Good Science Victorian Curriculum Year 7

CHAPTER 8: FORCES
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On the surface of Earth, acceleration due to gravity is constant.
The speed of a falling object gets faster and faster, but it gets faster
and faster at the same rate. That’s because the gravitational force
acting on it is always the same – the force of Earth’s gravitational field.
Why do objects speed up when they fall?
Gravity can cause objects to slow down
or change direction
Sometimes when an object is forced upwards, gravity can cause the
object to slow down or change direction, rather than speed up.
Imagine that you pick up the basketball you dropped, and you take
a shot at the basket. What do you expect to happen? First, the ball flies
up and out, gaining height while moving closer to the basket. At some
point, it stops going up and starts coming down, even though it’s still
moving away from you, until (hopefully) it swooshes through the basket.
The changes in the ball’s flight are all due to unbalanced forces.
After you release the ball, you can’t put any additional force on it – it’s
literally out of your hands! When the ball is in flight, the largest force,
gravity, will pull the ball back to the ground. It doesn’t matter how much
force you initially give the ball – after it has left your hands, gravity will
immediately start slowing it down and eventually pull it back to Earth.
Gravity is always an attractive force, pulling objects towards the
centre of Earth. But it’s the overall sum of unbalanced forces that
determines how objects move, and when they fall to the ground.
What is an example of gravity causing an object to slow down
or change direction?
Figure 8.9 When you shoot
a basketball, you need to
account for gravity acting
on the basketball.
CHECKPOINT 8.7
1 On the surface of Earth, in
which direction does a force
due to gravity act?
2 How would an object with
overall unbalanced forces
acting on it move differently
to an object with overall
balanced forces acting on it?
3 Give an example of an object
on Earth that has balanced
forces acting on it, and
describe how it would move.
4 Give an example of an object
on Earth with unbalanced
forces acting on it, and
describe how it would move.
5 Provide an example where
unbalanced forces allow an
object to move away from the
ground, and an example where
unbalanced forces move an
object towards the ground.
6 You experience unbalanced
forces any time you walk
upstairs or drop something.
For one of these situations,
describe the source of the
forces at work.
RESEARCH
7 Drones, rockets, space shuttles
and aeroplanes all generate
an upward thrust in order to
lift off the ground. Choose
one example to research and
describe the mechanism it
uses to move against the force
of gravity. Outline any positives
and any negatives of your
chosen method of lift-off.
SUCCESS CRITERIA
I can describe how gravity
acts on objects when they
are moving and when they
are stationary.
I can give examples
of everyday situations
where gravity acts as an
unbalanced force.
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