# TOPIC 4: FORCE | PHYSICS FORM 1

### FORCE

Force is a push or a pull which changes a body’s state of rest or uniform motion in a straight line. Force is a vector quantity which has both magnitude and direction.

Pushing or pushing a door with force is an example. Force is a vector quantity, meaning it has both magnitude and direction. Newton’s second law defines force as the “product of a body’s mass and acceleration.”

The SI unit of force is newton (N).

A newton is a force which gives a mass of 1kg an acceleration of 1ms-2.

### Effects of a force

Force on a body has the following effects;

i. It makes a stationary body move when pushed or pulled.

ii. It changes the direction of a moving body.

iii. It deforms or changes the shape of a body.

iv. It makes a moving body move faster or slower.

v. It stops a moving body.

vi. It changes the speed of a moving body.

### Types of forces

The following are types of forces;

#### 1. Frictional force

This is a force that opposes relative motion between two surfaces.

#### 2. Centripetal force

This is the force that keeps a body moving in a circle and is directed towards the centre.

#### 3. Up-thrust force

This is an upward force acting on a body immersed in a fluid.

#### 4. Magnetic force

This is the force that attracts or repels magnetic substances.

#### 5. Electrostatic force

This is a force between two charged bodies.

#### 6. Cohesion force

This is the force of attraction between molecules of the same kind.

This is the force of attraction between molecules of different substances.

#### 8. Viscous drag force

This is the force which opposes motion of body moving in a fluid.

#### 9. Tensional force

This is the pull or compression of the string at both ends.

#### 10. Gravitational force

Note:

Acceleration due to gravity depends on the distance from the centre of the earth and since the earth is not a perfect sphere then acceleration due to gravity is greater at the poles than at the equator because the polar radius is less than the equatorial radius.

### RESULTANT FORCE

The resultant force is a single force which has the same effect as of two or more forces acting on a body.

When two or more forces act on a body there is a resultant force, the resultant force may be greater or equal to zero. When the resultant force on the body is zero the body is said to be in equilibrium.

### 1. Forces acting in a straight line

###### A. Forces acting in the same direction.

Examples:

1. Two forces of 120N and 250N act on body in the same direction. Find the resultant force on the body

= 370N to the left

2. Two forces of 29N and 34N act on a body as shown below. Find the resultant force on the body

##### B. Forces acting in the opposite direction

Examples:

1. Two forces of 900N and 250N act on body in the opposite direction. Find the resultant force on the body

Resultant force, FÂ Â Â Â = 250 â€“ (70 + 90)

= 250 â€“ 160

= 90N to the left

### 2. Forces acting in the opposite direction

Examples:

• Two forces of 900N and 250N act on body in the opposite direction. Find the resultant force on the body

250NÂ Â Â Â 900N

### Non – Fundamental Forces

Are the forces in which the two interacting objects are in physical contact with each other

Examples are:-

(a) Kicking a ball

(b) Air resistance

c. Pulling a door

(d) Tension

(e) Compressing a spring

(f) Friction

(g) Elastic forces etc.

### Effects of Forces

Forces have several effects on objects. These effects include: –

i. Stretching (tensile)

ii. Torsion

iii. Attraction

iv. Friction

v. Compression

vi. Viscosity

vii. Air resistance

viii. Repulsion

#### i. Stretching and Restoring

Stretching occurs when an object increases its length when the force is applied to it.

For some objects there is a tendency to return to their original shape and size. This is called restoring force.

For example, when spring is pulled the stretching force elongates the spring

ii. Compression and Restoring

Compressional force is the force which when applied to an object results in decreasing in its volume.

Example when you compress the spring

Restoring force is the force which causes a body to return to its original shape and size

#### iii. Attractive force

Is the force that pulls objects toward each other.

For example, A Magnet always attracts other objects like iron.

#### iv. Repulsive force

Is the force that pushes objects against each other.

For example, when the same poles of magnets are closer to each other, they repel

The figures below show an example of attractive and repulsive force

#### v. Torsional Force

Is a force produced when a solid matter is twisted

#### vi. Frictional force

Is the force that prevents a body from sliding.

For example, an exercise book cannot slid on top of a table due to friction exists between exercise book and table

#### vii. Viscous force

Viscosity is the resistance of a fluid to flow.

For example, water has a lower viscous force than cooking oil, since it has less resistance to flow

#### viii. Air Resistance

Is the force that resists the movement of an object through the air.

Example of this force is viscosity

#### Factors affecting Air Resistance

a. Size and shape of the body

b. The speed of fluid

c. The density of the fluid

#### Normal Force

Is the force that acts in equal and opposite direction to the weight of a body

#### Applied Force

Is the external force that causes the system or body to change position.

The Writer