Good Science Victorian Curriculum Year 7

CHAPTER 8: FORCES
Imagine a mattress with a bowling ball in the centre, making a hollow.
If you place some marbles around the mattress, they’ll eventually roll
into the hollow made by the bowling ball. They might roll slowly when
they’re near the edge of the mattress, but they will roll faster as they
get closer to the bowling ball, where the slope gets steeper. This is very
similar to the way that objects are affected by Earth’s gravity.
Why is gravity on Earth constant?
INVESTIGATION 8.6
Measuring gravity
KEY SKILL
Identifying limitations
to the method
and suggesting
improvements
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An object’s weight depends on gravity
You may have heard people talk about weight in terms of grams and
kilograms, but these are actually units of mass. What’s the difference
between mass and weight?
An object’s mass is the amount of matter it contains. That bowling
ball on the mattress contains a specific amount of matter. Putting the
ball in different places – on a mattress, up a mountain, on the Moon or
floating in space – won’t change its mass. Mass is measured in grams
and kilograms.
An object’s weight is a measurement of the pull of gravity on an
object. Changing the gravity acting on an object will change its weight.
This is why astronauts on the Moon weigh less and can move around
with huge jumps. It’s also why astronauts on the International Space
Station float weightlessly, even though their bodies still have mass.
The metric unit used to measure weight is the newton (N).
Why don’t we usually use newtons when talking about weight?
It’s because there are very few circumstances on Earth where the force
of gravity changes. It stays constant, so we just use kilograms instead.
What is the difference between mass and weight?
Figure 8.8 Mass doesn’t change,
wherever you are, but weight does.
CHECKPOINT 8.6
1 Explain what gravity is, in your
own words.
2 What two things affect the
amount of gravity something
has?
3 Explain the difference between
mass and weight.
4 If objects are attracted to one
another due to gravitational
forces, provide a reason why
planets orbit around the Sun
rather than crashing together.
5 What metric unit is weight
measured in?
6 How can an object’s weight
change if its mass remains
constant?
7 On the Moon, an object weighs
90 N. Objects weigh 6 times
more on Earth than they do
on the Moon. Calculate the
object’s weight on Earth.
CONNECTING IDEAS
8 If you were to visit three other
planets in our solar system,
your weight would be different
on each planet. Explain why
your weight would change.
Trevor's mass
on Earth is 60 kg.
Trevor's mass on the
Moon is also 60 kg.
His weight on the
Moon is about 100 N.
His weight on Earth
is about 600 N.
SUCCESS CRITERIA
I can describe gravity,
including what causes
gravity.
I can explain the difference
between mass and weight.
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