Newton’s Laws Of Motion

Laws of physics are universal. They help you to understand why everything around us follows a pattern. In this article let’s make an attempt to understand how things move? What makes them stop? Why do the rest? What governs all of these?

Imagine you are sitting on a cozy sofa, watching your favorite movie on a lazy Sunday, and your mother calls you and asks you to shop for groceries. You can imagine how difficult it is for you to get up and do the work. Now, assume you are practicing for a 100-meter race on the track and somebody asks you to stop immediately within a second? Will you be able to do it? What if you are asked to turn towards right? Will you be able to turn immediately? The answer to both the question would be “NO”. You can stop, Yes but gradually. You can also turn but not sharply! And guess what? Basically, it’s the same story with every object.

Every object will continue to do what it is doing. If it is at rest, it will remain at rest. If it is in motion it will stay in motion. What can change this? Yes, only an unbalanced force can change this. If it is at rest and an unbalanced force acts on it, it will move. If it is in motion and an unbalanced force acts on it, it may accelerate it, or change direction. This is nothing but Newton’s First law of motion.  It states that “An object at rest continues to be at rest, an object which is at motion continues to be in motion with the same direction and same speed. Unless acted upon by an unbalanced force”

In simpler words, it means, Object continues to do what they are doing unless some external unbalanced force acts on them. Follow the chart given below for a simple visualization.

Laws Of Motion

This tendency of an object to resist change in its state of motion is called Inertia. Thus, Newton’s first law of motion is also called as Law of Inertia.

Do you think all objects have the same inertia? All objects resist changes to the state of motion, right? So, all object should offer the same inertia correct? Well, Not really! Because Inertia tells us How strong an object’s tendency is to resist change in its state of motion. For simple understanding purpose, imagine there is a small brick kept on the table, another relatively larger and heavier brick is kept next to small brick, which one you think would be easier to push? Without a doubt, it is tiny brick correct? Yes, why is that? Because Heavier brick has the tendency to resist motion more. The heavier the object… The more is its tendency to resist the change in motion. More mass an object has, Greater will be its inertia. So, if we apply equal force to both these bricks, the tiny brick will move further as compared to the larger brick. The heavier brick has more inertia.

How can we relate Inertia and mass? Inertia is the tendency of an object to resist changes to its state of motion and Mass is the measure of its inertia.

What happens when there is ‘net force’ acting on an object? This is precisely what the Second law speaks. The forces are said to be balanced when the Net force acting on an object is zero. If the forces are unbalanced there will be acceleration. Let me ask you a question, on which two factors do you think acceleration will depend on?

Let’s say there is an object on the table. You apply a force of 10 units on it and then you apply a force of 20 units on it. In which you think the acceleration will be more? Of Course, the one in which the force is more. So, we can say one of the things on which the acceleration depends on is the Net force. If it is more, more will be acceleration, if it is less, then less will be acceleration. Acceleration depends directly on the Net Force. Now let us think about the next quantity on which the acceleration will depend on.

Let’s say there are two objects on the table. One has a mass of 5 kg and another has a mass of 10 kg. Say, if you apply a Net Force of 10 units to each of them. Which one do you think will accelerate more? Surely, it’s the one 5kg one, because it has less mass. Lesser the mass more is the acceleration. More is the mass, less is the acceleration. Hence, we can say acceleration depends inversely on the object’s mass. If you understand this concept, then understanding the Second law will be very easy.

Newton’s second law states that ‘“Acceleration is directly proportional to the net force applied and inversely proportional to the mass of the object”
a\propto \frac{F_{net}}{m}

It is popularly written in the form F_{net}=ma

We can also understand this as Net force is the product of mass and acceleration. With this can you write the unit of force? Yes, that should be easy. The standard unit of mass in Kg and that of acceleration is m/s2 Thus, the unit of force is kgm/s2 and this is what we call newton or N.

Derivation of physics formulas is crucial to understand the subject with good depth. The above-mentioned formula F=ma can be derived easily. The force(F) acting on the body is directly proportional to change in its velocity, assuming its mass(m) to be constant. Thus, we can write

F\propto m\frac{v_{f}-v_{i}}{t}

Where vf is the final velocity and vi is the initial velocity.

We know that change in velocity with respect to time is acceleration. Thus,


F\propto ma

On removing proportionality symbol we get-


Where k is called the constant of proportionality and its value in SI Unit is 1

Thus we arrive at F=ma

Now you know what force an object can carry.

Now let us know what happens when two objects interact with each other. Imagine a ball is placed on the table. The ball exerts a force on the table. In return, the table exerts an upward force on the ball, there are two forces in this interaction, A force on the table and the force on the object. The magnitude of both the forces are the same but the directions are opposite. These forces are called action and reaction forces. The force exerted by the ball on the table is action force and the force exerted by the table on the ball is the reaction force. This scenario is the display of Newton’s third law of motion.

It states that “Every action has an equal and opposite reaction”. It means that when two objects interact, there is a force on each object. The magnitude of the force on the first object equals the magnitude of the force on the other object. Here the directions of the forces are opposite. For example, while a frog is swimming, it pushes the water back and water pushes its body forward. Another perfect example is when a bullet is fired from the gun, the Gun exerts a force on the bullet and bullet exerts an equal and opposite reaction force on the gun this results in recoiling of the gun.

To conclude, now we have understood how laws of physics play behind every motion. Click the link below for interactive video lecture on laws of motion.