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Interactions Ch 1 - Forces
Effects of a force
Learning outcomes:

Identify a force as a push or a pull

Show an understanding of the effects of a force:
- A force can move a stationary object
- A force can speed up, slow down or change the direction of motion
- A force can stop a moving object
- A force may change the shape of objects
A force is a push or a pull. It is any action which changes or tends to change the state of rest
or motion of a body. Actions such as lifting, squeezing, throwing, tugging, kicking or twisting
involve forces. We cannot see a force but we can observe its effects. A force can change the
shape or size of a body. It can make a stationary object start moving or a moving object
change direction, go faster (accelerate), slower (decelerate) or stop.
A force is a vector quantity. This means that it has both magnitude and direction. The
magnitude of a force tells us the size of the force and it can be measured using a forcemeter
or spring balance. A force is usually represented by an arrow with the arrow head indicating
its direction and the length of the arrow representing its magnitude.
F2 = 18 Newtons
F1 = -20 Newtons
Net force = -2 Newtons
Two unequal and opposite forces acting on an object
The SI unit of force is the newton (N), named after Sir Isaac Newton, who studied the
relationship between forces and motion and formulated the Laws of motion. One newton is
the force required to give a mass of 1 kg an acceleration of 1 m/s2.
According to Newton’s first law of motion, ‘Every body stays in its state of rest or uniform
motion in a straight line unless it is compelled to change that state by forces acting on it.’ This
is commonly known as the Law of inertia. Therefore, an object at rest will only start to move
when there is a force acting on it to make it move. The speed of a falling object increases as it
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falls towards Earth because gravitational force is acting on it. A rolling ball slows down and
stops after some time because frictional force is acting against it.
Different types of forces
Learning outcomes:

Recognise and give examples of the different types of forces:
- Magnetic force
- Gravitational force
- Elastic spring force
- Frictional force

Recognise that objects have weight because of the gravitational force between them and
the Earth

Investigate the effect of friction on the motion of objects and communicate findings

Investigate the effects of forces on springs and communicate findings
Magnetic force
Magnetic force is the force of attraction (pull) or repulsion (push) exerted by magnets.
Magnetic force can act at a distance. The magnetic field of a magnet is the space around the
magnet where its magnetic force has effects on magnetic materials and other magnets. The
pattern of the magnetic field lines of a magnet can be observed by sprinkling a thin layer of
iron filings around the magnet. The iron filings will align themselves according to the magnetic
field lines.
Magnets exert forces of attraction on magnetic materials like iron, steel, nickel and cobalt.
These materials are known as ferromagnetic materials. The individual atoms of ferromagnetic
materials act like very tiny magnets (dipoles) which are grouped together in clusters called
domains. In the unmagnetised state, the dipoles in each domain line up in a certain direction
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but the dipoles in different domains are not aligned. As a result, the material does not show
any polarity. In the magnetised state, the dipoles in different domains are aligned.
Domains before magnetisation
Domains after magnetisation
The poles of a magnet are the parts of a magnet where the magnetic force appears to be
concentrated. The poles of a bar magnet are near its ends. Like poles of magnets exert a
force of repulsion on each other, while unlike poles exert a force of attraction on each other.
Gravitational force (Gravity)
Gravitational force is the force of attraction between two bodies. The effect of gravitational
force is not noticeable between two bodies with small masses. However, it is easily observed
if one or both masses are great. We can observe, for example, the effects of gravitational
force between the Sun and a planet in the solar system, or between the Earth and objects
near the Earth. The gravitational force between the Earth and an object near the Earth’s
surface pulls the object towards the centre of the Earth. The gravitational force between the
Earth and the Moon pulls the Moon around the Earth, while the gravitational force between
the Sun and the planets in the solar system pulls the planets around the Sun.
Objects on Earth have weight because of the gravitational force between the objects and the
Earth. The terms “weight” and “mass” are often used interchangeably in our daily life.
However, in science, weight is different from mass. Weight is a force and its SI unit is newton
(N). Mass is the amount of matter and its SI unit is kilogram (kg). The mass of an object does
not change when the location changes. The weight of an object changes depending on the
gravitational force present in the location. A mass of 1 kg, for example, has a weight of about
10 N on Earth but its weight on the Moon is about 1.7 N because the Moon’s gravity is only
about 1/6 that of the Earth’s.
The weight of an object can be calculated using Newton’s second law of motion which can be
expressed as follows:
‘For a body with a constant mass (m) and acceleration (a), the force (F) acting on it is equal to
the product of its mass and the acceleration. That is, F = ma.’
The weight (W) of a body on Earth is the product of its mass (m) and its acceleration due to
the Earth’s gravity (g), which is about 10 m/s2. (W = mg)
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Elastic spring force
An object is said to be elastic if it is able to go back to its original shape and size after being
pulled or pushed. When a spring is stretched (pulled), its length increases. When the
stretched spring is released, it returns to its original length as long as the force applied does
not stretch it beyond its elastic limit.
Newton’s third law of motion states that ‘For every action, there is an equal and opposite
reaction.’ If a force is applied on a spring to stretch or compress it, the spring will exert an
equal force on whatever is stretching or compressing it. The equal and opposite force exerted
by the spring is the elastic spring force.
Robert Hooke discovered that the extension of a spring (x) is proportional to the stretching
force (load) or the tension in the spring as long as the spring is not stretched beyond its
elastic limit. This means that to stretch a spring twice as far, twice as much force is needed.
Hooke’s law can be expressed as:
F = -kx where k is the numerical constant for the material
Force-extension graph
Stretching force
Elastic limit
This is reached when the
graph line starts to curve
Extension
Frictional force (Friction)
Frictional force is the force which opposes motion. It slows down and stops motion. Friction
exists between two surfaces in contact with each other. There is friction between two rough
surfaces because these surfaces have projections which can catch against each other. When
an object moves through air, there is friction between the object and the air. This friction is
called air resistance. Aeroplanes have streamlined bodies to reduce the effect of air
resistance. When an object moves through water, it experiences friction between the water
and its body. This water resistance slows down its motion. Most fish have a streamlined
shape to reduce the effect of water resistance. When a hovercraft is moving over water, there
is a cushion of air between the bottom of the hovercraft and the water surface to reduce the
water resistance.
Friction is useful in our daily life. It enables us to walk or write without slipping. Without
friction, vehicles will not be able to slow down and come to a stop. Friction is also used to
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start fires. However, friction can cause wear and tear and produce unwanted heat in the
moving parts of machines. This reduces the efficiency of machines. Friction between surfaces
can be reduced by using lubricants such as oil or grease, or by using ball bearings. In
magnetic levitation (maglev) trains, magnetic force of repulsion is used to lift the train slightly
above the train track when the train is moving. This reduces friction between the train and the
track, making the train ride faster, smoother and quieter!
Internet links

Interactive website with introduction to different types of forces and their effects
http://www.zephyrus.co.uk/forces1.html

BBC website with activity, quiz and revision on forces
http://www.bbc.co.uk/schools/ks2bitesize/science/activities/forces_action.shtml

BBC website with activity, quiz and revision on friction
http://www.bbc.co.uk/schools/scienceclips/ages/8_9/friction.shtml

PBS interactive website which shows how forces affect structures like bridges
http://www.pbs.org/wgbh/buildingbig/lab/forces.html

National Geographic website on forces of nature (earthquakes, hurricanes, tornadoes and
volcanoes)
http://www.nationalgeographic.com/forcesofnature/
Projects/Field trips

Make a scrapbook (electronic or otherwise) on forces in nature, such as the forces of
earthquakes, hurricanes, tornadoes and volcanoes). Newspaper clippings may be
included to show the effects of these forces.
Recommended reading

“The Way Science Works” by Robin Kerrod and Dr Sharon Ann Holgate (Publisher: DK in
association with the Science Museum). Discover science in action from the principles that
explain the world around us to the theories behind today’s fast-moving technology.
Hands-on projects and beautiful visuals.

Eyewitness Books “Force and Motion” by Peter Lafferty (Publisher: DK). See, learn and
discover about forces and motion.
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