Newton's First Law of Motion

 Introduction 

Before Galileo and Newton, many people thought objects slowed down because they had a natural built in tendency to do so. They ignored the invisible forces—friction, gravity, and air resistance—here on Earth that cause objects to change their velocity. 

In deep interstellar space, we could observe the natural tendencies of an object's motion free from any external influences (like forces on earth). There we would observe what Sir Isaac Newton rightly stated as his first law of motion that an object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force (net force should not be 0).

Inertia

In short, Newton’s first law states that an object tends to do what it’s doing: it resists a change in its state of motion or rest. This property is nothing but inertia. Thus Newton's first law is also called the law of inertia. An example to explain this is as follows -:

Suppose that you filled a bowl to the rim with water and walked fast around an oval track. The water would spill:

• At the start

• At the end

• Around a curve

The water has a tendency to resist a change in its state of motion. The container was moved from rest to a high speed at the start while the water remained at rest and spilled (It's as if the container went ahead and the water was left behind). The container was stopped at the end while the water kept moving and spilled over the container's edge. The container was forced to move in a different direction to make it around a curve, while the water kept moving in the same direction and spilled over its edge. 

Inertia is commonly experienced when riding in cars. Consider for instance the unfortunate collision of a car with a wall. Upon contact with the wall, an unbalanced force acts upon the car to abruptly decelerate it to rest. Any passengers in the car will also be decelerated to rest if they are wearing seat belts. This is because, being strapped tightly to the car, the passengers share the same state of motion as the car - as the car decelerates, the passengers do too.

But what would happen if the passengers were not wearing the seat belt? The passengers would no longer share the same state of motion as the car. They would continue in motion. The passengers would likely be propelled from the car and be hurled into the air. The law of inertia has at least convinced us to wear seatbelts!

The inertia of an object is directly proportional to its mass. A more massive object has a greater tendency to resist changes in its state of motion. In other words, it is more difficult to move a heavier body from rest or to stop its motion. 

It is difficult (a larger force is required) to set a loaded trolley (which has more mass and offers larger resistance) in motion than an unloaded trolley (which has less mass and offers smaller resistance). Similarly, it is more difficult to stop a loaded trolley than an unloaded one, if both are moving initially with the same velocity. 

When we say inertia, it is not only the tendency to resist change in state of motion or rest but also change in direction. In other words, we can say inertia is the tendency to resist change in velocity (motion, per unit time, with direction). Since acceleration is the change in velocity (per unit time), inertia is also the tendency to resist acceleration. 

Inertia of rest is the tendency of a body to resist change in its state of rest. For example a bullet fired against a window glass makes a clear hole in the glass without cracking it. This is because the surface surrounding the place where the bullet strikes, cannot share the fast motion of the bullet and remains at rest due to inertia. 

Inertia of motion is the tendency of a body to resist change in its state of motion along a straight line. For example when a bus or train stops suddenly, the passenger inside bends forward. This is because the feet of the person, being in contact with the body of the bus, come to rest along with the bus, but the upper body is still in forward motion due to inertia of motion. 

Inertia of direction is the tendency of a body to resist change in its direction of motion. For example a stone tied to one end of a string, revolving in a circle, flies tangentially when the string breaks. This is because while revolving in a circle, at any point, the direction of the stone is along the tangent drawn at that point. When the string breaks, the stone doesn't continue circular motion but moves in the same direction (along the tangent) due to inertia.  

There is one small but important aspect of Newton’s first law of motion that we haven’t talked about yet - unbalanced force. Consider a physics book at rest on a table. The Earth's gravitational pull exerts a downward force. The push of the table on the book (normal force) exerts an upward force. 

Since these two forces are of equal magnitude and in opposite directions, they balance each other. The book is said to be at equilibrium. There is no unbalanced force acting upon the book and thus the book maintains its state of motion. It will not accelerate. 

Now consider a book sliding from left to right across a tabletop. The book is in motion and at the moment there is no force pushing it to the right (a force isn’t needed to keep a body in motion rather force does the opposite. It’s inertia that keeps a body in motion). The force of gravity and the force of the table balance each other. 

Yet there is no force present to balance the force of friction. As the book moves to the right, friction acts opposite - to the left. There is an unbalanced force; and as such, the book changes its state of motion. The book is not at equilibrium and subsequently accelerates. Unbalanced forces cause accelerations. In this case, the unbalanced force will cause it to slow down. 

An object is said to be acted upon by an unbalanced force only when there is an individual force that is not being balanced by a force of equal magnitude and in the opposite direction.

To summarise the read, I’ll restate Newton’s first law of motion - an object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force. This law is also known as the law of inertia - you can’t resist it! It has numerous applications - your coffee continuing a swirling motion even after you stop stirring, your cycle not stopping immediately after you stop pedaling and so many more small things that are part of our daily life…

Check your understanding with a few quick questions-: 

Q A 2-kg object is moving horizontally with a speed of 4 m/s. How much net force is required to keep the object moving at this speed and in this direction?

Q  Ben is being chased by an enormous bull. Yet, if he makes a zigzag pattern, he will be able to use the large mass of the bull to his own advantage. Explain this in terms of inertia and Newton's first law of motion.

Q If the forces acting upon an object are balanced, then the object

a. must not be moving.

b. must be moving with a constant velocity.

c. must not be accelerating.

d. none of these