Good Science Victorian Curriculum Year 7

CHAPTER 8: FORCES
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Kinetic friction makes moving objects
slow down or stop
When an object starts to move or slide, that doesn’t mean friction has
gone away. Whatever the object is moving along or through – whether
it’s a solid surface, water or just air – a kinetic friction force is acting on it.
Have you ever ice-skated, or slid across a polished floor in socks? If
so, you know that sometimes the kinetic friction forces can be pretty
small, and you can keep moving for a long time. But you come to a stop
eventually, even without running into something. This is because of the
continual action of kinetic friction against your movement.
The kinetic friction acting on a moving object is less than the static
friction needed to keep it still. If you were pushing a box up a ramp,
getting it moving requires slightly more force than keeping it moving.
Which is larger: static friction force or kinetic friction force?
1 When the box is stationary,
2
static friction force =
applied force.
Applied force
Static friction force
The box won’t slide until the
applied force is more than the
static friction force.
Heat is a product of friction
When the box is moving,
applied force only needs to
overcome the kinetic friction
force, which is less than the
static friction force.
Applied force
Kinetic friction force
The kinetic friction force depends
on the speed of the objects and
the size and materials of the
surfaces in contact.
Place your hands together, apply a little bit of pressure and then quickly
rub them back and forth. It shouldn’t take long for a bit of heat to build
up. This heat is caused by the friction between the particles of your
hands as they rub against each other.
If you have ever seen a video or picture of a space shuttle re‐entering
Earth’s atmosphere, then you’ve seen how much heat friction can
generate at high speeds. This is because of the high number of collisions
between particles in the air and the shuttle. The air gets hot enough to
form a glowing, fiery cloud around the shuttle.
What causes surfaces to produce heat when they interact?
INVESTIGATION 8.4
Heat from friction
KEY SKILL
Identifying the
variables and
formulating a hypothesis
Go to page 177
CHECKPOINT 8.4
1 Explain the difference between
friction and static friction.
2 Describe some ways that you
could increase the friction of
the surface of the floor your
feet are on right now.
3 What does friction oppose?
4 What are two types of friction?
5 Which friction force is larger:
the maximum static friction
force or the kinetic friction
force?
6 What type of energy is the
main by-product of friction?
7 Outline three factors that affect
the kinetic friction force.
8 A lot of machines use special
materials and substances to
minimise the friction between
moving parts. Explain why this
is done.
INQUIRY
9 Compare the soles of a pair
of sneakers to those of a pair
of school shoes. Identify the
key differences and explain
why they might be different
with regard to friction.
SUCCESS CRITERIA
I can explain the difference
between kinetic and static
friction forces.
I can explain why friction
can produce heat.
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