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.

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.

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).

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

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).

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.

Mass is constant everywhere. Weight depends on acceleration due to gravity.

In space, g=0, on the moon g = 1.6m/s2.

exercise

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

FINDING THE RESULTANT FORCE BY CONSTRUCTION

Consider the diagram below:

Draw parallelogram and solve vectors

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.

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

Expt: extension (y) vs load (x)

Show points X and E

p.60

Ex. P.61