Questions to think about…
What can a force
do?
What is a
force?
What are the
useful and
harmful types
of forces?
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Yo-yo activity
Actions to Actions to
move the move the
yo-yo
yo-yo
towards away from
you
you
Pull
Push
Push and pull
• A force is a push or a pull.
Pulling
Wemovements
push a badminton
A bicycle moves when
are used
in
a
game
of
A
pram
moves
racket to hit a A swing moves by
the pedals are pushed.
tug-of-war.
when
it
is
pushed.
shuttlecock.
pushing or pulling it.
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Push and pull
• We cannot see a force, but we can
observe the effects of a force when it
interacts with an object.
• We can tell if a force is weak or strong by
observing what happens to the object
during the interaction.
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Forces
• When a soccer ball is kicked, rolled, headed or
blocked, these movements are possible due to
the forces the players apply on it.
• A force is able to:
o
o
o
o
o
move a stationary object
make an object move faster or slower
stop a moving object
cause an object to change direction
change the shape of an object
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Different types of forces
Types of forces
friction
elastic spring force
gravity
magnetic force
My Pals are Here!
Science
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Marshall
Cavendish (Singapore) Private Limited
© 2009
Marshall
Cavendish
International
Friction
• Whenever two surfaces are rubbed together,
friction is produced.
• Friction between the surface of the book and the
desk causes the book to slow down and stop.
• Friction is a force that opposes motion.
• Friction acts in an opposite
direction to the movement
of an object.
• Thus, friction can cause
moving objects to slow
down and stop.
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Friction
• There is more friction on a rough surface
compared to a smooth surface.
• An object moves more slowly on a rougher
surface.
A gravel-covered floor
has a rough surface.
A marble floor has
a smooth surface.
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Friction
• Friction also produces heat.
• When the surfaces of objects are rubbed
against each other, heat is produced.
Try doing a foot rub
using your hands. Do
your hands feel warm
soon after?
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Friction can be useful
• Friction is useful in our daily lives as it
provides grip.
• Friction allows us to hold on to objects
without them slipping and sliding away
from each other.
• For examples, friction allows us to hold a
pencil and write.
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Friction can be useful
What are
the other
uses of
friction?
Friction between the
Friction
the and
soles between
of your shoes
soles
your shoes
the of
ground
allows and
you the
to
ground
stops
you from
run fast
without
slipping.
slipping when you walk.
sole
Friction is very helpful
when the ground is
slippery or wet.
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Friction can be useful
• The patterns on the tyres of a vehicle help
the vehicle to stay on the road.
• They prevent the vehicle from skidding
under wet, slippery conditions.
• They also allow the
vehicle to slow down
and stop when the
brakes are applied.
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Friction can be a problem
• Although friction is useful, it can also be a
problem as it causes objects to:
o
o
o
move slowly
overheat
wear out
• For example, there is friction between the
moving parts of a machine because the parts
are always rubbing against each other.
• Machines need extra energy to work against
friction, causing heat to be produced.
• After some time, the parts become worn out.
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Friction can be a problem
• When the soles of your shoes are worn
out, you cannot walk properly and may slip
on a smooth floor.
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Friction can be a problem
• Old and worn-out
tyres give very little
friction between the
tyres and the road.
• This is dangerous as
the car can skid more
easily.
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Ways to overcome the problems of
friction
• Lubricants
Lubricants like oil or grease on the moving
parts of a machine help to reduce friction.
o With less friction, machines last longer.
o
A can of grease
Oil to be placed in
between the gears.
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Ways to overcome the problems of
friction
• Rollers and wheels
Rollers and wheels
reduce friction between
the surfaces of objects
and the floor.
o This allows us to move
heavy objects easily.
o
wheel
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Elastic spring force
• Elastic materials are those
that have the ability to return to
their original shapes after a
force is applied to them.
• Clothes such as t-shirts and
socks are elastic.
• You stretch them to put them
on, but they return to their
original shapes after that.
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Elastic spring force
• Plasticine and dough
are not elastic.
• You can change their
shapes but they will
not return to their
original shapes on
their own.
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Spring
• A spring is an object that is elastic.
• It returns to its original shape after it is
stretched or compressed.
• When we pull a spring, we apply a force
that causes the spring to stretch.
spring is
stretched
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Spring
• When we push a spring down, we apply a force
that compresses the spring.
• When springs are stretched or compressed, they
exert a force called elastic spring force.
• When an elastic spring force is exerted, the spring
stores E.P.E.
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Spring
• When a force acts on a
spring, the spring
stretches.
• The larger the force, the
longer the springs gets.
spring
A spring balance
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Gravity
• Gravity (Gravitational Force)
acts at a distance.
• It pulls everything down towards
the Earth.
• Gravity is acting on you all the
time, pulling you downwards even
when you stand still.
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What gravity does
Gravity
makes things
in the air fall
to the ground.
Gravity causes
water to flow
downwards.
Gravity speeds you up as
you go downhill.
Gravity
pulls you
back as
you try to
climb
uphill
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Weight
• All objects on Earth have weight.
• They have weight because of the
gravitational force between them and the
Earth.
• Spring balances can be used to measure
weight.
• The spring is stretched when an object is
hung on it.
• The greater the weight, the longer the spring
stretches.
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Weight
• Weighing machines such as bathroom scales
and kitchen scales can also measure weight.
• The springs in bathroom scales and kitchen
scales are compressed by the weight placed on
top of them.
kitchen scale
bathroom scale
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Weight
• Weight is not the same as mass.
• Weight is a force, while mass is the amount of
matter an object has.
Weight
Mass
Weight is
• a force
• the force of gravity acting on
an object
Mass is
• not a force
• the amount of matter in an
object
Weight is different at different
places. The weight of an object
is different on the Earth, the
Moon and other planets.
Mass is constant. The mass of
an object is the same wherever
the object is.
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Weight
Moon
Earth
• If a person weighs
himself on the Moon, his
weight would be only
one-sixth of what it is on
Earth.
• His mass, however,
remains the same
whether he is on the
Earth or the Moon.
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Magnetic force
• There are forces that act at a
distance.
• They make an object move
without actually touching
them.
• Magnetic force is a type of
force that can act at a
distance.
• Magnetic forces are caused
by pushing and pulling
actions of magnets.
magnetic objects
Magnetic force
• A magnet pulls or attracts magnetic materials.
(NICS)
• Unlike poles of two magnets attract each other.
• The two magnets exert a pulling force on each
other.
• This pull is also known as the force of attraction.
force of attraction
S
NS
N
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Magnetic force
• Like poles of two magnets repel each other.
• The two magnets exert a pushing force on from each
other.
• This push is also known as the force of repulsion.
force of repulsion
S
N
N
S
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