Higher Education

DESCRIBING MOTION

Introduction

An object is said to be in motion if its position changes with time. Motion can be seen everywhere. From the micro level to the macro level, everything is in motion. Atoms, molecules, the sun, planets, etc.—all are in motion. In some situations, the motion can be clearly seen. For example, birds fly, cats run, etc. In some other situations, motion is inferred through indirect evidence. For example, the movement of air can be observed through the movement of dust and leaves. Do you know the cause of the phenomena of sunrise, sunset, and seasonal change? Sunrise and sunset are caused by the rotation of the earth, while the changing of seasons is due to the revolution of the earth. Now it is clear that the earth is moving. We are on the earth’s surface. Do you feel the motion of the earth? No. It is because when the earth moves, objects on the earth are moving together with the earth. An object seems to appear to be moving for a person. It may be stationary for some others. For example, a man in a moving bus appears to be moving for a boy standing on the roadside. The same man appears to be stationary for a person inside the moving bus. It is now clear that most motions are complex. Different types of motions are:

Linear motion: motion along a straight line
Circular motion: motion along a circular path
Rotational motion is the motion of a particle about a fixed point.
Vibratory motion: the to-and-fro motion of a particle

Let us discuss more about motion. Activity Discuss whether the walls of your classroom are at rest or in motion.

The walls of the classroom are at rest for a stationary object on the earth.
The walls of the classroom are in motion for a moving object.

Activity Have you ever experienced that the train in which you are sitting appears to move while it is at rest? When the train in which you are sitting stopped and you saw another train running on the adjacent track, you would feel that the train in which you were also moving was moving in a direction opposite to that of the other train. Think And Act Erratic and uncontrolled motions observed in a flooded river, a hurricane, or a tsunami, cause serious damage to objects around us. On the other hand, the controlled motion that can be observed in the generation of hydroelectric power is a service to humans. Do you feel the necessity to study the erratic motion of some objects and learn to control them? Erratic motion is a motion in which the particles move randomly. Its direction cannot be specified. It may sometimes go out of control. For example, flooded rivers. They flow in an uncontrolled manner. The direction and power of flow changes due to changes in forces. It will cause damage to buildings, houses, roads, etc. So it is necessary to study and control such erratic motions.

Describing Motion

Reference Point (Origin): This is a point or object used to specify the location of another point or object.

Fig. 1: “The point ‘A’ is 5m north of the point ‘P’. Here point ‘P’ is the reference point”. Fig. 2: "The point ‘B’ is 10m east of the point ‘Q’. So here, ‘Q’ is the reference point”.

Motion along a straight line

Motion along a straight line, also called linear motion, is the simplest type of motion.

Distance and Displacement

Consider a ball rolling over a flat surface. Let the ball rest at a place that is marked O. Take this as a reference point. Now gently push the ball. The ball stops at a point that is marked as A. Mark a position as B through which the ball passes. Again, push the ball, which is at A. It stops at C and moves through B.

Distance

The distance covered by an object is a physical quantity, which is the total path length traveled by the object. Distance can be described using only the numerical value, not the direction of motion. That means distance has only magnitude and no direction. In the above case, Distance traveled by the ball = Total path length covered by the ball = OA + AC = 60 + 35 ( OA – OC = 60 – 25) = 95 km

Displacement

Displacement is a physical quantity that is the shortest distance covered by an object. It has both magnitude and direction. It is the distance between the initial and final positions of the object. In the above case, Displacement of the ball = final position of the ball - initial position of the ball = At C – At O = 25 – 0 = 25 km From this, it is clear that the magnitudes of distance and displacement are not equal. Both of these physical quantities are required to describe the motion of an object. Activity Consider a child standing at one of the corners of a basketball court. He started to walk from that corner to its opposite corner along its sides.

Measure the distance and displacement using a meter scale and a long rope.

Let A be one of the corners of the court and C be the opposite corner. The child starts from A and reaches C through B. Distance covered by the child = AB + BC = 3 + 4 = 7 m displacement = AC =

\sqrt{A B^{2} + B C^{2}}

= $\sqrt{3^{2} + 4^{2}}$

= 5 m

What is the difference between the two in this case?

The distance and magnitude of displacement are not the same.

Distance – Displacement = 7 – 5

= 2 m

Uniform Motion and Non-Uniform Motion

Uniform motion: If an object travels equal distances in equal intervals of time, then the motion is said to be in uniform motion.

Example: revolution of the earth around the sun, revolution of the moon around the earth, etc.

Non-uniform motion: If an object covers unequal distances in equal intervals of time, then the motion is said to be non-uniform motion.

Time	Distance travelled by object A in m	Distance travelled by object B in m
9:30 am	10	12
9:45 am	20	19
10:00 am	30	23
10:15 am	40	35
10:30 am	50	37
10:45 am	60	41
11:00 am	70	44

According to the data given in the table, object A is in uniform motion, and object B is in non-uniform motion.

The table below shows the distance traveled by two different objects, say A and B.

Time	Distance travelled by object A in m	Distance travelled by object B in m
9:30 am	10	12
9:45 am	20	19
10:00 am	30	23
10:15 am	40	35
10:30 am	50	37
10:45 am	60	41
11:00 am	70	44

State whether the motion of objects is uniform or non-uniform.

Let us depict the data in the table using a straight line. Mark the difference between the distance traveled by the objects in two adjacent time intervals.

Object A travels 10 m in every 15 minutes. So Object A travels in uniform motion.

Object B covers unequal distances in each 15 minutes. So object B is in non-uniform motion.

Frequently Asked Questions

Q1: What is motion? What are the different types of motion?

Ans: Motion is the change in an object's position concerning its surroundings in a given period of time. The various types of motion, are uniform motion (constant speed in a straight line), non-uniform motion (changing speed or direction), and circular motion (motion along a circular path).

Q2. What is the difference between distance and displacement?

Ans:

Distance is the total length of the path traveled, regardless of direction.
Displacement is the change in position from the starting point to the ending point, considering direction.

Q3: How are distance and displacement related?

Ans: Displacement is a vector quantity that takes into account both distance and direction. It can be equal to or less than the distance traveled, depending on the path.

Q4: What is Uniform motion? Is the speed constant in uniform motion?

Ans: Uniform motion is when an object covers equal distances in equal intervals of time. It moves at a constant speed in a straight line.

Yes, in uniform motion, the speed remains constant, which means the object doesn't speed up or slow down.

Q5: What is Non-Uniform motion? Can Non-uniform motion involve changes in direction?

Ans: Non-uniform motion is when an object's speed changes over time, either increasing or decreasing.

Yes, non-uniform motion can include both changes in speed and changes in direction, making it a more complex type of motion.