FORCES 2 : WEIGHT vs MASS

1
FORCES 2 WEIGHT vs MASS

• By the end of this presentation, you should be
  able to
• Explain the difference between weight and mass?
• Describe the difference between the weight of an
  object on the earth, compared to the same object
  on the moon (or on another planet), or in outer
  space.
• Calculate the weight of an object if given its
  mass and the gravitational field strength.

2
What is gravity?
Gravity is a natural force field that exists
around ALL objects. Yes, you have your own
gravitational field around you! The strength of
this field is controlled by the mass of the
object. The heavier it is, the stronger the
field. Gravity is always attractive, and tries to
pull you towards the centre of the Earth,
wherever you are standing.
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Some more facts about gravity
The closer you are to the planet, the stronger
the gravitational pull. There is no gravity in
space. So objects are called weightless. This
is why things float around in spaceships. There
isnt anything to pull it! The Moons
gravitational field is 6 times weaker than the
Earths.
More about the moon and planets in our Solar
System
4
The strength of gravitational fields in different
places

• The gravitational field strength on the Earth is
  10N/kg. This means a 2kg bag of flour on your
  foot, will exert a 20N force on your foot. This
  is its WEIGHT.
• The table shows how the gravitational fields of
  the other planets compare to that of the Earth.

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5
Mass v.s. Weight

• Your mass is determined by how
• much matter or atoms make up
• your body. It is measured in
• Kilograms (kg). This will not
• change with location i.e. where
• you are in the Solar System, but
• it might change
• if you go on a food binge!
• Your WEIGHT depends on how much you are being
  pulled by the planet. So it depends on your MASS
  and the GRAVITY on the planet.
• We can write Weight Mass x Gravity
• Weight is a force so is measured in Newtons

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Changing mass to weight (using gravity)

• Remember the equation Weight Mass x Gravity
• Put the quantities into a triangle for easy
  manipulation
• The astronaut has
• a mass of 60kg.

On the moon, g1.7N/kg
On Earth, g10N/kg
W m x g

• What is his weight on Earth?
• W 60 x 10 600N
• 2. What is his weight on the Moon?
• W 60 x 1.7 102N
• 3. In space, W 60 x 0 0N

In space, g0 N/kg