Application of Newton’s Laws of Motion

Isaac Newton is one of the most well-known names in physics, and the most significant one when studying ‘Motion’. Newton, who lived from 1642 to 1727, formulated the three laws of motion in the year 1686. 

Newton’s three laws of motion are: 

• Newton’s First Law of Motion: Law of Inertia

• Newton’s Second Law of Motion: Law of Momentum

• Newton’s Third Law of Motion: The Action-Reaction Law.

These three laws determine the behaviour of objects when they are in motion, when they are still, or when some force is applied to them. First published in Newton’s book Principia Mathematica Philosophiae Naturalis, these laws have shaped the foundation of the study of motion in physics, after Galileo Galilei’s findings. 

Newton’s First Law of Motion: Law of Inertia

Inertia is the property of an object to remain unchanged and unaffected unless otherwise affected by an external force. This is the concept that determines Newton’s First Law of Motion: the Law of Inertia. 

In other words, it could be explained as:

• Any object in the position of rest remains at rest. (Here, the application of external force is zero.)

• When an external force is applied to the object, only then does it gain motion, or undergo any change.

• If an object is already in motion at a certain speed in a particular direction, it will continue to be in motion at the same speed in the same direction, unless an external force is applied to the object to change either its direction or speed or both. 

Thus, any object either at rest or in motion remains in its original state until it experiences an application of a net external force.

Application of Newton’s laws of inertia: This can be understood through the following example. The example of the book on the table is one of the most classic examples you will hear in a physics classroom. This is because it explains this concept in the simplest, yet most effective manner possible. 

This example states how a book remains unaffected when lying on a table unless an external force is applied to it to cause movement. 

Newton’s Second Law of Motion: Law of Momentum

Newton’s Second Law of Motion – the Law of Momentum – states that the rate of change of the momentum of an object and the force exerted on the object share a directly proportional relationship. Here, the direction of the net force applied decides the direction of momentum.

This law, also known as the mass-acceleration law, can also be understood in another way. The amount of force applied to the object as well as its mass are important entities that decide its acceleration. The force to be applied to the object can be calculated through the product of its mass and acceleration.

The formula is:

F = m.a

Force = mass.acceleration

Here, a higher force results in a higher acceleration for an object with the same mass.

As an example of an everyday application of Newton’s law of mass and acceleration, let us look at this situation: why is it that lighter objects move with a faster rate of acceleration? It is because of the application of Newton’s laws of mass and acceleration.

A typical example would be: A shopping cart, when empty (that is when the mass is less), requires a smaller force to move it smoothly and fast. However, as it acquires mass by being filled up with objects, it takes a much larger amount of external force to gain the same rate of acceleration as earlier.

Newton’s Third Law of Motion: The Action-Reaction Law

Newton’s Third Law of Motion – the Action-Reaction Law – states the relationship between action and reaction. It says that for every action, there is always an equal and opposite reaction.

For the application of Newton’s laws of motion, let us take an everyday example of this concept. When lying on your bed, there are two kinds of forces being applied. The first force is the one applied by your body on the mattress when you lay down on it. The second force is a reaction mechanism to this force that the mattress pushes back to hold your body up against its surface while adjusting to the shape and increased pressure points.

If these pressures were unequal, for example:

·  If the body applies more pressure on the mattress than the mattress applies on the body, the mattress would deform, or flatten out.

·  If the mattress applies more pressure on the body than the body applies on the mattress, it will act as a hard surface, not taking the shape of the body and retaining its original shape throughout.

Therefore, we can say that the applied forces from both objects are equal.

We can also say that the applied forces are applied in the opposite direction to each other, to create a balance and hold up against each other. The force applied by your body on the mattress is in the downward direction, while the force applied by the mattress on your body is in the upward direction.

This pairing of forces is thus not only equal but also in the opposite direction to each other.

Everyday applications of Newton’s Laws of Motion

Other than the previously mentioned examples of the applications of Newton’s laws of motion, here are some other common everyday examples:

1. An external force like the force of water on the rocky paths in mountainous regions during rainfall can cause the movement of the otherwise still rocks unless they hit either a bump in the path for them to stop, or the force, that is the rain, stops. This can be categorised under the first law of motion – the Law of Inertia.

2. Racing cars are specially designed with a light-weight design. So, the lower the mass, the lesser force required to gain a high acceleration. Therefore, this is an example of the Law of Momentum – the second law of motion.

3. The swimming mechanism involves the water pushing the person forward, while the person pushes back on the water to gain momentum. Here these are both equal and opposite forces, thus the Action-Reaction Law is in work. 

Conclusion

After completing this lesson, you should be well-versed in the following concepts: Newton’s three Laws of Motion; Newton’s First Law of Motion – Law of Inertia; Newton’s Second Law of Motion – Law of Momentum; Newton’s Third Law of Motion – The Action-Reaction Law.