Laws of Motion are the foundation of classical mechanics in physics given by the famous English physicist Sir Isaac Newton. These laws describe the behavior of moving objects and how they interact with forces.
What are Newton’s Laws of Motion?
Newton’s Laws of Motion in physics are the fundamental laws which describe the relationship between the motion of an object and the forces acting on it. There are three laws under Newton’s Law of Motion:
- First Law of Motion
- Second Law of Motion
- Third Law of Motion
Newton’s Laws of Motion
Newton’s First Law of Motion (Law of Inertia)
An object continues to be in its state ( Rest/Motion) unless and until an external force greater than its inertia is applied to the object.
- Newton’s First Law of Motion also called the Law of Inertia. It is one of the fundamental laws of physics that deals with the inertia of any object.
- For example, Suppose we see a man standing on a running bus and if the driver applies the brake then the man leans forward because his upper body is in a state of motion and no extra force is applied to it, and the man tends to lean forward.
If the total (net) force acting on an object is zero, the object will either stay at rest or keep moving in a straight line at a constant speed.
Inertia
Inertia is the property of an object that describes its resistance to any change in its state of motion. Inertia is directly related to mass: the greater the mass of an object, the greater its inertia. For example:
- A heavy truck is harder to start moving and harder to stop once it’s moving compared to a small car, because the truck has more mass and, therefore, more inertia.
Inertia is a key concept in Newton’s First Law of Motion (the law of inertia), which states that an object will remain in its current state (whether at rest or in uniform motion) unless acted upon by an external force.
Newton’s First Law of Motion Examples
Various examples of Newton’s First Law of Motion observed are,
Example 1: Applying brakes to the car, the passengers lean forward as the body want to continue its motion due to the presence of inertia. Inertia tries to keep the passenger moving.
Example 2: A book lying on a table will remain at rest unless an external force (like someone picking it up) acts on it. Without a push or pull, it stays still due to its inertia.
Example 3: A rolling football eventually stops due to the friction force on the ground. ( Change in State: Motion to rest , Force Applied: Friction)
Example 4: If the branches of the tree are shaken vigorously, the fruits attached to it fall down as they continue to be in the state of rest and hence the sudden motion detaches the fruits from the branches and falls on the ground.
Example 5: A satellite orbiting Earth will continue moving at the same speed and in the same direction unless a force, such as gravity from another body or a propulsion system, acts on it. Since there’s no air resistance in space, the satellite continues its motion due to inertia.
Newton’s Second Law of Motion
Newton’s Second Law of Motion states that,
The rate of change of momentum of an object is proportional to the applied unbalanced force, in the direction of the force. The rate of change of momentum of an object is proportional to the applied force.
Newton’s second law of motion provides a precise explanation of the changes that a force can make to a body’s motion.
Mathematically it is shown as,
Force ∝ (Change in Momentum) / (Time Taken)
[Tex]F ∝ d(mv)/dt[/Tex]
[Tex]F = kd(mv)/dt[/Tex]
[Tex]F = k\{v.dm/dt + m.dv/dt\}[/Tex]
As mass is always constant,
[Tex]\{dm/dt = 0\}[/Tex]
[Tex]F = km.dv/dt[/Tex]
Experimentally, k = 1
[Tex]F = m.dv/dt[/Tex]
[Tex]we \ know \ that, \ dv/dt = a[/Tex]
[Tex]F = ma[/Tex]
where,
- F is the Force Applied
- m is the Mass of Object
- a is the Acceleration of Object
Newton’s Second Law of Motion Examples
Various examples of Newton’s Second Law are:
Example 1: Catching a ball on the cricket field by a fielder is the best example of Newton’s second law of motion. When the fielder catches the ball it moves its hands backward to increase the time of the catch resulting in lowering the force by the ball on the hands of the catcher.
Example 2: Airbags in a car increase the impact time of the accident and then reduce the impact of the accident, etc.
Newton’s Third Law of Motion
For every action there is always an equal and opposite reaction.
This means that whenever one object exerts a force on a second object, the second object exerts a force of equal magnitude but in the opposite direction on the first object.
This law is also explained as, when an object A applies a force of F1 on object B then B applies a force of F2 on the object such that,
F1 = -F2
Third Law of Motion by Newton is also called the Law of Action and Reaction.
Examples
Various examples of Newton’s Third Law of Motion observed are,
Example 1: Recoil of the gun when a bullet is fired is an example of Newton’s Third Law of Motion.
Example 2: The launching of the rocket is also an example of the Third Law of Motion.
Example 3: When you push off the boat to jump onto the dock, the force you apply to the boat causes a reaction where the boat pushes back on you with an equal and opposite force. This mutual interaction causes the boat to move backward as you move forward.
Solved Examples on Newton’s Laws of Motion
Here are some numerical on Newton’s Laws of Motion for you to practice and learn.
Newton’s First Law (Inertia)
Question 1: A skateboarder is riding at a constant speed on a smooth road. Suddenly, she hits a rock, and the skateboard stops, but the rider flies forward. Explain why this happens using Newton’s first law.
According to Newton’s first law, an object in motion will stay in motion unless acted upon by an external force. In this case, the rock acts as an external force stopping the skateboard. However, the skateboarder’s body, due to inertia, continues moving forward at the same speed, which is why she flies off the skateboard.
Question 2: A soccer ball is at rest on the field. It remains at rest until a player kicks it. Which law of motion explains why the ball doesn’t move on its own?
This is an example of Newton’s first law of motion (the law of inertia). The ball remains at rest because no external force is acting on it. It will only move when a force, such as the player’s kick, is applied.
Newton’s Second Law (Force and Acceleration)
Question 3. Find out how much net force will be needed to accelerate a 2500 kg truck at 5.50 m/s2.
Solution:
Given,
- Acceleration (a) = 5.50 m/s2
- Mass of the Truck (m)= 2500 kg
Hence,
Force = Mass × Acceleration
F = 2500 × 5.5
F = 13750 N
Net force will be needed to accelerate a 2500 kg truck at 5.50 m/s2 is 13750 N.
Question 4. What will happen If a net force of 6 N is applied on 0.5 kg object. Calculate the acceleration of the material.
Solution:
Given,
- Force (F) = 6 N
- Mass (m) = 0.5 kg
Acceleration (a) = ?
Force = Mass × Acceleration
F = m × a
a = F/m
a = 6/0.5
a = 12 m/s2
The acceleration of the object is 12 m/s2
Question 5. If a racing car driver is on the race track in order to overtake accelerates his racing car first at the rate of 8 m/s2 and then at the rate of 16 m/s2. Find the ratio of the forces exerted by the engine for the acceleration change.
Solution:
Given,
We have to find the ratio of F1/F2
F1/F2 = ma1/ma2
Mass of the racing car is same in both the cases,
F1/F2 = a1/a2
F1/F2 = 8/16
F1/F2 = 1/2 = 1:2
Newton’s Third Law (Action and Reaction)
Question 6: When a swimmer pushes the water backward, the water pushes her forward. Which of Newton’s laws does this demonstrate, and how?
This demonstrates Newton’s third law of motion: for every action, there is an equal and opposite reaction. When the swimmer exerts a force on the water by pushing it backward (action), the water pushes the swimmer forward with an equal force (reaction), allowing her to move through the water.
Question 7: When a bullet of mass 20 gm is shot from a gun that has an initial velocity of 40 m/s the mass of the gun is 5 kg. What is the initial recoil velocity of the gun?
Solution:
Given,
- Mass of Bullet (m1) = 20 gm or 0.02 kg
- Mass of Gun (m2) = 5 kg
- Initial velocity = 40 m/s
Let final velocity is v m/s
By Law of Conservation of Momentum,
0 = 0.02 × 40 + 5 × v
5 × v = -0.8
v = -0.8 / 5
v = -0.16 m/s
Practice Questions on Newton’s Laws of Motion
Here are some practice problems on Newton’s Laws of Motion for you to solve:
Problem 1: A book is placed on a table. Describe what happens to the book when:
a) No force is applied to it.
b) A gentle horizontal force is applied to the book.
c) A vertical force is applied to the book.
Problem 2: What will happen to a stationary soccer ball if no external forces are applied to it?
Problem 3: A bicycle is moving on a flat road. What force causes it to eventually stop if no one keeps pedaling? How does this relate to the first law?
Problem 4: An object with a mass of 5 kg is subjected to a net force of 20 N. Calculate the acceleration of the object.
Problem 5: A force of 10 N is applied to a 2 kg object. What is the object’s acceleration?
(Hint: Use the formula F=ma)
Problem 6: If a force of 15 N causes an object to accelerate at 3 m/s², what is the mass of the object?
Problem 7: When you jump off a boat, the boat moves backward. Explain this observation using Newton’s third law.
Problem 8: When a rocket launches, it expels gas downwards, yet it moves upwards. How does this demonstrate Newton’s third law?
Problem 9: Two ice skaters, one with a mass of 60 kg and the other with a mass of 80 kg, push off each other on an ice rink. If the 60 kg skater exerts a force of 200 N on the 80 kg skater, what force does the 80 kg skater exert on the 60 kg skater?
Conclusion
Newton’s Laws of Motion are essential for understanding the mechanics of objects in motion. They provide a framework for analyzing forces and predicting the behavior of objects under various conditions. By mastering these principles, we gain insights into both everyday phenomena and complex engineering challenges.
Newton’s Laws of Motion – FAQs
Who discovered 3 laws of Motion?
The 3 laws of Motion were discovered by Sir Isaac Newton.
What are Newton’s Laws of Motion?
Newton’s Laws of Motion in physics are the fundamental laws of Physics that are given by famous English scientist, Sir Isaac Newton. There are three laws under Newton’s Law of Motion, that are,
- Newton’s First Law of Motion
- Newton’s Second Law of Motion
- Newton’s Third Law of Motion
All three laws are explained in detail in the article above.
What is Newton’s First Law of Motion?
Newton’s First Law of Motion is also called the Law of Inertia. It states that, “An object at the states of rest or motion states in its state of rest or motion until a a force is applied to it.”
What is Newton’s Second Law of Motion?
Newton’s Second Law of Motion states that, “Rate of change of momentum is the force applied to the object.”
What is Newton’s Third Law of Motion?
Newton’s Third Law of Motion also called the Law of Action and Reaction as it states that, “Every action has its equal and opposite reaction.”