Motion: Definition, Types, Terms, Equations, and Laws

Motion in Physics is defined as the change of orientation or position of a body with a time change. All the things in our universe perform the movement from a starting point. Suppose you and your friend are sitting in a moving car, then your position with respect to your friend is not changing.

Motion is explained by displacement, distance, velocity, acceleration, speed, and time. Let us learn more about types of Motion and the Laws in the article below. 

What is Motion?

When the position or orientation of an object changes with respect to a reference frame, with time, then we say that the object is in motion.
For example, a boy starts from his house at time \(t = 1\;{\rm{min}}\) and after time \(t = 15\;{\rm{min}},\) he reached his school. In this example, we can see that the boy is changing his position with respect to time, so we can say that the boy is in motion during this time duration.

Note: A reference point is an object or place that we assume is stationary or fixed, and we compare the object’s position in motion with respect to this point. For example, suppose a pole located at the platform is taken as the reference point, and the distance of the moving train from this fixed pole at different time intervals will represent a change in its position.

Modes of Transport

There are different modes of transport that are used based on the situation, like:

Air:
Aircraft, Helicopter, Hot Air Balloon

Land: 

• Road: Bikes, Bus, Car, Trucks
• Railways: Trains, Trams, Metros

Water: 
Ship, Sailboats

Other Modes: 
Cable cars, Space Crafts, UAVs, Pipelines

Types of Motion

There are different types of motion based on the path taken by the object. These are as follows:

1. Translatory Motion: all points of an object moving uniformly in the same direction is called translatory motion. For example, a bus moving and a man walking. Here the distance covered by all points of the object will be the same. There are two types of translatory motion, namely, rectilinear and curvilinear motion.

(i) Rectilinear Motion: If a body moves along a straight-line path, it is said to be in rectilinear motion. For example, the motion of a car along a straight path. Light also follows a rectilinear path.

(ii) Curvilinear Motion: If an object moves along a curved path, it is said to be in curvilinear motion. For example, the motion of a basketball thrown towards the basket and the movement of a snake.

2. Rotational Motion: When an object moves in a circular path around a fixed axis. For example, the movement of the earth around its axis and merry-go-round.

3. Periodic Motion: The motion that repeats itself after a fixed interval of time. For example, the movement of a simple pendulum and the hand of a clock.

4. Circular Motion: An object along a circular path is called circular motion. The distance of the object from the centre of the circular path remains constant. For example, a man sitting on a giant wheel and the revolution of the moon around the earth.

5. Oscillatory Motion: An object is said to execute oscillatory motion when it moves to and fro about its mean position. For example, the bob of a simple pendulum and a swing.

Terms Related to Motion

When we see any object in motion, many of us observe the kind of motion executed by it, and sometimes we try to assume the rate of change in the object’s position. There are some commonly used terms that we use to relate to the motion of the object.

1. Distance: The distance covered by an object is defined as the total path length travelled by the object. It is a scalar quantity. Its SI unit is meter \(\left( {\text{m}} \right)\) .

2. Displacement: It is the shortest distance between the final and initial position of the object undergoing motion. It is a vector quantity. Its SI unit is meter \(\left( {\text{m}} \right)\) Suppose an object follows a curved path to reach point \(B\) from \(A.\) Then the distance travelled by the object will be the length of the curved path, but the displacement of the object will be given by the shortest length between points \(A\) and \(B.\)

3. Speed: The speed of an object tells how fast or slow an object is moving. It is defined as the distance covered by an object in a unit of time. Let the distance covered by the object is \(d\) and the time taken by the object to cover this distance is \(t\), then the speed of the object is given by \(S = \frac{d}{t}\). The speed of an object is measured in meter per second \(\left( {{\rm{m}}\,{{\rm{s}}^{{\rm{ – 1}}}}} \right){\rm{.}}\)

4. Velocity: It is defined as the rate at which an object changes its position with respect to a frame of reference or the displacement per unit time. It is a vector quantity, and it can be measured in meter per second \(\left( {{\rm{m}}\,{{\rm{s}}^{{\rm{ – 1}}}}} \right){\rm{.}}\)

5. Acceleration: It is defined as the rate of change of velocity. If the initial velocity of the object is \(u\), final velocity of the object is \(v\) and \(t\) is the time taken, then acceleration of the object is given by a \(a = \frac{{\left( {v – u} \right)}}{t}.\) It is a vector quantity, and its SI unit is \(\left( {{\rm{m}}\,{{\rm{s}}^{{\rm{ – 2}}}}} \right){\rm{.}}\)

6. Position Vector: A vector that indicates the position of a point in a coordinate system is referred to as a Position Vector.
Assuming we have a fixed reference point \(O\), we can specify the position of a given point \(P\) with respect to point \(O\) using a vector whose magnitude and direction are represented by a directed line segment \(OP\). This vector is known as the position vector.

Equations of Motion

The three equations of motion give the relationship between the initial velocity \(\left(u \right)\) of the object, the final velocity \(\left(v \right)\) of the object, distance \(\left(s \right)\) covered by an object, time taken \(\left(t \right)\) and the acceleration \(\left(a \right)\) attained by the object undergoing constant accelerated motion.

1. \(v = u + at\)
2. \(s = ut + \frac{1}{2}a {t^2}\)
3. \({v^2} – {u^2} = 2as\)

Laws of Motion

Sir Isaac Newton studied the ideas of Galileo that objects move with a constant speed when no force acts on them, and he gave three fundamental laws that the motion of an object follows. Newton’s laws of motion are:

1. Newton’s First Law: An object remains in a state of rest or of uniform motion in a straight line unless compelled to change that state by an applied force. This law is also known as the law of inertia. The word inertia refers to the tendency of an object to resist any change in its existing state of rest or motion.

2. Newton’s Second Law: This law states that the rate of change of momentum of an object is directly proportional to the applied unbalanced force in the direction of the force. Let ({\rm{m}}) is the mass of the moving body, (v) and (u) be the final and initial velocities, respectively, and (F) is the applied force, then we get

\(F \propto \frac{{\left({mv – mu} \right)}}{t}\)
\(F = kma\)
\(F = ma\)

Where \(k\) is the constant of proportionality whose value is one, \(a\) is the acceleration of the body.

3. Newton’s Third Law: This law states that every action has an equal and opposite reaction. For example, if the weight \(\left( {{F_1}} \right)\) of an object exerts a force on the ground, then the ground will also exert an equal and opposite force \(\left( {{F_2}} \right)\) on the object.

Solved Examples of Motion

Q.1. A car accelerates uniformly from \(18\,{\text{km}}\,{{\text{h}}^{ – 1}}\) to \(36\,{\text{km}}\,{{\text{h}}^{ – 1}}\) in \(5\,{\text{s}}\) .Calculate the acceleration of the car.
Sol: Given, the initial velocity of car, \(u {\rm{ = 18\;km\;}}{{\rm{h}}^{{\rm{ – 1}}}}{\rm{ = 5\;m\; }}{{\rm{s}}^{{\rm{ – 1}}}}\)
The final velocity of car, \(v = {\rm{36}}{\mkern 1mu} \,{\rm{km}}\,{\mkern 1mu} {{\rm{h}}^{{\rm{ – 1}}}}{\rm{ = 10}}\,{\mkern 1mu} {\rm{m}}\,{{\rm{s}}^{{\rm{ – 1}}}}\)
Time taken \(t = 5\,{\text{s}}\)
By applying the first equation of motion, we get
\(v = u + at\)
\( \Rightarrow a = \frac{{v – u}}{t}\)
\( \Rightarrow a = \frac{{10 – 5}}{5} = 1{\mkern 1mu} {\rm{m}}\,{{\rm{s}}^{ – 1}}\)

Q.2. A ball is gently dropped from a height of \(20\,{\text{m}}\). If its velocity increases uniformly at the rate of \({\rm{10}}\,{\rm{m}}\,{{\rm{s}}^{ – 2}}\), with what velocity will it strike the ground? After what time will it strike the ground?
Sol: Given, initial velocity as the ball is dropped, \(u = 0\)
The height from where the ball is dropped, \(h = 20\,{\text{m}}\)
Acceleration due to gravity, \(a = g = 10\,{\mkern 1mu} {\rm{m}}\,{{\rm{s}}^{{\rm{ – 2}}}}\)
The velocity with which it strikes the ground \( = v\)
Let the time taken in striking the ground be \(t\)
Using the third equation of motion under gravity, we will get
\({v^2} = {u^2} + 2gh\)
\( \Rightarrow {v^2} = 0 + 2 \times 10 \times 20\)
\( \Rightarrow v = 20{\mkern 1mu} \,{\rm{m}}\,{{\rm{s}}^{{\rm{ – 1}}}}\)
To calculate the time of fall, we can use the first equation of motion under gravity.
\(v = u + gt\)
\( \Rightarrow 20 = \,0 + 10t\)
\( \Rightarrow t = 2\,{\text{s}}\)

Summary

From this article, we can conclude that whenever the position of an object changes with respect to time, the object is said to be moving or in motion. Motion can be classified into different types depending upon the path taken by the object. There are seven types of motion, namely, rectilinear, curvilinear, periodic, circular, rotational, oscillatory, and vibratory motions. All objects continue to be in their state of rest or of uniform motion along a straight line unless compelled to change their state by an externally applied force.

Rectilinear, Curvilinear, Periodic, Circular, Rotational, Oscillatory, and Vibratory

Related Topics to Motion

FAQs on Motion

We have listed out some of the commonly asked questions related to motion:

Q.1: What are the \(7\) types of motion?
Ans: The \(7\)  types of motion depending upon the path taken by the object are given below.
(a) Rectilinear
(b) Curvilinear
(c) Periodic
(d) Circular
(e) Rotational
(f) Oscillatory
(g) Vibratory

Q.2: What is meant by motion?
Ans: Motion is the state of an object when it changes its position with respect to time or with respect to a reference point.

Q.3: What are motion and time? How it is related to time?
Ans: When an object changes its position with respect to a reference point with time, it is said to be in motion. Time is the duration of the ongoing event. Time is the measure of the interval between two events. The change in the position of an object with respect to its surrounding always requires time to spend.

Q.4: What is rectilinear motion?
Ans: When an object moves along a straight line is called rectilinear motion. For example, the motion of aly falling ball from a certain height.

Q.5: What is translatory motion?
Ans: When all points of an object move uniformly in the same direction is called translatory motion. For example, the motion of a car running on the road.