Force and Laws of Motion

July 17, 2020
Basic Science 
Author: Naivedya Moondra

Rest and Motion

We all know what is rest and what is motion. But when we are asked to define them, we just say that when an object is not in motion, it is at rest and vice versa. This gets relative and tends to confuse us. So, that’s why it is so important to define rest and motion. These definitions will help us during this chapter.

Rest

A body is said to be at rest when it does not change its position with respect to time.

Motion

A body is said to be in motion if it is changing its position with respect to time.

Now I hope you have understood the definition. 🙂 Let’s move on.

Force:

Now, we all use the word “Force” a lot in our daily life. Your idea of force may be a bit different from what we define force in physics.

A Force is any interaction between two or more bodies that may bring a change in the state of motion of that object.

For example: When we apply force on an already rolling ball, it moves faster.

The SI unit of Force is Newton (N). 1 N = 1 Kg m/s2

Let’s move on to the properties of forces…

Properties of Forces

  • Force may make a stationary object move.
  • It may stop a moving object.
  • It may make a moving object move faster or slower.
  • Force may change the direction of a moving object.
  • It may change the shape of an object.
  • It is a vector quantity.

Types of Forces:

There are many kinds of force. But on this blog, we will focus on the following types of forces:

  1. Balanced Forces
  2. Unbalanced Forces
  3. Frictional Forces
Balanced Forces:
Balanced Forces: Image Courtesy: Google
Balanced Forces (Image Courtesy: Google)

When two or more forces are applied such that the resultant force produces no acceleration, the force is called a balanced force.

As the name suggests, these forces balance out each other and produce no acceleration.

Note that the minimum amount of forces required to balance the resultant is two.

Unbalanced Forces:
Unbalanced Forces (Image Courtesy: Google)
Unbalanced Forces (Image Courtesy: Google)

When a force is applied to a body such that the resultant force is nonzero produces acceleration, the force is called an unbalanced force.

In this type of force, there is always acceleration.

Any number of forces can be used to create an unbalanced force.

Frictional Force:
Frictional Forces (Image Courtesy: Google)
Frictional Forces (Image Courtesy: Google)

The Force which opposes motion is called frictional force.

In the image above, Ff is the frictional force if we push in the opposite direction with a force of magnitude FA.

Examples: Tyres of vehicles wear out due to friction. We can write on paper due to friction.

There are many more examples of frictional forces in our daily lives.

Let’s move on…

Newton’s First Law of Motion:

Yup, now you’re gonna Newton’s famed laws of motion. The first law of motion states that:

“An object will remain in its state of rest or uniform motion unless acted upon by an external, unbalanced force.”

Newton's First Law of Motion (Image Courtesy: Google)
Newton’s First Law of Motion (Image Courtesy: Google)

Basically, this law tells us that an object will remain in rest or uniform motion in a straight line until an external unbalanced force is put upon it.

Examples:

  • A pen kept on a table doesn’t move on its own. It only moves when somebody tries to move it.
  • A ball in rest starts to move when we kick it.

Simple, right! Here comes the second law…

Newton’s Second Law of Motion:

The second law of motion is very important. It goes like this:

“The force applied on an object is directly proportional to the product of its mass and acceleration.”

So, mathematically:

F ∝ m x a

Therefore,

F = k x m x a

Where:

F is the force applied.

m is the mass of the object

a is the acceleration of the object

k is the constant of proportionality

Note: In this law, the constant of proportionality is 1 if you take the unit of force as Newton (N).

Thus,

F = m x a

Newton’s second law can also be written in terms of momentum (mass x velocity):

“The force applied on an object is directly proportional to its rate of change of momentum.”

So, mathematically:

F ∝ Δ p / Δ t

Therefore,

F = (pf − pi) / t 

Newton’s Third Law of Motion:

This law is the most popular law of motion. Everybody knows this one. You might have guessed by now. Here goes:

“Every action has an equal and opposite reaction.”

So, this law states that if a body A1 applies a force on another body A2, then A2 also applies an equal and opposite force on A1.

Newton's third Law of Motion
Newton’s third Law of Motion

So, according to Newton, if we apply F1 force on the object, the object will also exert force F2 on us, which is equal and opposite to F1­.

Thus,

Magnitude of F1 = Magnitude of F2

and F1 and F2 act in opposite directions.

On this note, I will bid you goodbye. Thank You!