Contents are appropriate for IGCSE Physics

2.  A force is a pull, push or twist exerted by one object
on another.
 It is measured in Newton (N),
 It has magnitude and direction. A force is a vector
quantity.
 Small forces can be measured using a spring balance.
 The greater the force, the more the spring is stretched.
 Forces can be in such a way that they produce motion
or do not produce motion.
 Forces can also change shapes of objects.

3. Newton’s Laws of Motion
 Sir Isaac Newton studied forces and came up with 3 laws:
1. Newton’s first law of motion
This law states that:
“… a body continues to be in its state of uniform motion
unless an external force is applied.”
This means that:
i. A stationary body will forever be stationary unless an external
force is applied.
ii. A body moving at a constant speed in a straight line
continues to move with the constant speed unless a force is
applied.

4. This law is also called the law of inertia. It can be stated as:
“… a body always resists a change in motion unless an
external force is applied.”
If a body is moving, it will always move.
For example:
 When a bus starts moving, passengers are pushed by their
seats.
 When a bus stops quickly, passengers tend to be pushed
forward.
 Inertia is related to mass. The larger the mass, the larger
the inertia.
 A big truck is harder to stop compared to a small car. This
is because of inertia (mass).

5. 2. Newton’s second law of motion
When a force is applied on an object, it can
produce motion (acceleration).
The larger the force, the greater the
acceleration.
Newton’s second law states that:
“… force is directly proportional to
acceleration.”
Derive the formula
Exercise

6. 3. Newton’s third law of motion
 This law states that:
“… to every force, there is an equal but opposite force
reacting.”
 It can also be stated as:
“… to every action, there is an equal but opposite reaction.”
 Examples include:
1. When sitting on a chair, your weight acts downward and the chair
pushes you up.
2. Rocket and Jets: the engine pushes a huge amount of gas very
quickly in one direction and the body moves in the opposite
direction.
3. When a free-falling object reaches terminal velocity, its weight
acts downwards and air resistance upwards. The two forces are
equal and motion is constant (constant speed downwards).

7. Gravity
 If you hang an object from a spring balance, it is pulled
downwards towards the Earth.
 The force that pulls objects towards the Earth is called
Gravitational Force.
 Gravitational force is called Weight. It is given by:
Weight (N) = mass (kg) x acceleration due to gravity (m/s2)
W = mg
 On Earth, acceleration due to gravity is 9.8m/s2. it is a
constant.
 In calculations, g = 10m/s2. It is the same as 10N/kg.

8.  Mass is constant everywhere. Weight depends on
acceleration due to gravity.
 In space, g=0, on the moon g = 1.6m/s2.
exercise

9. Resultant force
 Two vectors acting at a point can be replaced by a
single vector with the same effect.
 In the diagram above, the effect produced in two
diagrams is the same.
 The two forces 15N and 5N produce the same effect as
10N. Therefore, 10N is the Resultant force.
15N 5N
= 1oN

10. FINDING THE RESULTANT FORCE BY
CONSTRUCTION
Consider the diagram below:
Draw parallelogram and solve vectors

11. Circular Motion
 The Moon goes round the Earth and the Earth round the Sun.
Their paths are circular.
 As a body goes round a circular path, there are two forces that act
on it:
i. Centripetal force:
This is a force that pulls the object towards the center of the
circle.
i. Centrifugal force:
This force pulls the object away from the center.
• For a vehicle moving round a bend, the centripetal force is
provided by the tires and centrifugal force by inertia (mass).
• If the centripetal force is greater than the centrifugal force, the
body moves towards the center. If the centrifugal force is
greater than the centripetal force, the object moves away from
the center.

12. Hooke’s law
 In 1660, Robert Hooke investigated how springs and wires
stretch when loads are applied.
 He found that the extension and load were in proportion
up to a certain point called Elastic limit.
 For example; if 5N produces an extension of 0.2cm, the 10N
would produce 0.4cm, 15N produces 0.6cm and so on.
Once the elastic limit has been exceeded, a load increment
of 5N increases the length of the spring by more than
0.2cm.
 Hooke’s law states that:
“… the load is directly proportional to the extension of
an elastic material provided the elastic limit is not
exceeded.”
Derive f=kx

13. Expt: extension (y) vs load (x)
Show points X and E
p.60
Ex. P.61