Motion is a very important topic for general science portion of Physics. Once you understand this chapter it becomes interesting too. You are required to study the chapter of motion in physics to understand the change in position of a body with respect to time and the equations related to it. In almost major exams of India such as UPSC, BPSC, UPPCS, SSC CGL, Railway RRB etc. questions on motion are asked. Read this post to understand the physics chapter motion and laws of motion.

## Rest and Motion

An object is in rest if its position is not changing with respect to time. And an object is in motion if its position changes with time. Example of an object at rest is a book lying on a table and an example of an object in motion is a bike running on a road.

State of rest or of motion is relative to each other. As for example, a bus running on a road appears to be in motion to a man standing beside a road and the same bus appears to be in rest for the man sitting inside the bus.

## Types of Motion

As per the nature of the movement or path travelled by an object, motion is classified into the following types:

- Rectilinear Motion
- Circular Motion
- Rotational Motion
- Oscillatory Motion

**Rectilinear Motion**: It is a motion in which the object moves in a straight line. For example a car running on its own lane.

**Circular Motion:** It is a motion in which the object travels in a circular path. Motion of the moon orbiting around the earth is an example of a circular motion.

**Rotational Motion**: When an object rotates around an axis, the rotation is known as rotational motion. The axis around which the object rotates is known as the axis of rotation.

**Oscillatory Motion: **When an object moves to and fro about its fixed position then that type of motion is known as an oscillatory motion. An object in such a motion oscillates about an equilibrium position.

## Terms Related to Motion

**Distance:** The length travelled by an object in a given amount of time during the motion is called as length. The value of distance is always positive and can never be negative. Distance is a scalar quantity. Its SI unit is metre (m). Odometer is used to measure distance.

**Displacement:** The shortest distance travelled by an object from start to finish is known as displacement. Its SI unit is also metre (m). It is a vector quantity i.e. it has both magnitude and direction.

**Difference between Distance and Displacement:**

Distance | Displacement |

Total distance travelled by an object. | Shortest path or length between initial and final position. |

It is always positive and can never be negative or zero. | It can be positive, negative or zero. |

Distance depends upon the nature of the path travelled by the object. | Displacement is independent of the path travelled by the object. |

For an identical motion of an object, its displacement is always less than or equal to distance.

A uniform motion is a motion in which an object travels equal distances in equal intervals of time. And a non uniform motion is a motion in which an object covers unequal distances in same or equal intervals of time. Example of a uniform motion is a man covers 1 km in first 15 minutes and another 1 km in next 15 minutes. Example of a non uniform motion is the motion of a man walking in a crowded street.

**Speed:** The distance travelled by an object in unit time is known as the speed of the body. Speed of the body is always positive and never be negative. Its SI unit is m/s or metres per second. Speed is a scalar quantity.

Various types of speed are uniform speed, non uniform speed, Average speed, and instantaneous speed.

Uniform speed is when an object covers equal distances in equal intervals of time. Non uniform or variable speed is when an object covers unequal distances in equal intervals of time.

**Average Speed:** Average speed of an object is the Total distance travelled by the object divided by the total time taken by the object to cover the distance. Avg. Speed = Total Distance Travelled/ Total Time Taken

Instantaneous speed is the speed of the object at a particular moment or instant in time.

**Velocity:** Velocity is often referred to as the directional speed of an object. It can also be defined as the displacement of an object in unit interval of time.

Velocity = distance travelled in a given direction/ Time Taken = Displacement/ Time

Velocity is a vector quantity. Its SI unit is m/s or metres per second.

Velocity can be of three types: Uniform Velocity, Non Uniform Velocity, Average velocity and Instantaneous velocity.

Uniform velocity is the velocity in which an object covers equal displacements in equal intervals of time. **Non uniform** velocity is the velocity in which an object covers unequal displacements in equal intervals of time.

Average Velocity is the ratio of total displacement of the object to the total time taken by the object.

Average Velocity = total displacement/ total time = [initial velocity + final velocity] / 2

Velocity is represented as v.

Speed | Velocity |

The rate of change of position of an object in any direction | The rate of change of displacement of an object in a particular direction. |

Its magnitude can be zero or positive but never be negative. | Its magnitude can be zero, negative or positive. Negative related to direction. |

Speed is a scalar quantity. | Velocity is a vector quantity. |

Average speed is constant throughout the motion. | Average velocity changes with the time interval and cann become zero. |

**Acceleration: **Acceleration of an object is the rate of change of its velocity with time.

Acceleration (a) = Change in Velocity/ Time Taken = (v-u)/t where v is the final velocity and u is the initial velocity.

SI unit of acceleration is m/s^{2} or metres per second square. It is a vector quantity.

When a body is moving in a straight line and its speed is rising over time, the acceleration will be in the same direction as that of speed. It is said that the motion is an accelerated motion. It resembles the motion of a freely falling body.

Retardation is the term for negative acceleration. Its direction is the inverse of the body’s direction of motion. For instance, when a body is thrown vertically upward, the opposing gravity causes it to accelerate downward.

There are again four types of accelerated motions. They are:

- Uniform Acceleration
- Non Uniform Acceleration
- Average Acceleration
- Instantaneous Acceleration

**Equations for Uniform Acceleration of a body**:

- v= u + at
- s= ut + 1/2 at
^{2} - v
^{2}– u^{2}= 2as

where, u = initial velocity, v = final velocity, a= acceleration, t= time, and s= distance

Acceleration due to gravity is termed as g. In scenarios where acceleration due to gravity is considered the new equations will be formed by just replacing a with g. Also, when the object is thrown vertically upwards, then the value of acceleration due to gravity g is -g. If you take g into account then the equations can be framed as:

- v= u + gt
- h= ut + 1/2 gt
^{2} - v
^{2}– u^{2}= 2gh

where, v= final velocity, u is the initial velocity, g is the acceleration due to gravity and h = height of the object.

## Graphs

There are distance time, velocity time and acceleration time graphs through which nature of motion of object is depicted.

## Relative Velocity

The relative velocity of an object refers to its velocity in relation to another object. It is the pace at which one object’s relative position changes in relation to another object over time. It is represented by the symbol Vr. Some example of relative velocity are a boat crossing a river that is flowing at some rate or an aeroplane encountering wind during its motion. In the above example both the objects in first case the boat and the river both are in motion and in the second one aeroplane and air both have some velocity with respect to each other.

If the objects move in the same direction, then relative velocities gets subtracted and if they are moving in opposite directions the velocities gets added.

## Circular Motion

A motion is considered to be circular when it follows the path of a circle, such as when an object rotates or moves around it.

**Uniform circular motion:** Uniform circular motion is defined as the motion of an object following a circular path at a constant speed or velocity.

Speed of the body moving in a circular path is 2πr/t where 2πr is the circumference of the circle and t is the time taken.

## Projectile Motion

Acceleration due to Gravity causes a body that is propelled upward in a direction other than vertical, to move along a curved path in the vertical plane. This kind of motion of the particle is known as projectile motion. The object under consideration is known as a projectile. The trajectory of the projectile is parabolic.

## Motion FAQ

### What is the definition of motion?

Motion is defined as the rate of change of position of a body with respect to time.

### What are the types of motion?

The types of motion are Linear motion, Uniform Motion, Non- uniform motion, Periodic Motion, Circular motion, Oscillatory motion. Rotational motion, Translational motion.