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What are Newton's three laws of motion?

Class 9 ScienceNCERT chapter: Force and Laws of Motionइसे हिंदी में पढ़ें

Exam-ready answer

Newton's three laws of motion describe the relationship between force and motion. The first law states that an object remains at rest or in uniform motion unless acted upon by an external force. The second law states that force equals mass times acceleration (F=ma). The third law states that every action has an equal and opposite reaction.

Explanation

Newton's first law, often called the law of inertia, explains why objects resist changes in their state of motion. For example, a book lying on a table stays at rest until someone pushes it. Similarly, a moving ball continues to move at constant speed in a straight line unless friction or another force stops it. This law introduces the concept of inertia, which depends on mass.

The second law quantifies force: it is the product of mass and acceleration. Mathematically, F = ma, where F is force in newtons (N), m is mass in kilograms (kg), and a is acceleration in metres per second squared (m/s²). This law tells us that a heavier object requires more force to accelerate at the same rate. For instance, pushing a empty cart is easier than a loaded one.

The third law states that forces always occur in pairs. If object A exerts a force on object B, then B exerts an equal and opposite force on A. These forces act on different objects, so they do not cancel out. Examples include a rocket pushing gases downward to move upward, or a swimmer pushing water backward to move forward.

Points to remember

  1. First law (law of inertia): An object continues in its state of rest or uniform motion unless acted upon by a net external force.
  2. Second law: The rate of change of momentum is directly proportional to the applied force and takes place in the direction of the force. F = ma.
  3. Third law: To every action, there is an equal and opposite reaction, acting on different bodies.
  4. Inertia depends on mass: greater mass means greater inertia.
  5. SI unit of force is newton (N), where 1 N = 1 kg·m/s².

Exam tip

Always write the formula F = ma with SI units and draw a simple diagram to show action-reaction pairs for the third law to score full marks.

Questions students also ask

What is inertia?
Inertia is the natural tendency of an object to resist changes in its state of rest or uniform motion. It depends on the mass of the object: heavier objects have greater inertia.
Why do we fall forward when a bus stops suddenly?
When a bus stops suddenly, our body tends to remain in motion due to inertia of motion. Thus, we fall forward. This is an example of Newton's first law.
How is Newton's second law related to momentum?
Newton's second law states that the rate of change of momentum of an object is directly proportional to the applied force and takes place in the direction of the force. Mathematically, F = dp/dt, which for constant mass gives F = ma.

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