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Forces and Motion

Mass, Weight and Gravity.

What is Mass?

Mass is an amount of substance. It is measured in kilograms.
The mass tells you how many particles you have,
not what they weigh. Particles can be atoms, ions or molecules.

What is Gravity?

Gravity is a force of attraction between masses.
Gravity is a property of mass,
the bigger the mass, the bigger the gravity.
The further away from each other the masses are,
the weaker the gravity between them
(similar to the forces between magnets and charges,
except that gravity always attracts).

On Earth the force of gravity is 10 N/kg.
The acceleration due to gravity
(how fast things accelerate when you drop them)
is approximately 10 m/s2.

What is Weight?

Weight is the force of gravity pulling on a mass.
Weight is a force, so it is measured in Newtons, not kilograms.

weight = mass x gravity

w = m x g

This equation is important!

Compare this with the general formula F = m x a.
Weight is the force, gravity is the acceleration.

What is the Difference between Weight and Mass?

If you go to the shops,
you will find fruit and vegetables weighed in kilograms.
In physics, this would be considered to be wrong.
On Earth the force of gravity is 10 N/kg,
so you can convert mass into weight by multiplying it by 10.
For example, 1kg of tomatoes weighs 10 N.

If you took your 1kg of tomatoes to the moon,
you would still have the same mass
(because you still have the same number of tomatoes)
but they would weigh less
because the moon has less gravity than the Earth.

People who sell tomatoes are not generally troubled
by the difference between mass and weight.
They are priced in kg, not Newtons, since on the whole
they do not try to sell tomatoes on different planets.

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