7
What
Forces and
Motion
you will learn
Friction
Simple machines
Lifting and turning
63
Forces and
motion
7a
Friction
Friction is the force that tries to stop materials sliding past each other. There is friction between
your hands when you rub them together. And there is friction between your shoes and the
ground when you walk. What happens when you run in the sea or try to walk in the sea?
Shaped for speed
It takes some effort to walk through water
because of friction with the water. You can
make the friction less by changing your shape.
If you swim, it is easier for the water to flow
around your body and so you can move more
quickly.
Many fish need to move fast through the water,
to catch their prey and to escape from their
enemies. They have a very streamlined shape.
Air resistance
Friction with air is a lot less than friction with
water, but it is still there. Friction with air is
usually called air resistance. It slows down all
your movements, though you probably do not
notice it because you are so used to it.
There is no friction in space, because there is no
air. Spacecraft can travel very fast because there
is no air resistance at all. The Earth can carry on
travelling round the Sun for millions of years
without slowing down.
Most big aeroplanes fly high up where the air is
thin and the air resistance is less. This helps
them to go faster.
Streamlined shapes can make air resistance less,
in the same way that they reduce friction in
water.
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Do
you know?
When a bicycle’s speed doubles, air
resistance increases four times.
Space rockets fly most of the time without
burning their engines. The rockets speeds
can very gradually change. However they are
affected by the pull of gravity from the nearest
planet.
Comparing water and air
You need a jar containing water and air supported by retort stands and clamp.
Make two balls of plasticine (clay) of the same size.
Drop one of the plasticine balls into each of the jars at the same time.
water
air
Which ball reaches the bottom of the jar first? Can you explain why?
?
1
What is friction?
2
Complete the following sentences using the correct
word from the pair given.
a. Air resistance is the force/energy of
friction when something moves through air.
b.
3
4
There is less/more friction in air than in
water.
c. Friction makes it easier/harder for cars
and planes to travel fast.
Why are wet brakes less effective in stopping
motion?
Look at these two fish.
A
B
K
Ideas
!There is friction when
something moves through air
or water.
!Air resistance is a force
that slows objects moving
through air.
!Friction can be reduced
by using a streamlined
shape.
a.
Which fish A or B is designed to move
faster?
b.
How does the design help it move fast?
!Friction is much greater
in liquids than in gases.
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Forces and
motion
7b
Simple machines
screwdriver
If you had to make a list of useful machines, the list might include a
washing machine, a mixer and a motor cycle. You probably would
not include a can opener, a spanner or scissors. You don’t normally
consider these tools to be a machine, but they are. In science a
machine is any device which helps to make work easier for us. Tools
such as a screwdriver, tongs and scissors are called simple machines.
It’s not difficult to believe that a screwdriver can
make it easier to do work. Just try unscrewing
one without a screwdriver! Opening a can,
pulling a nail out and cutting a piece of cloth are
other jobs which are impossible to do with your
bare hands. These jobs can be done quite easily
using a can opener, hammer and scissors.
scissor
can opener
Force magnifiers
Some machines make our work easier by changing the size or direction of a force.
A small force is needed to lift a car using a jack, but its handle has to be moved a
longer distance. Machines which can overcome a large force using a small force are
called force magnifiers.
input
force
output
force
input
output
pliers
input force
car jack
output
force
Do
you know?
The oldest machine still in use is a
water raising machine called a dalu.
It was invented around 3500BC.
pulley
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Movement magnifiers
Other machines make our work easier by producing
more movement than we put in. However, the output
force is less than the input force. Step on a pedal bin.
Observe and compare the movements of the pedal
and the bin. Which one moves more? Machines
which produce a bigger movement from a smaller
movement are called movement magnifiers or
distance multipliers.
?
1 What is a machine?
2 What is the advantage of a fishing rod?
3 Give three examples of household machines which
are force magnifiers.
4 Machines help to make our work easier. Using a
crowbar to lift a rock is much easier than lifting it with
your bare hands. Does this mean that less work is
done when you use a crowbar to lift the rock than
using your bare hands? Explain your answer.
K
Ideas
!A machine is anything
which makes it easier to do
work.
!Movement magnifiers
are machines which put out
more movement than you
put in.
!Force magnifiers are
machines which put out
more force than you put in.
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Forces and
motion
Lifting and turning
7c
Levers can be very useful. They can help you
to do a job which would otherwise be
difficult. The longer the lever, the better. This
child should push down as far from the pivot
as possible to be able to lift the heaviest
weight.
Getting balanced
A see-saw is a kind of lever. If a boy and girl weigh the same, they will balance if they sit at
opposite ends.
Which way will the see-saw tip if the man moves
Which way should the women move to
further away from the pivot?
balance the see-saw?
Aisha weighs half as much as Ali, so Aisha has to sit twice as far from the pivot. Ahmed is their dad. He weighs
800N. Draw a diagram to show where Ahmed should sit, if he is to balance Ali without Aisha sitting on the see-saw.
Ahmed
1m
Ali
2m
Aisha
Do
you know?
The most powerful crane in the
world can lift 350 of the heaviest
blocks.
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800 N
400 N
200 N
More levers
Cyclists make use of levers. The brake handle has a
pivot. You pull on the end of the handle with a small
force. The lever pulls the brake cable with a bigger
force.
If you have a puncture, you may need to take
the tyre off your bicycle wheel. It is much
easier to do this if you have a tyre lever. You
push down on the end of the lever to get as
much leverage as possible. Then you can
provide enough force to remove the tyre.
A wheel barrow is a kind of lever which helps
you to lift heavy loads. The pivot is at the middle of
the wheel. Your hands are at the ends of the handles,
as far as possible from the pivot.
A crow bar helps you lift a heavy rock. You use a
small force to lift a heavy weight.
?
1 Copy and complete the sentence below. Choose
the correct word from each pair.
A lever can be useful if you are not heavy/
strong enough to lift a heavy/strong load. You
can lift more if you push down near the pivot/
end of the lever.
2 You can use a miniature see-saw to compare
weights. A ruler pivoted on a pencil will allow you
to compare the weights of two coins.
a.
b.
Suppose you had three coins, and you knew
that one was a fake. Its weight is more than
the other two, because it is made of different
metal. How would you use your see-saw to
find the fake?
If you had seven six coins, and one was
fake, how would you find it? You are only
allowed two weighings!
K
Ideas
!A
lever is useful for
lifting heavy objects.
!The further a force acts
from the pivot of the lever,
the more it can lift.
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Forces and
IR SAA
C
motion
SAAC
S I
NEW
TON
EWT
(1
6 42-1
72
7)
(16
42-172
727)
Isaac Newton was born on December England the same year Galileo
died. His father, who had died shortly before Newton was born,
was a yeoman farmer, and it was intended that Newton should
carry on the paternal farm. He was sent to school at Grantham,
where his learning and mechanical proficiency excited some
attention. In 1656 he returned home to learn the business of a
farmer, but spent most of his time solving problems, making
experiments, or devising mechanical models. His mother noticing
this, sensibly resolved to find some more congenial occupation
for him, and his uncle, having been himself educated at Trinity
College, Cambridge, recommended that he should be sent there.
Newton is clearly the most influential scientist who ever lived. His
accomplishments in mathematics, optics, and physics laid the
foundations for modern science and revolutionized the
world.Newton’s greatest achievement was his work in physics and
celestial mechanics, which culminated in the theory of universal
gravitation. By 1666 Newton had early versions of his three laws
of motion. He had also discovered the law giving the centrifugal
force on a body moving uniformly in a circular path.
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