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Force and motion are fundamental and integral parts of our daily lives.

When we see an acrobat flawlessly performing her act at the circus (Figure 1), have we ever thought how she can do that? It requires lots of hard work and right amount of force to do that.

A force is a pushing or pulling action that can make things move, change direction, or change shape (Figure 2). Understanding how motion works was one of the great discoveries of science by Sir Isaac Newton.

Balanced force keeps the motion at the steady sate, whereas unbalanced forces changes the motion of objects. These changes can be predicted and described.

This course covers position, reference point, distance, displacement, force, motion, and acceleration.

To understand force and motion, we first must understand concepts, such as position and reference point.

## Position, Reference point, Distance and Displacement

Position is the location of an object. It is a single point. Position of an object is its distance and direction from another point, known as a reference point. It is a vector quantity.

A reference point of any object is to which another object’s location is compared. It is used as a comparative point which helps to analyze motion of a body. This is because the motion is relative to a subject.

Distance is a scalar quantity that describes the length of the total path covered by a moving object.

Displacement (∆d) is the change in the object’s position. It can be defined as distance between the initial point and final point of an object.

It is a vector quantity, and thus has both magnitude and direction. Distance traveled by an object can be greater than its displacement (Figure 3).

Table showing difference between distance and displacement

 Distance Displacement Definition Length of actual path traveled by an object irrespective of the direction It is the shortest distance between the initial and final position of an object in a  direction Quantity Scalar Vector Value Distance can be positive or zero. It can never be negative Displacement can be positive, negative and zero Measurement Reference point is not used for the measurement Displacement is measured using a reference point Property Distance does not decrease with time Displacement decreases with time SI Unit Meters Meters Path Not Unique Displacement between two locations, is the unique path Symbol d s Formula Speed × Time Velocity × Time

## Force

### What is a force? Do we experience force in everyday life?

Force is a pushing or pulling action that make things move, change direction, or change shape. Force is a vector quantity, which means it has both magnitude and direction.

Galileo Galilei and Sir Isaac Newton described how force works mathematically. A force is usually denoted by the symbol F.

An example is an equation from Newton’s second law (Figure 4). The SI unit of force is the Newton (N).

F = m·a

where, F = force, m = mass, and a = acceleration

## Types of Forces

In nature there are several types of forces. Following are the examples of types of forces in nature.

### Frictional

It is the force exerted by a surface as an object moves across it or tries to move across it (Figure 5).

There are at least two types of friction force – sliding and static friction. It works in the opposite direction of the main force.

The force of friction on a resting object, called static friction, and that on a moving object, called sliding friction.

### Gravitational

A force caused by a large body, such as the Earth. Gravity pulls objects toward the Earth with an acceleration of “g” which equals 9.8 m/s2.

The magnitude of the gravitational force objects exert on each other depends on the masses of the objects and the inverse of the square of the distance between them (Figure 6).

G = m1 m2/r2

### Electromagnetic

Electromagnetic force is a force associated with electric and magnetic fields. Electricity exerts a force by the attraction or repulsion of charged particles, whereas magnetic force is due to the attraction or repulsion caused by magnetic poles (Figure 7).

### Nuclear

Nuclear force is a force that hold atoms and their particles together. It is one of the four basic forces in nature.

### Tension

A pulling force that is exerted by a string, rope, cable, or chain on another object when it is pulled tight by forces acting from opposite ends (Figure 8).