Good Science Victorian Curriculum Year 7
Digital sample of Matilda's newest publication, Good Science Victorian Curriculum Year, authored by Emma Craven and Aaron Elias. For more information visit www.matildaeducation.com.au or email Katrina Tucker, katrinatucker@matildaed.com.au
Digital sample of Matilda's newest publication, Good Science Victorian Curriculum Year, authored by Emma Craven and Aaron Elias. For more information visit www.matildaeducation.com.au or email Katrina Tucker, katrinatucker@matildaed.com.au
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CHAPTER 8: FORCES<br />
2<br />
Other forces in an interaction can change<br />
a friction force<br />
Imagine you’re packing equipment into a sled on a ramp. Which do you<br />
think would be more likely to slide away: a sled with a little equipment<br />
on it, or a fully packed sled?<br />
The fully packed sled would be more likely to slide off. This is because<br />
friction is due to the material’s coefficient of friction and how much the<br />
objects are being pushed together by other forces.<br />
At the start of this chapter you learnt about the forces acting on a<br />
stationary object. A book on a table is pulled down by gravitational<br />
forces, but it stays in place because the normal force applied by<br />
the table pushes it back up.<br />
Figure 8.6 shows how the friction force keeps the stationary sled<br />
in balance.<br />
Figure 8.6 A sled on a ramp is affected by gravity,<br />
the normal force and the friction force. If these<br />
forces balance, the sled will not move.<br />
Normal force<br />
Force of<br />
gravity<br />
If you loaded the sled with more equipment, the force of gravity and<br />
the normal force would increase. This means the friction force would<br />
also increase to keep the sled still. But there’s an upper limit to the<br />
friction force, depending on the materials of the sled and the ramp.<br />
Eventually the friction force wouldn’t be strong enough to balance<br />
the other forces, and the sled would slide away.<br />
Let’s look at this another way – how do you use your brakes when<br />
riding a bike? When you need to slow down or stop quickly, what do you<br />
do with the brakes? You apply a larger force on them. Why? You do this<br />
because the larger force applies more friction on your wheels from the<br />
brake pads, causing them to slow down more quickly.<br />
How does the normal force affect the friction force?<br />
Friction<br />
force<br />
INVESTIGATION 8.5<br />
Friction of materials<br />
KEY SKILL<br />
Explaining results<br />
using scientific<br />
knowledge<br />
Go to page 178<br />
CHECKPOINT 8.5<br />
1 What two aspects of materials<br />
determine the level of friction<br />
they have when they interact?<br />
2 What does the coefficient of<br />
friction measure?<br />
3 How can you make an object<br />
have less friction when<br />
interacting with another?<br />
4 Why does more mass<br />
generally result in a greater<br />
friction force?<br />
5 On bicycles, brakes are<br />
generally made out of two<br />
rubber pads that apply<br />
pressure on the rim of the<br />
wheel to slow it down.<br />
Why are they made out<br />
of rubber?<br />
6 Explain why people need to<br />
drive more carefully when it<br />
is raining.<br />
7 Why do tyres on cars and<br />
bikes have to be replaced<br />
occasionally?<br />
RESEARCH<br />
8 Curling is a sport where a<br />
large stone is pushed down an<br />
icy track. Players use brooms<br />
to manipulate the friction<br />
between the ice and the stone,<br />
speeding it up or slowing it<br />
down. Research and describe<br />
another activity that involves<br />
manipulating friction, and<br />
explain how the participants<br />
do this.<br />
SUCCESS CRITERIA<br />
I can describe some causes<br />
of differences in friction.<br />
129
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