Mass vs weight, determining g

1. 6.5 HSW: What lies beneath?
28 January 2015
What is the difference
between mass and
weight?
Quantity Symbol Unit Unit symbol Description
Mass
Weight

2. Quantity Symbol Unit Unit symbol Description
Mass m kilogram kg Amount of stuff
Weight W newton N Gravitational pull on amount of stuff

3. Quantity Symbol Unit Unit symbol Description
Mass
Weight
Quantity Symbol Unit Unit symbol Description
Mass
Weight
Quantity Symbol Unit Unit symbol Description
Mass
Weight

6. Gravity map
• You may think that the force of gravity of the
Earth is the same all over the Earth but it isn’t
• At the Earth’s surface, the Earth pulls object
towards its centre with a force of 9.8 N on every
kilogram of mass (9.8 N/kg)
• Another way is saying that one kilogram will
accelerate by 9.8 m/s every second (9.8 m/s2)
• This value can change depending on where you
are standing and what you are standing on

7. Gravity surveys
• Rocks are made from different materials.
• Some are made of sand while others contain
denser material like iron
• A dense materials is one which is heavy for its
size
• Think of a block of iron compared with a block of
polystyrene. When you stand on the rock with
iron you are pulled down more strongly than
where you are standing on rock made of sand

9. Gravity surveys
• Geologists carry out gravity
surveys to investigate these
differences in gravitational
attraction
• They use a spring with a
mass on the end to measure
the gravitational attraction in
each place
• This is called a gravimeter
• The stronger the force, the
more the spring stretches

11. Hidden secrets
• Gravity surveys help us work out what is
below the surface without having to dig or
drill
• They can reveal underground springs, rivers
and volcanic bases as well as evidence that
the Antarctic ice sheet is losing ice

13. Examples
• Geophysicists:
– Look for underground rock movements that might
show an earthquake is about to occur
• Oceanographers
– Look for changes in ocean currents and climate
• Archaeologists:
– Look are remains of walls and buildings to help
inform them of where to investigate

14. The view from space
• Satellites can now
measure gravity
variation from Space
to produce gravity
maps like the one of
the Earth
• The red parts are
areas of high gravity

15. Experiment: Determine the value of g graphically?
• You have a number of
different masses
• Weigh the masses
using a newton meter
• Draw a table for your
results
• When you have
collected all the data
(including repeats)
construct a graph
• Determine g
Mass / kg Weight / N
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0

16. 0
1
2
3
4
5
6
7
8
9
10
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Weight/N
Mass / kg
A graph to determine value of g

17. 0
1
2
3
4
5
6
7
8
9
10
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Weight/N
Mass / kg
A graph to determine value of g

18. 0
1
2
3
4
5
6
7
8
9
10
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Weight/N
Mass / kg
A graph to determine value of g on Earth
kg
Ng
kg
N
m
mass
weight
m
massmweight
mxy
cmxy
10
1
10







20. Questions
• A block of iron and a block of polystyrene are
the same size
– Which block has more mass?
– Which provides a greater force of gravitational
attraction?
(4)

21. Questions
• The force on a 1 kg block at the Earth’s surface
is on average 9.8 N
– What is the force on a 50 kg pupil?
– What are the weights of the block and the pupil?
(5)

22. Question
• Suggest why scientists use colours rather than
numbers on the map to represent changes in
gravity. (qualitatively)
• What is the advantage of presenting the data
numerically? (quantitatively)

23. Questions
• Create a key to go with the gravity map of
Antarctica
• Suggest which colours should represent
medium, low and very low gravity areas.
(7)

24. Question
• As well as showing areas of high density,
gravity maps also show the locations of
mountains and valleys. Why would a
mountain appear different to a valley?
(8)