6.5 hsw what lies beneath

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

Quantity Symbol Unit Unit symbol Description

Mass m kilogram kg Amount of stuff

Weight W newton N Gravitational pull on amount of stuff

Quantity Symbol Unit Unit symbol Description

Mass

Weight

Quantity Symbol Unit Unit symbol Description

Mass

Weight

Quantity Symbol Unit Unit symbol Description

Mass

Weight

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

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

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

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

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

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

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 / N0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

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

We

igh

t /

N

Mass / kg

A graph to determine value of g

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

We

igh

t /

N

Mass / kg

A graph to determine value of g

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

We

igh

t /

N

Mass / kg

A graph to determine value of g on Earth

kgNg

kg

Nm

mass

weightm

massmweight

mxy

cmxy

10

1

10

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)

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)

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)

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)

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)