# Best Dynamics Notes (Forces) | GCE O Level Physics (5054)

In Physics, dynamics refers to forces and their impact on the motion. In dynamics notes, we will learn about the laws of Issac Newton that explain different types and effects of forces.

So, let’s start from the very beginning of this topic without further introductions and learn about force and the terms related to this topic.

## What is a force?

We usually hear and read that force is something that causes an object to change speed, direction and even shape (in some cases) due to push or pull. It is a vector quantity because it has a magnitude and direction.

We can learn about forces in even more depth through the concept of contact and non-contact forces. Let me elaborate this with the help of an example:

### Contact force examples:

1. Friction: It is the phenomenon in which a force resists motion between a surface that is in contact with another surface.
2. Tension: This force is experienced by an object that is being stretched (extended or expanded). For example, when a rope is pulled, it experiences tension force.
3. Reaction: The normal reaction force is when a surface exerts a force on the object that is placed on it such as glass on the table.

### Non-contact force examples:

1. Gravity: This is the force which earth exerts on everything in the universe depending upon the mass of a body. This pull is also related to a term, weight.
2. Electromagnetism: You would have heard the similar charges repel and opposite charges attract (such as positive and negative attract each other).

In the same way, these attractions and repulsion also exist between magnets. This is one of the examples for non-contact forces.

You should also be familiar with a very simple topic which is about the effects of a force. Some of the effects are that force changes direction decreases speed, increases speed and makes a stationary object move.

This was the basic about dynamics and when you know something about the topic, it is time to discuss the three laws of Newton that are related to forces.

What does the Newtons first, second and third law say? It is time to find out!

## Newton’s first law:

This law of Newton is related to balanced forces and the law states that:

The situation of rest or uniform movement (in a straight line) will resume for an object unless a resultant force acts on it.

What does this mean? Let’s understand this in more depth and it is to note that the wording of the laws will be changed in order to ensure better understanding.

Case 1: Let us suppose that a book is placed on a table, what will happen? The book will remain at the exact position unless we apply some force to either change its position or to move it.

Case 2: Similarly, the book will continue to move unless a force acts on it and stops it (such as friction). If you apply force on a book then why does it stops eventually?

The reason is that the friction between the book and table results in that because the book has to face a resistive force. Remember that if the forces acting on an object are equal (resultant force zero) then, the forces are balanced.

This is the first law of Issac Newton about “balanced forces“.

## Newton’s second law:

This law of Newton deals with “unbalanced forces” which means that the resultant force is not going to be zero. Let us first look at the law first and then we will discuss it.

The law states:

On an object of constant mass, the resultant force will make the object accelerate in the direction of the resultant force. This resultant force can be calculated through the product of the mass and acceleration of the object.

In short, the formula which this law is referring to is:

F = ma

Here F is the resultant force, m is the mass and a is the acceleration of the object. Therefore, it be said that:

Resultant force = (Mass) (acceleration)

Now, it is time to explain this law. You should know that when a force is applied on an object, it will accelerate (move in the direction where the force is applied). If you double the resultant force, the acceleration will double but if the mass is double then, the acceleration will be halved.

This means that force and acceleration and directly related while mass and acceleration are inversely related. In other words, the alternation in momentum is related to the force (net force).

Note that if a force of 1 Newton (N) is provided to an object of 1 Kilogram (kg) then, the acceleration produced will be 1 m/s2. This example can help you understand the second law of newton. The questions related to the topic will be mentioned at the end of the article.

This takes us to the third and last law of Newton so let us discuss it.

## The third law of Issac Newton:

The law states that:

If one body (X) applies a force on another body then that body (Y) will also apply a force which will be equal in magnitude but opposite in direction.

This means that if X applies a force of 10 Newtons on Y (towards the right) then, Y will also exert a force of 10 Newtons on X (towards the left). This refers to action and reaction that every action has an equal and opposite reaction.

Let us understand this law with the help of some examples:

• When you jump, your legs exert a force on the ground and the ground applies a force on your legs.
• Similarly, when you run, you apply a backward force on the ground and the ground exerts a forward force on you.
• The force of bat on ball and ball on a bat is also an example.
• The force of a book on the table and of the table on the book also explains Newton’s third law.

Thus, we can learn that forces produce action and reaction that are equal in magnitude, opposite in direction, occur in pairs (action and reaction) and they always occur at the same instant.

Fun Fact: Have you ever applied force on the walls while swimming to give you an initial boost? Congratulations, you have practically applied the third law of Newton.

This is because the rule of action and reaction has been applied (you have applied a force on the wall and the wall has applied a force on you).

What next? It is time to practise some questions regarding the above-mentioned topics. Let us test your understanding.

## Questions:

1. A rope is stretched by two 50 N (Newton) forces that are opposite in direction, what is the resultant force?

A. 15 Newton

B. 25 Newton

C. 50 Newton

D. Zero Newton

1. Every action has a reaction that is equal in magnitude and opposite in direction. This statement is presented by which law of Newton?

A. First law of Newton

B. Second law of Newton

C. Third law of Newton

D. Law of Inertia

1. If a external force is absent, a moving object will:

A. Turn to right

B. Move in a straight line with a constant speed

C. speed up

D. Slow down

1. What is the acceleration produced by a 15 N force that is acting on a mass of 8 kg?

A. 1.30 m/s2

B. 1.57 m/s2

C. 1.87 m/s2

D. 2.03 m/s2

1. A boy throws a football down a tall building. What will be the acceleration of the football immediately who it is released from the hand of the boy (you can neglect air resistance in this question).

A. 1.7 m/s2

B. 5 m/s2

C. 10 m/s2

D. 20 m/s2

1. D

2. C

3. B

4. C

5. C

The topic has almost ended but we have to discuss one more important topic which is friction. You should know that friction is a force that resists an object and there are various advantages (such as cars stop easily) and disadvantages (energy is wasted as it is converted to heat).

The topic of friction was also discussed in the topic of contact forces and you can refer back to that to revise your concepts.

## Conclusion:

With this, the topic about dynamics notes has come to an end. Thank You very much for reading and staying with me till the end.

In the article, we have discussed Newton’s three laws of motion and the concept of friction, contact forces and non-contact forces. Remember to practise the above-mentioned questions so that you ace this topic.

This topic is very important from Multiple Choice Questions (MCQ’s) point of view and several long questions regarding this topic can also appear in your examination.

The three laws of Issac Newton, concept about balanced and unbalanced forces and the advantages and disadvantages of friction are some of the important topics for this chapter, dynamics (forces).

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