What is a force?
force is a push, a pull or a twist. A force can change the speed,
direction or shape of an object. For example, when a racquet
strikes a tennis ball, it can force the ball to change speed and
direction. It can temporarily change the shape of the ball too.
A
Types of force
There are names given
to the many types of
force that are around us.
Here is a short description
of some of the most
important ones.
Contact forces
Often, forces can act between two
objects that are in contact with
(touching) each other. Familiar
examples of contact force
include the force of your hand
pulling your shoelaces, the
force of your friend’s hand on
a bottle top as he unscrews it,
and the force of your
schoolbag pulling down on
your shoulders.
Friction is a contact force
between two surfaces that are
sliding, or attempting to slide, over
one another. For example, there is
friction between the tyres of your
bike and the ground when you
pedal. Without friction, the tyre
would just slip and you would not
move forward.
You cannot walk on water, but . . .
water does provide an upward
force on you when you step in.
This upward force is called
buoyancy. It is the force that
enables you, and ships, to float.
Divers in the water need to wear a
heavy belt to be able to sink to the
bottom, otherwise they would keep
bobbing up to the surface.
Non-contact forces
Forces can occur even between two
objects that are not touching each
other. These forces are called
non-contact forces. Like all other
forces, non-contact forces can
The air in this tank
enables the diver to
breathe under water.
➡
➡
The weight of the water
pushes down on the diver.
➡
Contact between
the water and the
flippers pushes
the diver forward
as she kicks.
➡
The push from under
the diver is a force
called buoyancy. To
overcome buoyancy,
divers wear weight
belts.
The Earth pulls down on the
diver with the force of gravity.
Forces can act between two objects
that are in contact with each other.
Other examples of contact
force include friction and
buoyancy.
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Science Alive for VELS Level 5
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Deep diving danger
affect the motion of an object — they can get the
object moving, slow it down or stop it altogether.
Gravity is a non-contact force. We can see the effects
of the force of gravity acting between an apple and the
Earth when the apple drops from a tree. The force
of gravity acting on us is often called our weight.
Gravitational forces also hold the Moon in orbit
around the Earth, and the planets in our solar system
in orbit around the Sun.
Magnetic forces can also act at a distance. These
forces act between two or more magnets, or between
magnets and some substances, such as iron. Magnets
have two ends or poles. When two magnets are
brought together they either attract (pull) each other
or repel (push), depending on the position of the
poles of the magnets.
Electric forces hold all of the material around us
together. Electric forces even hold together the
material we are made from. It is responsible for plastic
wrap sealing our sandwiches and for your hair standing
on end if you hold onto a Van de Graaff generator.
experienced a force on it from the car in the
picture, perhaps causing similar damage. We can call
this force the reaction force. Every force has an action
on one object and a reaction on the other. The force
on either object can be called the action, making the
other the reaction. The point to remember is that
forces always act in pairs.
Action and reaction forces
This damage must
have been caused
by another object.
All forces act between two (or more) objects, whether
they are in contact or not. You never see something
being forced forwards all by itself; something else
must be present to cause the force.
The damaged car in the photo on the right must
have collided with some object, such as another car
or a pole. We can describe the force of the object on
this car as the action. The object would also have
REMEMBER
1. What is a force?
2. What are the three possible
➡
results of a force acting on
an object?
3. List two types of contact
forces and three types of
non-contact forces.
4. What is buoyancy?
THINK
5. Look at the drawing on the
➡
➡
Water pushes on the diver
from every direction.
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✓ learning
previous page of the student
opening the drink can. We can
call the force applied by the
boy’s hand to the pull-tab the
‘action’. What is the ‘reaction’?
Justify your answer.
6. Air resistance is the force that
results as objects move
through the air. Is air
resistance a contact or a
non-contact force? Explain
your answer.
I CAN:
explain what a force is
list several types of forces
recognise and distinguish
between contact and non-contact
forces.
9. Forces and motion
Representing forces
he skysurfer on page 191 would not fall down to Earth if it weren’t for a force.
In fact, without forces, the aeroplane he jumped from wouldn’t even be in the
sky. Knowledge of everyday forces can often be a matter of life or death for people
who participate in extreme sports. Scientists have helped us understand forces by
creating ways to represent them.
T
Drawing forces
Balanced forces
Scientists use arrows to represent
forces. The direction of the arrow
shows which way the force is
acting. The length of the
arrow shows how big the
force is. A long arrow
represents a larger,
stronger force than a short
arrow.
The arrows that represent forces
acting on an object should be
drawn from the object’s centre of
gravity. All objects,
including your body,
have a centre of gravity.
Picture a point in
your body where
your weight
would be
concentrated if
your body was a
single point. That
point is called
your centre of
gravity.
The arrows describing the up and down forces acting on the kayaker are the same
length. That means that the forces are the same size. But these forces are acting in
opposite directions. The weight of the kayaker and the kayak pushes down, but a
push from the water holds him up. The two forces are balanced and so the kayaker
does not move up or down.
Your centre of gravity
changes with your
position. When
standing, your centre
of gravity is at about
bellybutton height.
Unbalanced forces
The arrows describing the forward and backward forces on the kayaker are not the
same length. The forward force comes from the kayaker using a paddle to push
forward. The backward force is the drag from the water slowing the kayak down.
These forces are unbalanced. The forward force is larger than the backward force,
so the kayaker and his kayak move forward faster and faster. Eventually they
cannot move any faster. This is because drag increases when speed increases.
Eventually these two forces balance out and the kayaker moves at a constant speed.
Balanced and unbalanced
forces
Forces act on us all the time: when
we are moving, when we are
stopped and even when we are
sleeping. More than one force is
acting on us all the time. The
forces acting on us can be
balanced or unbalanced.
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Go to worksheet 9.2:
Balance in sport
The effect of balanced and unbalanced forces
Unbalanced forces can get something moving, like
the kayak in the water. Unbalanced forces can
increase or decrease the speed of the kayak as well.
If the forward and backward forces are balanced,
there is no increase or decrease in speed; the kayak
moves at a steady speed or stays at rest.
REMEMBER
1. How is the size of a force represented in a
diagram?
2. Imagine a moving object. List three things that an
unbalanced force could change about the object
and its motion.
3. What is the name of the force that slows down
movement in water?
Balancing
weights
THINK
4. There are four forces acting on the person in this
diagram.
You will need:
washer
fishing line or strong string
50 g masses
ruler
marker pen
paper
sticky tape
2 pulleys
2 clamps (if the pulleys don’t have them).
• Set up the equipment as shown.
Pulley
Pulley
Washer
in centre
Paper
Clamp
Clamp
Hanging
masses
Hanging
masses
• Use a ruler to find the middle of the string.
Mark this spot on the paper under the string.
• Line up the washer with the central spot marked on
the paper.
(a) Which forces are balanced?
(b) Which forces are unbalanced?
(c) Is the rider’s speed increasing, decreasing or
constant?
5. Redraw the diagram from question 4 without the
force arrows. Instead, draw force arrows that
show the forces acting when the bike rider is
slowing down.
• Hang a 50 g mass from each end of the string.
Support the two masses until they are both secured
to the string.
• Gently let both masses go at the same time.
1. What happens to the position of the washer?
2. Are the forces on the washer balanced or
unbalanced?
• Gently release the masses at the same time.
3. What happens to the position of the washer when
the masses are not equal?
4. Were the forces balanced or unbalanced in this part
of the experiment?
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9. Forces and motion
✓ learning
OBSERVE
6. Choose two objects around you. Use arrows to
draw the forces you think are acting on the
objects. (Hint: You can tell if forces are balanced or
unbalanced by looking at the object’s motion.)
• Repeat the process. Begin with the washer in the
centre of the string. This time, secure one 50 g mass
at one end and two 50 g masses at the other.
I CAN:
explain what a force is
describe balanced and unbalanced
forces
describe the effects of balanced
and unbalanced forces.