Chapter 9

Force and Laws of Motion

Force and laws of motion is an important chapter in the class 9 NCERT syllabus that covers the most basic concepts in physics: the laws of motion and the concept of force and momentum. The solutions are provided to ensure that students can save time while studying. Students can refer to the laws of motion class 9 NCERT solutions provided by MSVGo as an overview of the content in the textbook and even use it for a quick revision before the exams. Make sure also to check out various visualisations, study material, and other resources to get a strong grasp of the subject. 

Topics covered in Chapter 9 – Force and Laws of Motion (Content Table)

  1. Introduction

  2. Newton's First Law of Motion

  3. Newton's Second and Third Law of Motion 

  4. Law of Conservation of Momentum

Introduction

When an object changes its position with reference to its surroundings, it is said to be in motion. Changes in the object's position can be measured by taking another moving or stationary body as a reference. For example, if a person is cycling from one place to another, the cyclist is said to be in motion with respect to the road. A movement can be in a straight line, a circular path, or a combination of different lines of paths. If an object moves in a straight line, it is said to be moving in a straight-line motion. For example, a train moving along a track.

Force can be defined as an external influence that changes the state of rest or uniform motion in a straight line. Force can bring about motion in a body, stop it or even change its direction. It can produce, modify or destroy energy.

Force can be applied in a push or pull direction upon an object, depending on its interaction with another body. Whenever two objects interact, a force acts upon each of the objects.

Force is a vector quantity that has both magnitude and direction. When two forces act on a body in opposite directions, they tend to cancel each other, resulting in zero. This is known as balanced force. But, when the two forces act along the same line of action, they combine to form a single force whose effect is equal to their sum. This is called an unbalanced force.

Newton's first law of motion, also called the law of inertia, states that a stationary object will continue to remain stationary unless acted upon by an external force. This means that an object at rest stays at rest, and an object in motion stays in motion at the same speed and in the same direction unless acted upon by an unbalanced force. It is a fundamental principle of classical mechanics that is used to describe the motion of objects.

For an object to have a constant velocity, it must have a constant speed in a specific direction. If the net force on an object is not zero, its velocity will change because of the acceleration produced by the net force. Inertia is the tendency of an object to resist changes in its state of motion. If an object has a higher mass, its inertia will also be more significant. 

Key points in the first law are:

  • A body continues its state of rest or uniform motion unless acted on by an external force.

  • For a body to be at rest, there is no external force acting on it. 

  • For a body to be in uniform motion, the resultant of all external forces is zero.

Newton's second law of motion can be answered as follows: 

The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the object's mass. 

The equation for this law is F=ma, where F is the net force acting on the object, m is the object's mass, and a is the acceleration of the object.

Newton's second law is about how much force it takes to move an object. If a truck has a lot of mass and you want to increase its speed, you need to apply a lot of force. But if you're trying to speed up a small car that has less mass, then you won't need as much force.

Newton's third law of motion states that there is an equal and opposite reaction for every action. In other words, for every force exerted in one direction, a force equal in size but opposite in direction will be exerted back.

Key points in the third law:

  • The magnitude of the force on the first object will be equal to the magnitude of the force on the second object. 

  • The direction of the force on the first object will be opposite to the direction of the force on the second object. 

  • Forces always come in pairs. These action-reaction force pairs will be equal and opposite.

The law of conservation of momentum is a fundamental concept of physics. The law states that the total momentum of a system remains constant over time. It means that the total amount of motion in a system – which can be measured as mass multiplied by velocity – does not change unless acted upon by an external force.

The law of conservation of momentum is often used to analyse the motion of objects in collisions and explosions. In such events, the total momentum of an object or a collection of objects changes when forces are applied or removed. 

For example, if two objects collide with each other, the forces they exert on one another are equal in magnitude but opposite in direction. This causes each object to exhibit a change in velocity (rate of change in position), resulting in a change in momentum. However, since the collision involves only two objects and no external forces, the total momentum will be equal before and after the collision.

The law of conservation of momentum can be stated as, 'In an isolated system, the total momentum before and after a collision is the same.' A system is considered isolated if no external forces are acting on it. In other words, when there are no net forces on an object, its momentum does not change as there is no acceleration and therefore no change in velocity.

From the following information, this equation can be inferred. 

m1u1 +m2u2 = m1v1 +m2v2

In the equation, m1 and m2 are masses of the two objects, u1 and u2 are their initial velocities before the collision, and v1 and v2 are their final velocities after the collision.

      1. What is the third law of motion?

Newton's third law of motion states that for every action, there will be an equal and opposite reaction. Thus, if a body exerts a force on another body, the second body will exert equal force on the first body but in the opposite direction. 

 

      2. What is meant by force in physics?

Force can be simply defined as a push and pull motion. A force can cause an object with a mass to change its velocity, which includes moving from a state of rest to a state of motion and accelerating. 

 

      3. Can I get force and laws of motion chapter 9 class 9 online?

Yes, NCERT provides all the chapters for class 9 science online. You can also find the solutions on the MSVGo app. 

 

       4. Can I download linear programming NCERT Solutions?

Yes, you can get the force and laws of motion NCERT solutions on the MSVGo app. You can download it through the Google Play Store or the App Store. This app has been developed by experts, and the solutions given here are verified.

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