Motion and Rest

Key Concepts

• Motion and rest
• Point and frame of reference
• Types of Motion

Introduction: 

We see many different things around us. Some of them appear to be moving, while others seem to be not moving. For example, in a park, we see people jogging, children playing, swimming, and exercising, birds flying, etc. All these are moving. However, we also see trees, grass, large rocks, which do not move. So, how do we precisely tell a moving object from a non-moving one? 

Explanation: Motion And Rest  

There are two states of motion in which an object can exist: motion or rest.  

“A body is said to be in motion if its position changes with time.” For example, a car moving on a road, a bird flying in the sky, the hands of a clock, the blades of a fan are in motion. It is because their positions change with time.  

On the other hand, “a body is said to be at rest if its position does not change with time.” For example, a tree, a mountain, a building on the ground never change their positions with time. Therefore, they are said to be at rest. 

Point and Frame of Reference 

How do we know if an object changes its position with time? There must be an object or a point, which is fixed from where the distance of the moving body can be measured to conclude whether the object is in motion or at rest. This fixed point is called the reference point. 

“A reference point is a place or an object used for comparison to determine if another object is in motion or at rest.” 

• An object is in motion if it changes position with respect to a reference point. 
• A reference point should be a stationary point. 
• It is the point from which the change in position of an object can be observed. 
• Objects fixed in their positions like a tree, a building, a monument, a milestone, a sign, etc., can be chosen as a reference point. 
• At times it is also considered as the point of observation of the event. 

Example: In the figure given below, for the car moving on the road, the fixed position of objects like the building and the milestone can be the points of reference. However, the moving truck’s position cannot be considered a reference point, as it is not fixed. 

The state of motion of an object can appear different with respect to two different points of reference. For example, in the figure given below, a man is driving a car on the road. Let us consider two reference points, viz., the position of the man standing by the road and the position of the seat next to the man in the car. 

In the former case, the man inside the car seems to be moving along with the car. On the other hand, the man seems to be at rest in the latter case, sitting on the driver’s seat. Therefore, the choice of reference point is crucial while concluding the state of an object. 

A frame of reference comprises the stationary environment/surroundings of the object, whose position is considered the reference point. In figure: 1.2, there are two frames of reference corresponding to the two points of reference considered, viz., the car’s frame of reference and the observer’s (the earth’s) frame of reference. In the car’s frame of reference, the driver is at rest. Whereas, in the observer’s frame of reference, the driver is in motion along with the car. 

Types of Motion 

There are many ways in which things move. For example, the motion of a car in a straight road is different from the motion of the blades of a fan, which is again different from the way a pendulum of a clock moves (to and fro). Broadly there are five types of motions, which are as follows: 

1. Rectilinear motion

2. Rotational motion

3. Circular motion 

4. Periodic motion

5. Random motion 

Type of motion	Description
Rectilinear motion	The motion in a straight line is called rectilinear motion. For example, a truck moving on a straight road, fruit falling from a tree to the ground, motion of a ray of light, etc.
Circular motion	Circular motion is a movement of an object along the circumference of a circle. For example, the earth’s motion around the sun, a person running on a circular track, etc.
Rotational motion	The circular motion of an object about a fixed axis is called rotational motion. For example, the earth’s rotation about its axis, the motion of the blades of a fan and the hands of a clock, etc.
Periodic motion	The motion that repeats itself after equal intervals of time is called periodic motion. It is classified into two types, namely, oscillatory and vibrational motion.  For example, the to and fro motion of a pendulum and a swing are oscillatory motions. Whereas the vibration of a phone and the strings of a guitar are examples of vibratory motion.
Random motion	The motion in which an object moves without following a particular pattern and continuously changes its direction is called random motion. For example, the motion of an ant and the motion of a roller-coaster, etc.

Summary:

• An object is said to be in motion if its position changes with time with respect to the point of reference.
• We require a point of reference to tell whether an object is in motion or not.
• The point of reference is a stationary point from where the object of interest is observed.
• Suppose the same object is observed from two different reference points. In that case, the state of motion of the object may seem different in each case.
• The stationary surrounding of the object is considered the frame of reference.
• There are different types of motions like rectilinear, rotational, circular, periodic, and random motions.