Motion

Every object in the universe is either in motion or at rest. You cannot decide which one it is simply by looking at the object on its own. You have to compare it with some point, some place or the surroundings around it. A flying bird, a moving bus and a walking person all change their position compared with the places around them, so they are in motion. A house and a tree do not change their position compared with the ground around them, so they are at rest. Our Earth is in motion too, because it keeps going round the Sun, although those of us standing on it never feel that motion.

When a force is applied to an object that is standing still and the object changes its place, we say the object has been brought into motion. Without the use of an outside force, a moving object cannot be brought to rest, and an object at rest cannot be set moving. A table shifts only when somebody pushes it, and a rolling ball stops only when somebody stops it or when the friction of the ground stops it.

A flying plane, a moving truck, a walking dog and a rolling ball are some examples of motion. As time passes, each of them keeps changing its position with respect to the places around it. To say that an object is in motion is to say that its position keeps changing as time goes on.

The motion of an object is of two kinds, uniform and non-uniform. Which kind it is, is decided by looking at how much distance the object covers in equal intervals of time.

• Uniform motion: the object covers equal distances in equal intervals of time. For example, a train that covers exactly 10 metres in every second.
• Non-uniform motion: the object does not cover equal distances in equal intervals of time. For example, a bus in traffic that moves quickly at one moment and slowly at the next.

A house, a tree and a bus that has stopped are some examples of rest. A tree that is seen at one place in the morning is seen at the same place in the evening. A stopped bus also stays at rest as long as it does not change its position with respect to the road.

One object can be in motion with respect to a second object and, at the very same moment, at rest with respect to a third one. When you travel in a moving bus, the passengers inside are at rest with respect to the bus, because they do not change their seats or their place inside the bus. At the same moment those same passengers are in motion with respect to the road, because their place on the road keeps changing along with the bus. Motion and rest are therefore relative. Whenever you say that something is moving or standing still, you must also say what it is moving or standing still with respect to.

Which runs faster, a leopard or a deer? Some things run fast and some run slowly. When we say that a car moves at 20 metres per second, we mean that the car covers 20 metres in every single second. In the same way, saying that the speed of sound is 332 metres per second means that sound covers 332 metres in one second. Whatever runs fast has a greater speed, and whatever runs slowly has a smaller speed.

Speed is found by dividing the distance covered by the time taken. Below, \( d \) stands for the distance covered, \( t \) for the time taken and \( v \) for the speed.

\[ v = \frac{d}{t} \]

Speed has only a value, that is, a magnitude, and it has no direction. Saying that a car moved at 20 metres per second tells you nothing about which way it went. Speed is therefore a scalar quantity.

Sometimes you need to know not only how fast something went but also which way it went. The distance covered in a definite direction in unit time, that is, in one second, is called velocity. Velocity too is measured in metres per second. Below, \( s \) stands for the distance covered in the definite direction and \( t \) for the total time taken to cover it.

\[ v = \frac{s}{t} \]

When you write an answer for velocity, you must write the direction along with the number and the unit. For example, the cycle moved with a velocity of 12 metres per second towards the east. An answer with no direction in it is an answer for speed, not for velocity.

In everyday talk, speed and velocity may sound like the same thing, but in physics their meanings are different. The comparison below makes the two easy to keep apart.