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Confused about weight and mass?

Have you ever heard anyone saying that “the scales don’t tell the truth about their weight”? You may be surprised to hear that they are right!

Scales give a reading in kilograms, which are the units of mass, not the units of weight.

WARNING: This lesson may alter your weight!

The confusion arises because most people use the word ‘weight’ when scientists would use the word ‘mass’.


What are mass and weight?

Mass is the amount of matter in an object and is measured in kilograms.

Weight is a force and is caused by the pull of gravity acting on a mass. Like other forces, weight is measured in newtons and has both magnitude and direction.

Weight has different values depending on where the object is in the Universe.

Mass and weight are not the same!

Mass is not a force and has the same value anywhere in the Universe, including outer space.


Weight and mass on different planets


Why does weight vary?

Gravity is the force that attracts objects with mass towards each other.

The Earth has a bigger mass than the Moon and so exerts a stronger gravitational pull on the apple.

For example, an apple will have the same mass on Earth as on the Moon, but its weight will be different.

The bigger the mass of the object, the stronger the force of gravity.


Weight and mass on the Moon


How do we calculate an objects weight?

The weight of an object depends on its mass and the gravitational field strength:

weight = mass x gravitational field strength

Weight is measured in newtons (N). Mass is measured in kilograms (kg). Gravitational field strength is measured in

newtons per kilogram (N/kg).

The gravitational field strength depends on the force of gravity. On Earth it is 10 N/kg, but it varies depending on planet size.

The units for these quantities are as follows:


Calculating the weight of a car

A car has a mass of 10,000 kg.

weight = mass x gravitational field strengthweight = 10,000 kg x 10 N/kg

weight = 100,000 N

What is the weight of the car?

(Use 10 N/kg as the value of the gravitational field strength.)


Calculating the weight of an astronaut

An astronaut and his equipment have a mass of 150 kg.

(Use 1.6 N/kg as the value of the gravitational field strength.)

What is the weight when he is standing on the Moon?

weight = mass x gravitational field strengthweight = 150 kg x 1.6 N/kg

weight = 240 N


Calculating weight and mass



So why don’t the planets clump together?

Why does the Earth move around the Sun?

The size of gravitational attraction depends on the size of an object’s mass.

Small everyday objects have such a relatively tiny mass that we do not notice their gravitational attraction.

What force can be large enough to change the direction of a planet? gravity

However, with objects as big as moons, planets and stars, the gravitational attraction is much more noticeable.


Newton’s thought experiment


Humans have also launched many artificial satellites into Earth’s orbit. These are used for communications, imaging and scientific monitoring.

What orbits the Earth?

What is the largest satellite to orbit Earth? Clue: it is natural.

A satellite is an object that orbits another object.

The Moon rotates about its own axis once every 27.5 days, which is the same time it takes for the Moon to orbit the Earth. This means that from Earth we always see the same side of the Moon.

The Moon.


How do planets orbit the Sun?

The planets travel in elliptical orbits around the Sun. This is due to the force of gravity.

The changing speeds of the planets as they orbit the Sun make it very difficult for the orbits to form a perfect circle.

The pull of the Sun’s gravity causes the planets to speed up when they move towards the Sun and slow down when they move away from it.

All the planets, except Pluto, orbit the Sun in the same plane.



Threats to Earth

Threats to Earth are often covered in the media.

Can you think of any potential threats to lifeon Earth from outer space?


As the Earth moves through space, it collides with lumps of material which burn up in the atmosphere.

A large meteor may not burn up completely and hits the Earth with a lot of energy. This is called a meteorite.

Meteors, meteorites and asteroids

Asteroids are very large rocks that normally orbit the Sun. Sometimes, they break free and collide with planets.

These are called meteors and can be seen as streaks of light in the night sky.


Effects of an asteroid impact

The effect of a large asteroid hitting our planet would be catastrophic, it could destroy many species, including the human race.

The damage that an asteroid might cause can be estimated by examining crater sites created by earlier asteroid impacts, both on Earth and other planets.


Did an asteroid wipe out the dinosaurs?

There is evidence that an asteroid impact may have wiped out the dinosaurs 65 million years ago.

In 1980, a layer of iridium, an element commonly found in asteroids, was discovered in rocks in Italy.

This layer was subsequently found in rocks all over the world. This suggested that asteroid material had spread out all over the world at the same time in history.


Then, in 1990, a scientist called Alan Hildebrand was looking at some old geological data.

Dinosaurs and asteroid impact

The theory of mass extinction from an asteroid impact lacked one major piece of evidence: a crater large enough to be the impact site.

There is still debate about whether this impact wiped out the dinosaurs, but it is one of the most popular theories.

He found evidence of a crater with a diameter of 100 km, at Chicxulub, Mexico.


Most of the planets travel around the Sun in slightly elliptical orbits that are almost circular. Comets also travel around the Sun but in very elliptical orbits.

What are comets?

For most of its orbit, a comet is a long way from the Sun.

The tail only appears when the comet’s orbit takes it nearest to the Sun.

The head of a comet head is a lump of ice and dust.

A comet’s tail consists of gas and dust released by the Sun’s heat.


What is a Near-Earth object?

Near-Earth objects (NEOs) are asteroids or comets that pass within a third of the distance of the Earth from the Sun.

The paths of NEOs are monitored by telescopes around the world and the risk of a collision assessed.

About 50,000 small NEOs fall on the Earth as meteorites each year without causing much damage. However, it is estimated that around 1000 NEOs are bigger than 1 km.

So far, observations have not found any large NEOs that will collide with Earth.


Objects in space – matching activity



How is space observed?

Humans have observed space for the last 5,000 years.

The view from optical telescopes is affected by the Earth’s atmosphere and light pollution.

Astronomy was revolutionised 400 years ago when the optical telescope was invented.

The optical telescope allowed scientists to see more of the Universe than ever before.

They are often located on high ground in very remote areas to minimise these effects.


What is the Hubble Space Telescope?

The Hubble Space Telescope (HST) is a satellite in orbit around the Earth. It was deployed from the Space Shuttle Discovery on 24 April 1990.

The HST is outside the Earth’s atmosphere and does not experience the same interference as Earth-based telescopes.

This means that its images of the Universe are much more detailed than images observed from Earth.


What is a radio telescope?

Astronomers do not only observe the Universe using optical telescopes that detect visible light. They also observe otherwavelengths of electromagnetic radiation.

Radio telescopes are also used to look for alien life in the Universe.

Radio telescopes are used to observe radio waves from other parts of the Universe, which are able to pass through the Earth’s atmosphere.


Search for extra-terrestrial intelligence

The Search for Extra-Terrestrial Intelligence (SETI) is the world’s attempt to communicate with other life forms in the Universe.

Humans cannot yet travel to other solar systems, so we rely on radio telescopes and transmitters to listen for communications from other planets.

Several major SETI projects have been established since the 1960s, some of which use participant's home computers to help analyse the radio waves they receive.

However, an authentic extra-terrestrial signal has yet to be detected.


Although many people believe that there is intelligent life on other planets, it is a great challenge to find this.

Should we spend lots of money looking for aliens when there are still huge numbers of plants and animals on our planet that we know nothing about?

Is there life out there?


Are we alone?



Why do we explore space?

Space exploration helps us to find out more about the Solar System and answer questions such as ‘Is there life on any of the other planets?’

As well as furthering our knowledge about space, research into space has also lead to a number of technological developments that may not have occurred otherwise.

Ski boots and smoke alarms are based on technology from the space programme.


What about crewed space missions?

Since humans first travelled into space in 1961, crewed space missions have played an important part in space exploration.

Crewed missions are very expensive and put lives at risk. The Challenger and Columbia space shuttle disasters are tragic reminders of the dangers involved.

In total, 15 astronauts and 4 cosmonauts have been killed in space missions.

However, atmospheric pressure, distance and temperature all limit the places that humans can visit.


What about uncrewed space missions?

Uncrewed space probes can travel further and endure more intense conditions than humans can.

Despite this, many people believe that a human geologist would still be more effective than any number of robots.

Astronauts need constant supplies of food and air on space missions. Imagine the size of the storage space that would be needed to take a human far out into space.

Some, like the Martian Rovers, have been so successful that they have had their missions extended.


Messages to other worlds

In 1977, NASA launched the Voyager 1 spacecraft to photograph the gas-giant planets.

Although it will take 40,000 years for Voyager 1 to even come close to another star, the record is an important symbol of our willingness to communicate with aliens.

Imagine that you were making your own ‘Golden Record’ to send into space. What information would you include?

On board was the Voyager Golden Record, a record that contains sounds and images from our culture and a map of the Solar System.


Advantages of space probes


The future of space exploration

As a result of these goals crewed missions to Mars may become a reality. However, in order to fund these plans, projects like the Hubble Space Telescope have to be cut back.

In 2005, new goals were set out for NASA:

develop a new spacecraft return to the Moon by 2020.

complete the International Space Station by 2010

What do you think the future of space exploration should be?


Opinions on space exploration


How can humans survive in space?

Outer space is a very harsh environment for humans to visit.

Spacesuits allow astronauts to control their oxygen supply, air pressure and temperature, which enables them to move around in space in relative safety.

Without protection, humans would rapidly fall unconscious due to lack of oxygen.

Blood would be boiled or frozen due to the lack of air pressure and extreme temperature.

DNA would also be destroyed by cosmic radiation.


Could humans live in space colonies?

In order for humans to survive long term in space, sustainable artificial environments need to be built.

The results of these experiments may one day prepare the way for more complex structures such as permanent Moon bases, or maybe even space colonies.

The International Space Station is home to a crew of at least two astronauts.

They conduct research and perform experiments about the long-term effects of living in space.



Glossary

asteroid – A lumps of rock up to 1,000 kilometres in size.comet – A lump of rock and ice, which has a very elliptical

orbit around the Sun.gravity – The force of attraction between all objects.mass – The amount of matter in an object. It is measured

in kilograms.meteor – A lump of material from space that burns up in

the Earth’s atmosphere.meteorite – A large meteor that does not completely burn

up in the Earth’s atmosphere and crashes to Earth.near-Earth object – An asteroid or comet whose orbit

brings it close to the Earth’s orbit.orbit – The path made by one body moving around another.weight – The force caused by the pull of gravity acting on

a mass. It is measured in newtons.


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