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FORCES AND MOTION
Forces are what
make things move,
like a push (kicking a
football), or a pull
(train)
THERE ARE DIFFERENT TYPES OF FORCES.....
Some of the forces are more obvious than others and they can be broken down in to two different types:
NON-CONTACT FORCES
GRAVITY - Gravity is a force that pulls objects and people towards the
Earth. It pushes things down wherever they are, whatever their size.
Gravity is the force that gives us weight.
MAGNETIC FORCE - We all have played with magnets and discovered
the invisible force that can either attract or repel another magnet or
object made of iron or steel. It is the Earth’s magnetic field that we track
on a compass.
ELECTRICAL FORCES - Static electricity is an example of an electrical
force. You will all have noticed that when comb your hair repeatedly that the
comb attracts stray strands of hair. Static electricity can cause sparks and
crackles if you take off a jumper made from man-made fibres (find out what
they are).
[1]
FORCES AND MOTION
CONTACT
FORCES
FRICTION - This is the force that resists
things rubbing against each other. It helps
you walk without sliding - except when
you walk on the ice, there is very little
friction and therefore you slip.
AIR RESISTANCE - This force slows things
down when they are moving. Have you
ever put your hand out of a car window
when you are travelling along? If so you will
have felt air resistance
PULL (TENSION) - Have you ever had to
compete in a tug-of-war competition or had
to drag a heavy object? Pulling is a force
that we come across every day
PUSH (APPLIED FORCE) - Again, this is a
force that we are all familiar with. Whether
you are pushing a bike up a hill or simply
shoving a plate across the table - you should
be able to think of lots of examples.
TWIST (SPRING FORCE) - Whether you are
screwing the top off a fizzy drink or turning a
tap on to brush your teeth, spring forces are
ones that you will be familiar with on a daily
basis.
UPTHRUST - This is the force that pushes
things up in the water. It helps boats float
and keeps swimmers from sinking.
(Who am I?)
MASS AND WEIGHT
MASS
WEIGHT
MASS AND WEIGHT
The mass of an object
is the amount of
matter that is in the
object, so mass is
related to how much
stuff there is.
The weight of an
object is the FORCE
OF GRAVITY that acts
on that object, so
weight is related to
the pull of the
Earth.
The mass of an object
(measured in Kg) is
the same no matter
where it islocated in
the universe. The
weight of an object
(measured in Newtons)
will vary depending
where in the universe
it is located.
[2]
BALANCED OR
UNBALANCED
Steady
Speed
BALANCED AND UNBALANCED FORCES
Forces can be described as being balanced or unbalanced.
If the forces on an object are balanced:
• an object that is not moving will remain still
or
• an object that is moving continues to move at the same speed and in the
same direction
If the forces on an object are unbalanced:
• an object that is not moving starts to move
or
• an object that is already moving changes speed or direction
UNBALANCED FORCES CAUSE CHANGES IN SPEED OR DIRECTION
Activity:
Have a look at the images in the panel down the side of the page. Name the
forces illustrated in each image and decide whether the forces are either
balanced or unbalanced.
MEASURING FORCES
Force is measured in Newtons. A force of one Newton will give a mass of one
kilogram, and an acceleration of one metre per second. This means that each
second, its speed will increase by one metre per second. When measuring a
force, you must state its direction, otherwise you do not know what effect it
has.
We can show the forces acting on an object using a force diagram. In a force
diagram, each force is shown as a force arrow. The arrow shows the size of
the force (the longer the arrow the bigger the force) and also the direction in
which the force acts.
What is the resultant force?
Standing Still
[3]
FORCES AND MOTION ACTIVITIES
If you drop an object it will fall to the floor due to the force of gravity. If you place the same
object on a a table it will not fall because the table produces an equal force upwards. However, if
WRITE THE NAME OF THE
FORCES IN THE BOXES:
you place the object on a weak paper support, the support crumples and the object will fall to
the floor because the force created by the paper platform is not as strong as the force of gravity.
So, when the object is on the table, the forces are balanced and when the object is on the paper
platform the forces are unbalanced (therefore the object moves/changes speed in the direction
of the bigger force).
Activity:
Push a model car along a table, so that it slows down and eventually stops. What forces are
acting on the car as it moves and stops? What are the opposing forces acting on the model car?
When are the forces balanced and unbalanced?
By reducing air resistance and friction forces that act on a car (i.e. stream lining), what effects will
these changes have on the speed, the amount of force required to accelerate the car, and fuel
consumption of the car?
Design activity:
Cars which create less friction use less petrol and are therefore less polluting.
Design and draw a car, labeling and describing features you have changed to reduce air
resistance.
DRAW AND LABEL YOUR
OWN ARROWS:
FORCES IN ACTION
Look at the images in the 4 boxes at the side of the page and label them appropriately.
Draw your own pictures in these two boxes showing examples of the following forces:
• Gravity
• Air Resistance
• Pull
• Upthrust
• Friction
• Push
SPEED
Speed is calculated by the number of metres
travelled per second.
Speed (metres per second) = distance (metres) x
time (seconds)
1.
Calculate the speed of a car which travels
600m in 30 seconds
2.
Calculate the distance travelled by a horse
which runs 1 Kilometre in 4 minutes
3.
Calculate the time it takes a man running
5m/s to run 450 m
[4]
SECOND LAW
NEWTON’S LAWS OF
MOTION
THIRD LAW
FIRST LAW
History
Apple Incident
Isaac Newton (1642-1727) was an English physicist,
It is said that Newton was inspired to write his theory of gravitation
mathematician and astronomer who many people consider to be the
after an apple fell from a tree and hit him on the head. This incident
greatest scientist of all time. In his book, Principia, published in 1687,
made him think about why the apple fell to Earth rather than fly off in
he described his 3 laws of motion which have dominated scientific
another direction. He concluded that:
study for over 300 years. English poet Alexander Pope wrote
‘the earth draws it...’ and that ‘...the sum of the drawing
a very famous epitaph for Newton after his death:
power must be in the Earth’s centre’.
“Nature and nature’s laws hid in night; God said ‘let
3 Laws of Motion
Newton be’ and all was light.”
FIRST LAW OF MOTION
Einstein kept a picture of Newton in his study.
An object continues in its state of rest or uniform
Gravity on the Moon
motion in a straight line unless acted upon by a net
Gravity is the force that gives us weight. Everything
external force
SECOND LAW OF MOTION
contains a certain amount of material called its mass. When
you measure your weight you are really measuring the force of the
When a net (or unbalanced) force acts on a body, the body is
Earths gravity pulling on your mass. On the Moon your weight would be
accelerated in the direction of the force
about one-sixth of your weight on Earth because the Moon’s gravity is
THIRD LAW OF MOTION
about one-sixth of the Earth’s.
For every action there is an equal and opposite reaction
FIRST LAW OF MOTION
GRAVITY
TERMINAL VELOCITY
This law is also known as
the law of inertia. If
forces acting on an object
are balanced, then:
- if it is still, it will
stay still
- if it is moving, it keeps
moving at a constant speed
and in a straight line
When something falls through the
air it feels friction due to air
resistance. Because of this, the
object will reach a terminal
velocity (maximum speed). This is
the velocity at which the air
resistance balances its weight and
so it doesn’t speed up but falls
at a constant velocity.
[5]
30 MINUTE STEM ACTIVITY
WHY NOT TRY THIS FUN ROCKET EXPERIMENT
IN YOUR SCIENCE LESSON OR STEM CLUB.
Most rockets create thrust, the force that
propels them forward, by an exothermic chemical
reaction.
Make sure you launch this rocket outside and
wear safety goggles.
It’s very messy and can fly a long way!
TRY THIS AT
SCHOOL
WHAT YOU NEED:
. One plastic specimen tube with
a tight push fit lid
. One soluble ‘Alka-Seltzer’
CAUTION
. Water
If it does not launch, wait at least 1
minute before examining your rocket. Often
the lid might not be on tight enough and the
CO2 gas has escaped.
. Safety Goggles
. Test tube rack
HOW DOES IT WORK?
The water dissolves the tablet and starts a
chemical reaction which produces carbon
dioxide.
The carbon dioxide starts to build up, which
increases the pressure inside the tube,
until eventually the cap is forced off and
the rocket blasts up. This principle of
thrust is exactly how real rockets work,
only they use rocket fuel instead of AlkaSeltzer tablets.
WHAT TO DO:
Set up the equipment outdoors.
Wear safety goggles.
1) Break the Alka-Seltzer tablet
into 8 pieces
2) Put one piece of the tablet
into the plastic test tube
FURTHER INVESTIGATION
3) Add 1 or 2 teaspoons of water
(5-10ml)
How do the following changes affect the time
it takes the rocket to fire and the amount
of thrust created?
DO THE NEXT STEP VERY QUICKLY
4) Put the lid on the test tube
Try using a greater and smaller amount of
tablet.
5) Put the test tube into the test
tube rack upside down(Lid on the
bottom)
Try breaking the tablet into smaller pieces
to increase the surface area.
6) Stand back 5 METRES.
Try using baking powder and vinegar rather
than water and Alka- Seltzer.
About 10 seconds later you will
hear a large pop and your tube will
launch in the air (up to 10 metres)
See if you can get the rocket to go
higher by adding fins to the bottom of
the tube to increase stability.
[6]
HOW DOES A JET
ENGINE WORK?
Remember Newton’s Third Law of Motion?
Jet engines create tremendous thrust and can propel
aeroplanes with such force that they can achieve
speeds in excess of 1000 MPH
Jet engines (or gas turbine as they are sometimes
called) all work in the same way. The engine sucks
air into the front with a fan. A compressor raises
the pressure and temperature of the air. The
compressed air is then sprayed with fuel and an
electric spark lights the mixture. The burning gases
expand and blast out through the nozzle at the back
of the engine, thrusting the aircraft forward.
Front of a
turbofan
engine
Not all of the air sucked in at the front of the
engine goes through the engine. Some of the air
flows around the core. This ‘cold’ air passes
through a low pressure compressor and is mixed with
the gas generator exhaust to produce a ‘hot’ jet.
The objective of this sort of bypass
system is to increase thrust without
increasing fuel consumption.
Another benefit is that it produces a
much quieter jet engine.
PARTS OF A TURBOFAN JET ENGINE
FAN - The large fan at the front of the engine sucks in large amounts of air. The air is speeded up and
split into 2 parts. One part goes through the core or centre of the engine. The second part goes through
ducts which surround the core, to the back of the engine where it produces much of the force that propels
the aircraft forward. This cooler air quietens the engine as well as add thrust.
COMPRESSOR - This is the first part of the engine core. The fans in the compressor squeeze the air into
smaller areas, resulting in an increase in the air pressure. This squashed air is forced into the
combustion chamber.
COMBUSTOR - The squashed air is mixed with fuel then ignited. As many as 20 nozzles spray fuel into the
airstream. As the mixture catches fire, this produces a high temperature, high energy airflow. The heat
can reach 2700 degrees
TURBINE - The high energy airflow coming out of the combustor goes into the turbine which causes the
turbine blades to rotate. These blades are linked to a shaft which drives both the fan at the front of
the engine and the compressor immediately behind it.
MIXER - This is where the ‘cold’ air that has flowed around the outside of the core and the ‘hot’ air
from inside the core are mixed together before being ducted to the nozzle.
NOZZLE - The nozzle is the exhaust duct of the engine. This is the part of the engine where the thrust
is created. The combination of the ‘hot’ and ‘cold’ air are expelled and produce an exhaust, which
causes forward thrust.