Book Review 👏👏

Gravity

• Highlight the main ideas. Summarize these main ideas  in one or two paragraphs.

• When does gravity decrease? When you get further away from the main source of gravity

• When does the force of gravity become noticeable? When the object has a large mass

• Gravity disappears when you are above the atmosphere. True or False?

• According to the text, how long does it take the International Space Station (ISS) to orbit Earth?
• It takes
• 90 minutes for the
• international
• space station (ISS) to orbit the earth.

• Who first stated that all objects are attracted towards each other by gravity? Isaac Newton

• Find six words in the text that have a suffix.  “Attracted”
• “Attraction” “
• Orbiting”
• “Fixed”
• “Noticeable”
• “released”

Gravity

Gravity is a force that attracts all objects towards each other. People are attracted towards

the Earth and the Earth towards people, the Moon and the Earth are attracted towards each

other, and the Sun and the Earth are attracted towards each other. All of these attractions

are caused by gravity. Gravitational attraction is greater for more massive objects. Gravity

decreases as distance between the objects increases.

Gravity attracts all things towards each other

Every object in the Universe is being attracted towards every other object by the force of

gravity. This means that there is nowhere you can go in the

Universe where gravity is not acting.

Examples of gravity in action:

• Gravity holds the atmosphere in place around the Earth.
• Gravity keeps people on the Earth’s surface.
• Gravity keeps the International Space Station in orbit around the Earth.
• Gravity keeps the Moon orbiting around the Earth.
• Gravity keeps the Earth orbiting around the Sun.

Isaac Newton was the first to come up with the idea that all objects are attracted towards

each other by gravity. Even people are attracted towards each other by gravity, but this force

is so small that it is not noticeable. Gravity only becomes noticeable if one (or both) of the

objects has a lot of mass, such as the Earth.

There is gravity in space

Earth-Moon system and gravity

Any two masses are attracted towards each other by gravity. This force of gravity

causes the Moon to change direction to make it orbit around the Earth.

Gravity doesn’t disappear just because you are above the atmosphere. Even if an

object is high above the Earth’s atmosphere, there will still be a strong force of gravity

pulling it towards

the centre of the Earth. At an altitude of 30 km, you would be above 99% of the Earth’s

atmosphere. At 100 km, you would officially be in space, yet the weight force of gravity

would still be nearly the same. You and the Earth would still be pulled together.

Isaac Newton worked out that, if the distance from the centre of the Earth doubles, gravity

becomes a quarter as much as it was on the surface. A satellite with a mass of

1000 kg has a weight

force of 9800 N at the Earth’s surface. The radius of the Earth is about 6366 km,

so at 6366 km above the Earth’s surface, the distance from the centre of the Earth

will have doubled.

The weight force pulling it towards the centre of the Earth will now only be a

quarter

as much but will still

be 2450 N.

So why doesn’t a 1000 kg satellite just fall back to Earth?

Sideways speed keeps satellites in orbit.

As a satellite moves around the Earth in a circular orbit, the direction of the force

of gravity is always towards the centre of the Earth.

At an altitude of 100 km, you would be so high that you would see black sky and stars if

you looked upwards. If you took a satellite to this height and released it, it would still fall

towards the Earth because the force of gravity is nearly the same as it is at the

Earth’s surface.

However, if the satellite is given speed in any direction horizontal to the surface of the

Earth, it will travel further before it hits the Earth. If it is given enough speed, it will travel

so far that, as it curves towards the Earth, it will miss the Earth altogether. At just the

right speed, it

will move around the Earth in a circular motion. This type of motion and the path that a

satellite moves in is called an orbit.

Close to the Earth at an altitude of 100 km, a satellite needs to be moving at 8 kilometres

per second (28 000 km/h) to stay in orbit. At higher altitudes, satellites do not need to be

travelling as fast. Television communication satellites are at a higher altitude of 36 000 km

and only need to travel at 3 km/s (11,000 km/h).

The Moon is 360,000 km from the Earth and only needs to be travelling at 1 km/s to stay in

orbit around the Earth.

If there is gravity in space, why do astronauts appear weightless?

Astronauts appear to be weightless for the same reason that a person on a trampoline

feels weightless when in the air. There is still the same amount of gravity acting, but

there is no floor pushing upwards on the astronaut, so the weight force cannot be felt.

If a person was in an elevator and the cables broke and the brakes failed (which is actually

quite possible), the person and the elevator would fall towards the Earth at the same rate.

The

floor would not be holding the person upwards, so the person could enjoy

the sensation of weightlessness (for a brief while before a painful death).

This is the same for astronauts high above the atmosphere on the International Space

Station (ISS) at an altitude of about 400 km. Gravity is still strong, but the astronaut and

the ISS fall towards the ground at the same rate. They are also both travelling

horizontally

at 28,000 km/h. As they fall towards the ground, they travel so fast horizontally that

they miss the Earth altogether and orbit the Earth once every 90 minutes.

Nature of science

Science ideas change over time. Isaac Newton’s gravity-based world view has since

been superseded by Albert Einstein’s ideas that all masses distort space and time.

This

highlights

the fact that science is not a fixed body of knowledge. Although Einstein’s theory

is widely accepted, Newton’s law of universal gravitation is still used for practical

situations

such as satellite motion.

Gravity attracts all things towards each other.

There is gravity in space

Sideways speed keeps satellites in orbit.

If there is gravity in space, why do astronauts

appear weightless?

Gravity is a force that attracts all objects towards each other.

Gravity doesn’t disappear just because you are above the atmosphere.

orbit

Space Inquiry

Taku Mōkai