What Do We Mean By Motion?

 

What Do We Mean By Motion?

 

As we all know, motion means movement. The motion or movement is one of the vital signs of life in an organism. For example, animals and humans can move their body parts or they can move from one place to another without any external help. The plants are fixed at a place in the soil. They can move parts of their body such as leaves, flowers and branches etc. Again, a bird moves when it flies in the air, a fish moves when it swims in water and so on.

 

Non living things cannot move on their own. We have to apply some external force to move them. For example, a scooter, a car, a bus and a truck move on the road when force is applied on them by their engines.

 

The motion of an object is perceived when its position changes continuously with respect to some stationary object.

 

However, there are situations where motion is inferred through indirect evidence. For example, motion of air is detected by movement of dust particles, leaves and branches of trees. Similarly, the phenomena of sunrise, sunset and changing of seasons are now understood to be due to rotation of the Earth. Atoms, molecules, planets, stars and galaxies are all in motion.

 

Further, an object may appear to be moving to one person and the same object may appear to be at rest to another person. For example, when a bus is moving on a road,

 

(i) the roadside trees appear to be moving backwards to the passengers, (ii) A person standing on the road finds the bus and the passengers moving forward, (ii) A person inside the moving bus finds his fellow passengers at rest.

 

Thus, the states of rest and motion are relative only. For example, our house is at rest with respect to other houses on the Earth. But if we were to look at our house from outside the Earth, it will be seen to share the motion of Earth.

 

Most of the motions are complex. Some objects may move along a straight line, others may follow curved paths, some may rotate and still others may vibrate. In certain cases, we may observe combination of two or more kinds of motion. Sometimes, the motion may be erratic and uncontrolled as observed in a flooded river, a hurricane or a tsunami. These are all dangerous motions, which may cause havoc. In this chapter, we shall confine ourselves to the study of motion of objects along a straight line. We shall use simple equations and graphs to represent such motions. Later on, we shall breifly discuss circular motion.

 

1.2. DESCRIBING MOTION : CONCEPT OF DISTANCE AND DISPLACEMENT

 

We have learnt that the motion of an object is perceived when its position changes continuously with respect to some stationary object. Therefore, in describing motion, we learn first to describe the location of an object. To locate an object, we have to specify or choose some suitable reference point. For example, when we say that our house is 1 km east of the railway station, we choose the railway station as the reference point and location of house is specified with respect to the railway station. The reference point is called the origin.

 

It is not necessary to choose the railway station as the reference point always. We can choose anything else as the reference point, which would serve as the origin. Let us now understand the concept of distance and displacement.

 

The distance travelled by an object is the length of the actual path traversed by the object during motion.

 

For example, suppose an object starts its journey from O, along a straight line OX. A, B, C are the positions of the object at different instants of time. Let the object go from 0 to C and come back to B along the same straight line. The total path length covered by the object = OC + CB = 100 km + (100 – 40) km = 160 km. This is the distance travelled by the object.

 

Thus, to describe the distance, we need to specify only its numerical value and the unit. The direction of motion need not be specified. Therefore, distance is a scalar quantity.

 

Obviously, during motion of the object, the value of distance can never be zero or negative.

 

The displacement of an object in motion is the shortest distance between the initial position and final position of the object.

 

In the example given above, when an object goes from 0 to C and comes back to B, its final position is B and initial position, of course, is 0. Therefore, displacement of the object = OB. Obviously, displacement is a vector quantity, having both the magnitude as well as the direction. In this case, the magnitude of =

 

displacement | OB 1 = 40 km and its direction is from initial position 0 to the final position B.

 

Distinction Between Distance and Displacement

 

(i) Distance is a scalar quantity having magnitude only. Displacement is a vector quantity having both, the magnitude as well as direction.

 

(ii) Both, the distance and displacement have the same units.

 

(iii) The distance travelled by an object in motion can never be zero or negative. The displacement of an object can be positive, zero or negative.

 

(iv) When final position of an object in motion coincides with its initial position, the displacement is zero, but the distance travelled is not zero. The reverse is not true, i.e., when distance travelled is zero, the object may be at rest. Its displacement would also be zero.

 

(v) Between two given positions, i.e., initial position and final position, distance can have any value depending on the actual path followed, but displacement has only one value, i.e., displacement is the shortest distance between the initial position and final position of the object. Displacement does not depend upon the actual path followed by the body.

 

(vi) Corresponding to a given displacement (between initial position and final position), distance travelled may be larger than the displacement. However, when motion from initial position to final position is along a single straight line, distance travelled is equal to magnitude of displacement. In no case, distance travelled can be less than the displacement.

 

(vii) Neither the distance nor the displacement gives any information about the nature of motion followed by the object.

 

So, that’s all for this blog we will meet in next blog with new topic related to Motion.

 

Thank You!!