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The Physics behind Motion
We are all moving since our birth, and we see a lot of things moving around us. Be it a living organism or a non living object. Although we are used to it, it is important to understand the reason behind motion to better utilise its cause and effect.
It's basic among scientists that we gotta explore and try to understand the minute details and basics, because these are the understandings and observations that lead us to new inventions for the betterment of our world.
Motion
An object is said to be in motion if it changes its position with respect to a fixed position.
That was just the formal definition of motion in most high school books. But you gotta understand what it actually is.
Motion is always relative. We always compare an object with something to make our perspective of it, that is - is it moving or standing still. And that something depends on our frame of reference.
Let’s understand this practically, imagine that you are inside a train. What do you observe there? You can see other passengers inside the train, reading books or passing their times and they look like they are not moving at all, standing still. And when you look outside through the window of the train, you can see the trees, poles, sign boards and the whole world outside the train moving, going backward.
But you know that, in reality, you are moving since you are inside the train and the train is moving for your destination. But what would happen if you don’t know that you are inside of the train and suddenly when you look outside through the window, the entire world is in motion for you.
For you, you are standing still from your frame of reference, while the entire world is moving for you. That’s what happens if we don’t understand the importance of the relation between two different frames of references.
What do you mean by Frame of Reference?
It is a coordinate system or some other system by which we determine the properties of an object like its position, velocity, acceleration, etcetera.
I hope, by now you have understood the meaning and importance of the frame of reference. Now we are getting a bit deeper into it.
We can classify frame of reference into two categories:
- Inertial frame of reference
- Non inertial frame of reference
Inertial frame of reference follows Newton's laws of motion while Non inertial frame of reference does not. Also the same goes for the three equations of motion.
Actually, non inertial frames are just the inertial frames in acceleration. Let’s understand the difference between Inertial and Non inertial frames clearly through the illustrations given below.
Here we are always talking about the system inside the truck, so think of the whole truck as your whole world, or as a frame of reference.
The above image shows a non moving truck, inside the truck there is a pendulum hanging from the top, it’s at rest and only the gravitational force is acting on it, that’s why it’s not moving or tilted by any angle other than 90 degrees.
And this image is taken when the truck is moving, we can clearly see that the pendulum is tilted by some angle theta in the opposite direction in which the truck is going. But do you think if a person is standing beside the pendulum inside the moving truck, he will be able to tell why the pendulum is tilting backwards.
This phenomena is not what is supposed to happen in an inertial frame, there is an extra force acting on the pendulum, tilting it backwards. This hidden force is known as pseudo force. This force is used to balance Newton's laws of motion in a non inertial frame.
The thing is if you observe the system from within the non inertial frame itself, then you will get confused with the pseudo force thing. On the other hand, observing this non-inertial frame itself from outside will make all the terms clear.
Distance
It is the length of the actual path covered by an object during motion between its starting point and destination. It’s a scalar quantity, that’s why it doesn’t have a direction component.
Displacement
It is the shortest distance between the starting point and the destination. It is a vector quantity, it has a direction component.
Speed
Speed is the distance covered by an object per unit time.
Speed = Distance travelled / Time taken.
I think you guys know about these terms already, but still going stepwise is a good idea to get the complete picture. Now onto the types of speed.
- Uniform speed
- Non uniform speed
- Average speed
- Instantaneous speed
Uniform speed is when an object covers equal distances in equal intervals of time.
Non uniform speed is when an object covers unequal distances in equal intervals of time.
Average speed = Total distance travelled / Total time taken.
Let’s understand the average speed a little more. Suppose a car moves at speed 20 mph for 30 miles and covers next 30 miles at speed 40 mph, the total distance covered would be 60 miles and the total time taken would be 1.5 hours and 0.75 hours, respectively. So, according to the formula, the average speed would be 60 miles divided by (1.5 + 0.75) hours, which is 26.67 mph.
This means that if the car moves with a constant speed of 26.67 for the entire distance of 60 miles, without stopping anywhere in the middle, it would reach the destination taking exactly the same amount of time.
Instantaneous speed is the speed of an object at any given instant. It may seem to be confusing if you have encountered the "instantaneous" term for the first time, but it is related to calculus and it’s very easy to understand.
One can also say that instantaneous speed is just the average speed with a very small time interval i.e, total time taken. Also, if we take smaller time intervals, the distance travelled would also be smaller.
The time interval can be very small from a second, half a second, or even smaller. The smaller the time interval, the better grasp we will have about the speed of any object at any instance.
Velocity
Velocity is the rate of change of displacement of an object with respect to time. It’s a vector quantity, which implies it has a direction component. That’s why it has displacement instead of distance, otherwise it is quite similar to the speed.
Velocity = Displacement / Time taken
Similar to speed, velocity can also be categorised into four different types.
- Uniform velocity
- Non uniform velocity
- Average velocity
- Instantaneous velocity
There is just a slight difference between the definitions of various kinds of speeds and velocities, velocity is speed with an additional component which is direction.
Uniform velocity is when a body travels equal displacements in equal intervals of time.
Non uniform velocity is when a body travels unequal displacements in equal intervals of time.
Average velocity is the uniform velocity with which a body would move to reach the destination in the exact same time, as with combined non uniform velocities.
It is calculated as the ratio of total displacement and total time taken.
Instantaneous velocity is the velocity of any given body at an instant of time interval.
Acceleration
Acceleration is defined as the rate of change of velocity with respect to time.
Acceleration = Change in velocity / Time taken
Types of acceleration are as follows.
- Uniform acceleration
- Non uniform acceleration
- Average acceleration
- Instantaneous acceleration
Uniform acceleration is when a body changes its velocity by equal amounts in equal intervals of time.
Non uniform acceleration is when a body changes its velocity by unequal amounts in equal intervals of time.
Average acceleration is the acceleration by which a body reaches its desired acceleration at the end of the total time interval. It is the ratio where the numerator is the sum of all the uniform accelerations multiplied by the time interval during which they are activated and the denominator is the sum of all the time intervals i.e, total time taken (only the ones when the body is accelerating).
I hope you don’t get confused by what we have just gone through.
Instantaneous acceleration is the acceleration of a body at any instant of time.
Next time, we should go through “One dimensional motion”, but before that you should recap the topic “Units and measurements”, which I have discussed in Physical quantities.
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