Introduction

Movement or motion is studied under a branch of physics known as kinematics. In this branch, we study only the “how” of the motion, but do not pay attention to the “why”. On the other hand, when the forces that are causing motion are studied alongside it, the field of physics involved is known as dynamics.

This tutorial discusses the concept of motion and lists Newton’s three laws of motion.

What Is Motion?

An object is said to be in motion if it is changing its position with respect to some fixed point in space. Hence, motion relates to movement. Mathematically, we use quantities like displacement, velocity, or acceleration to describe exactly how fast and in what direction the object is moving in.

It is interesting to note that motion is always defined with respect to a fixed point or origin. In other words, unless we have a frame of reference, we cannot specify whether an object is moving or not. This can be understood physically as well. Imagine you and your friend moving side by side on the road with the same speed. Naturally, it would appear to you that your friend is not moving at all because you are moving alongside him. Hence, the description of motion always requires a reference frame.

Types Of Motion

Motion is widely categorized into the following:

1. Linear motion

This is the simplest type of motion wherein, an object is moving in a straight line. It can change its direction but the path it traces out consists of straight line segments, not curves. Handling motion of this sort is quite easy and does not require too much analysis.

2. Rolling motion

As the name implies, this type of motion is related to the rolling movement of certain objects. A wheel rolling down a slope or a car’s tire rolling on the road are two examples of rolling motion.

3. Circular motion

Once again, the name is self-explanatory. While rolling motion involves an object rolling in itself, circular motion refers to an object moving around a fixed point in a circular path. Interestingly, when a wheel is rolling, all the points on its circumference are performing circular motion around its center.

Laws Of Motion

1. An object at rest or in motion tends to remain so unless it is acted upon by some external force.

You have seen that a spinning top comes to a halt; a ball rolling on the ground stops after some time; all these phenomena occur because of friction. The first law of motion states that if there are no external factors involved, then an object will not change its state of motion. If a rocket is moving in space, it will continue moving on its path until it encounters some external factors like debris or a planet’s gravity. If there were no friction, the spinning top would spin forever, and more interestingly, you wouldn’t be able to walk without slipping!

2. The rate of change of momentum is directly proportional to the applied external force.

Mathematically, we can write it as:

Momentum is a physical quantity that is related to the effect of motion and is mathematically defined as mass times velocity. The second law states that the rate at which momentum changes in time is directly proportional to the external force applied on the object.

For ease of calculations, the SI system of units takes the constant of proportionality in the above relation to be 1 unit. Also, in most cases, the mass of the object does not change with time and thus, we can write:

F=ma

3. To every action, there is an equal and opposite reaction.

This is a very interesting and unique law that we might not be able to see clearly in real life, but without which, physics would cease to function. In simple terms, this law says that if a body applies 10 units of force on an object, the object applies a force of the same magnitude (10 N) in the opposite direction, i.e., on the first body. If you push on the wall, you will feel yourself pushed back. If you hit a ball, you will feel the opposite reaction on the bat. Mathematically,

Linear Momentum

Momentum measures the effect of motion. Imagine a feather moving with a speed of 30 km/h. While this speed is a lot, the feather won’t affect you if it hits you because of its very small mass. 

On the other hand, a very heavy truck or bus can be very dangerous if it hits you even at a speed of just 4 or 5 km/h. Thus, mass plays as much a role in the effect of motion as velocity. This is quantified by momentum, which is mathematically defined as mass times velocity. Since velocity is a vector quantity, momentum is also a vector and we can write:

When a system is isolated, i.e., no external force is acting on it, its total momentum does not change. This also plays into the second law of motion. Since the rate of change of motion is proportional to the external force, momentum should remain constant if the external force is absent.

Examples

• Driving on a highway in a straight line is an example of linear motion.

• Every point on the wings of a ceiling fan performs a circular motion about the fan’s center.

• Earth is constantly performing circular motion around the sun and at the same time, it is also rolling, since it rotates about its own axis.

Applications of Newton’s laws

1. Basic Mechanics

Problems related to the motion of objects can be solved easily via Newton’s laws, especially when the systems aren’t too complex. For instance, Neptune was discovered only because when we studied Uranus, it didn’t seem to be following Newton’s laws.

2. Calculating the motion of planets

Newton is also responsible for positing the theory of gravitation. Scientists use its concepts along with his laws of motion for calculating the motion of planets, which is essential for space travel.

3. General applications

There are a large number of scenarios where Newton’s laws come in handy. For instance, according to the second law, a heavier object requires more force to get accelerated. Thus, automobiles and machines are designed with materials that weigh less so that less energy is spent in making them work.

Summary

Motion is related to movement and in physics, motion arises when a body is changing its position with reference to a fixed reference point. Without a reference point or frame, the concept of motion does not make sense.

Motion is generally classified into linear, rolling and circular types, depending on the path taken by the object. Other classifications like simple harmonic, vibratory, and projectile motion also exist. Motion of any type is governed by three laws proposed by Sir Isaac Newton, which are given below:

1. An object does not change its state of motion or rest unless compelled by an external force.

2. The rate of change of momentum is directly proportional to the applied external force. 

3. To every action, there is always an equal and opposite reaction. 

These laws of motion are universal and interestingly, the first and third laws can be derived from the second law itself.

Frequently Asked Questions

1. If there is no absolute frame of reference, is the motion of every object in the universe undefined?

Even without an absolute reference frame, scientists agree that everything in the universe is in motion since the universe is constantly expanding.

2. Circular motion involves angular momentum and torque. Are the laws of motion valid in such a scenario?

Yes. The laws of motion are always valid. And they can even be slightly modified to suit circular motion by considering angular momentum as the analogue of linear momentum and torque as the analog of force.

3. According to the third law, don’t all forces cancel each other out?

No. The opposite reaction exerted by the second object acts on the first object, not on itself. When you push the wall, the wall pushes you back, not itself.

4. Does an object remain in its state of circular motion due to the first law?

No. Circular motion is performed when centripetal force keeps an object in a circular trajectory. The first law of motion requires the absence of an external force.

5. Do we ever have violations of Newton's laws?

No. However, if our frame of reference itself is being accelerated, then Newton’s laws aren’t directly applicable and instead, we must perform certain modifications.