtons (N).

Gravity – the force of attraction between all objects that have mass. Earth’s gravity is the force that keeps us firmly planted on the ground, it is the reason objects fall when we drop them, and it determines planetary motion around the sun.

Gravitational field – a physical pattern, an invisible net around any body with mass. It explains the effect that a body exerts in space to create a force on another body.

Gravitational field strength – the force with which the gravitational field attracts a unit mass (mass of 1 kg). It is measured in newtons per kilogram – N/kg.

Gravitational constant (G) – more commonly referred to as the “big G”, is a number used in gravity calculations, i.e. it determines the force of attraction between two bodies. The value of the gravitational constant is very small. This means that gravity is a relatively weak force (compared to other fundamental forces in nature, e.g. electromagnetic forces) that needs very massive objects to create a detectable gravitational field.

Why do all free bodies fall to the ground?

Why can’t people fly?

Why does the moon go around the earth and not the other way around?

These and other interesting questions are answered in this lesson.

Gravity – what is it and how does it work?

Gravity is a fundamental force that describes the general property of all bodies with mass in the universe to attract each other. Two huge ships at sea, two pens placed on a mass, and a proton with an electron in an atom are attracted by gravity. The heavier the bodies, the greater the gravitational force they attract. Conversely, as the distance between the centres of gravity of the two bodies increases, the magnitude of the gravitational force decreases until it gradually disappears completely.

Gravity is the force that pulls all objects that have mass towards the centre of the Earth. This means that anything that goes up must come down due to gravity. Gravity is everywhere. It is intangible. You cannot even see it. It is a force of attraction that exists between objects and the Earth. It acts in the water, on land and in the air.

Gravity is not just an attraction between objects and the Earth. It determines the strength of mutual attraction between two objects and between all cosmic bodies in the universe, from the smallest particles to the largest galaxies.

Earth’s gravity is the force that keeps us firmly planted on the ground, it is the reason objects fall when we drop them, it affects the motion of satellites and objects near the Earth.

Two bodies do not need to touch at all to gravitationally attract each other. This is possible thanks to the gravitational field that arises around each body.

The gravitational field is a physical model of the invisible force that is created around any body that has mass. It explains the effect that one body exerts in space to create a force on another body at any point in space.

The strength of the gravitational field is the force with which the gravitational field attracts a unit mass (mass of 1 kg). It is measured in newtons per kilogram – N/kg. It is a vector quantity, which means that it has both magnitude and direction. The direction of the force is always directed towards the centre of the body that creates the field.

The motion of the planets is affected by the gravitational forces between them, and the gravitational interaction between the Moon, Earth and Sun causes the tides in the oceans. Gravity attracts all objects, large and small. It is everywhere – in the water, on land and in the air. Without it, life on Earth would not exist because it holds back the atmosphere and prevents water from escaping into space.

There is an essential difference between gravitational forces and electric or magnetic forces. Gravitational forces are only ever attractive – bodies are never gravitationally repelled. Whereas two electric charges of the same sign or two magnets facing each other with the same poles can repel each other.

The gravitational force between two bodies depends on the masses of the bodies and the distance between them. The greater the mass of the body, the stronger the gravitational field it creates. As the distance from the centre of the body increases, the strength of the gravitational field decreases.

Newton’s law of universal attraction

The mutual gravitational attraction of two bodies was described by Isaac Newton in 1687. He summarized the results of numerous astronomical observations of the motion of the Moon and other planets and formulated his Law of Gravitation, which is still in force today.

Source: https://humanachievementalliance.org/wp-content/uploads/2020/10/2-Newton.jpg

This law, known as the Law of Universal Attraction, mathematically describes the relationship between the force of gravitational attraction, the masses of bodies, and the distance between them. It states:

Between any two bodies (material points) of the universe act forces of mutual attraction, called gravitational forces, whose magnitude is directly proportional to the product of the masses of the bodies and is inversely proportional to the square of the distance between them.

From this law we can derive a formula for calculating the strength of the gravitational field

Source:
https://bg.wikipedia.org/wiki/%D0%A4%D0%B0%D0%B9%D0%BB:
NewtonsLawOfUniversalGravitation.svg

where:

F1 and F2 are the forces of attraction between any two bodies 1 and 2 (the force of attraction between the two material points)
G is the gravitational constant
m₁ is the mass of the first material point
m₂ is the mass of the second material point
r is the distance between the centre of the two masses (material points)

The gravitational constant (G) – more commonly referred to as the “big G”, is a number used in gravity calculations, i.e. it determines the force of attraction between two bodies. The value of the gravitational constant is very small: G = 6.673 * 10^-11 N m² /kg². This means that gravity is a relatively weak force (compared to other fundamental forces in nature, e.g. electromagnetic forces) that needs very massive objects to create a detectable gravitational field.

Gravitational forces are measurable and can only be observed when at least one of the two bodies has a large enough mass.

For example: We observe no attraction between two apples placed on the table. But the gravitational force of the Sun determines the motion of the planets in the solar system; the gravity of the Earth attracts the Moon, and the gravity of the Moon attracts the Earth. Since the Earth has a much greater mass than the Moon, its gravitational pull is stronger.

Source: https://cdn.britannica.com/26/157426-131-16C2E991/moon-Planet-Earth-space.jpg

The tides in the oceans are the result of the interaction between the gravitational forces of the Moon and the Earth, causing sea levels to rise and fall periodically.

Take a look at the short videos below and answer the following questions:

What do you think are the effects of the Moon’s weaker gravity than Earth’s?
Would it be harder or easier for astronauts to move on the Moon, how high can they jump compared to Earth?
And how will objects fall?

Justify your assumptions.