 # Motion in Physics General Knowledge

Motion in Physics General Knowledge for UPSC, IAS, Railway, Banking, SSC, CGL, MTS and other competitive govt. job examinations. Motion and its equations are very important and I am sure some of questions from this topic will come in above exams. So read this topic carefully.

Mechanics is a branch of physics. In this branch of physics, we deal with the motion of an object.

Contents:

## What is motion in physics?

When an object changes its position with respect to time, we can say, the object is in motion. Or in other words, as time goes on, the body or object moves from one particular position to another.

### Type of motion:

Mechanical motion can be classified into two types, –

Translational or Liner Motion: In Translational motion an object moves linearly. Example: If a car moves on road, its motion is translational.

Rotational Motion: In rotational motion, an object rotates or spins about a fixed point or an axis. Example: Moving of top about its axis, moving earth around sun etc are example of rotational motion.

To explain or understand the motion of an object we need to introduce some physical quantities, like distance, displacement, speed, velocity, acceleration, momentum etc are required to explain the linear motion. And angular velocity, angular acceleration etc are required to explain rotational motion. Note that for both kind of motion, time is necessary. Now let us define each physical quantity briefly.

Distance: Distance is the length of actual path covered by an object which is in motion in some interval of time. Distance is a scalar quantity as it has no particular direction.

Let us consider an object moves 5 m along north, then it goes to 6 meter along east and again goes to 8 m along south. The total distance covered by the object is (5+6+8) m = 19 m.

Displacement: Displacement is defined as the shortest distance covered by a moving object in a particular direction in a given interval of time. Displacement is a vector quantity as it has a particular direction i,e displacement take into the direction also. If an object goes 3 m in north (AB) and then 4 m in east (BC), then its Displacement will be the distance from starting point to end point i,e shortest distance between two points. In this case the distance is simply (3+4) m = 7 m, but its displacement will be √(32+42) = 5 m along AC. Both distance and displacement have same unit i.e. meter in SI system. Another important thing to remember is that the displacement may be positive, negative or zero, but distance is always positive quantity. Thus magnitude of displacement ≤ distance as usual.

Speed: Speed is defined as the distance travel by an object in an unit interval of time. Speed is a scalar quantity as distance is. Generally speed is denoted by ‘s’. In SI system, unit of speed is meter/second.

`Thus Speed = (distance)/(time) or s = d/t.`

Suppose an object move 50 m distance in 10 second. Its speed is 50/10 = 5 m/s

Velocity: Displacement per unit time is called velocity. Velocity is a vector quantity, because displacement is vector ( If we divide or multiply a vector by scalar we get vector). Unit of velocity is same as speed i.e meter/second (m/s). Usually velocity is denoted by ‘v‘. (Bold text v to denote vector).

`Thus, Velocity = (Displacement)/ Time. Or v=D/t`

From the above figure displacement is 5 m along AC. Now this displacement covered by the object in 2 second, then its velocity will be (5/2) = 2.5 m/s.

Acceleration: The rate of change of velocity i,e velocity change per unit time is called acceleration. Acceleration is a vector quantity. Its SI unit is meter/(second)2.  Thus

`Acceleration = Velocity/time. or a=v/t.`

Now consider an example. Initial velocity (v1) of a car is 4 m/salong north. After 5 second it final velocity (v2) is 19 m/s2. Then,

Acceleration a = (v2 – v1)/5 = (19-4)/5 = 15/5 = 3 m/salong north.

Acceleration due to gravity: One of the most familiar acceleration is due to gravity. If we drop some object from a height it does not fall with a uniform velocity. Initially its velocity is zero. As long as it goes to downward its velocity continuously or uniformly increases. This is because earth exerts gravitational force on the object and subsequently its velocity increases uniformly producing acceleration. The magnitude of gravitational acceleration (g) of earth is 9.8 m/s2.

## Equations of Motion (Linear):

Equations of motion are very useful in solving problems. Let displacement, initial velocity, final velocity, acceleration and time are denoted by S, v, u, a and t respectively, then the following equations of motion are very important to solve problems.

`Equation-1>` v = u + at

`Equation-2>` S = ut + (1/2)*at2

`Equation-3>` v2 = u2 + 2aS.

## Circular Motion:

When an object move around a point in circular path, its motion is called circular motion. In circular motion, the direction of object changes continuously constitutes an acceleration. Thus circular motion always has acceleration. If the object moves circularly with constant speed it constitute an angular velocity, which is always a vector quantity. If it rotates anticlockwise, direction of angular velocity is upward.

Angular velocity: When an object rotates it always subtended an angle ‘θ’. The rate of change of angle with respect to time is called angular velocity. It is denoted by ‘ω’. `Thus Angular velocity = angle/time or ω = θ/t.`

Time Period: Time Period is defined as the time taken to complete one revolution or one oscillation. It is denoted by ‘T’. It is scalar quantity.

Frequency: Number of complete oscillation or complete revolution per unit time is called frequency. It is denoted by ‘ν’ and SI unit of frequency is Hertz (Hz).

Relation between time period and frequency:

`T = 1/ν`