Updated on April 25, 2021 03:36 PM IST by Lakshmi Arya

when an object changes its position with respect to other object along with time. This state of an object is called motion Or change in the position of an object with time is called motion.

In physics there are two states of an object: stationary (at rest) and in motion (moving state). when an object does not change its position with respect to other stationary object, the object is said to be at rest. i.e tree, pillar, building etc. when an object changes its position with respect to other stationary object, the object is said to be in motion. i.e walking, moving car etc.

**Generally motion is of three types:**

- Linear/rectilinear motion-when an object moves along a straight line.
- Oscillatory motion-when an object moves to and fro from its mean position.
- Circular motion- when an object moves in a circular path.

we know that linear motion (motion along straight line) is the simplest type of motion. To describe the position of an object we need to specify a reference point called the origin. The reference (origin) point of an object decides whether the object is in motion or at rest. Whenever an object moves from its reference point it covers a distance in a time interval. The length of actual path travelled by an object is called distance. It is a scalar quantity. It is always positive and cannot be 0(zero).As per SI system it is measured in meter. The distance travelled by an object from its initial position (origin) to its final position is called total distance. Whereas the shortest distance between the initial position (origin) to the final position is called Displacement. It is a vector quantity. It can be positive, negative and 0(zero). It's SI unit is also meter.

**Laws of Motion**

- First Law of Motion:- Any object will remain in its existing state of motion or rest unless a net external force acts on it.
- Second Law of Motion:- If an object has a certain mass, greater the mass of this object, greater will the force required be to accelerate the object. It is represented by the equation F = ma, where ‘F’ is the force on the object, ‘m’ is the mass of the object and ‘a’ is the acceleration of the object.
- Third Law of Motion:- For every action, there is an equal and opposite reaction.

The distance covered by an object per unit time is called Speed. It is a scalar quantity. As per SI system it is measured in meter per second (m/s). In most of the conditions an object usually moves with a non -uniform speed. The speed of such an object is described in terms of average speed in this way:

**Average speed(v) = total distance covered(s) / total time taked(t)**

**Velocity:-- **Speed of an object in a particular direction is called as its velocity. Like speed it is also represented by v. It shows the direction of motion of the object along with its speed. Velocity of an object can be determined by this equation:

velocity(v) = displacement(s) / time(t)

It is a Vector quantity, whose magnitude changes with change in speed and direction. It’s S.I. unit is also metre per second (m/s).It can be Zero (0).e.g if you covers the distance from your home to school and back from school to home by the same route in 10 m, in this situation you will have speed. But here,the displacement is 0(zero).Therefore your velocity will be zero. When an object is moving along a straight line with the changing (variable) speed/velocity, we can calculate the magnitude of its rate of motion in terms of average velocity by the following equation:

**Average velocity (v) = ( initial velocity(u) + final velocity(v) ) / 2**

**Acceleration:- **In a uniform motion there is no change in the velocity of an object with respect to time. But in a non uniform motion like a car running on road in traffic, its velocity keeps on changing with time. We represent the change in velocity with respect to time with another physical quantity known as Acceleration (a).In its formula it is represented by (a). The change in velocity of an object per unit time is called acceleration .

This can be calculated by : -

Acceleration (a) = Change in Velocity / time taken

SI unit of Acceleration (a) is ms^{-2}

- First equation of motion (velocity-time relation) V = u + at
- Second equation of motion (time-position relation) s = ut + (1/2)at
^{2 } - Third equation of motion(velocity-position relation) 2a s = V
^{2 }- u^{2}

**Q.** What is the difference between distance and displacement?

**Q.** Define speed and write its SI unit.

**Q.** If a car travels 100m in 25s, then find out its velocity.

**Q.** When you apply break to the moving bicycle, then what type of motion it has?

**Q.** What is Acceleration ? Explain its formula and SI unit in brief.

**Q.** Explain uniform and non uniform acceleration.

**Q.** A car accelerates uniformly from 5m/s to 10 m/s in 5 s. Calculate its acceleration?

**Q.** An aeroplane attains a velocity of 200m/s in 30 seconds on its track. Calculate its acceleration?

**Q.** State two differences between uniform and non-uniform speed.

**Q.** what is the slope of distance time graph ?

**Q.** What does V, u, s, t and a stands for in equations of motion ?

**Q.** Give three examples of Uniform Circular Motion.

**Q.** Why does a passenger jumping out of a rapidly moving bus fall forward with his face downwards?

**Q.** Why is it difficult for a fireman to hose, which ejects large amount of water at a high velocity?