NCERT Gist: Force and Laws and Motion | Additional Study Material for UPSC PDF Download

- An object moves with a uniform velocity when the forces (pushing force and frictional force) acting on the object are balanced and there is no net external force on it.
- If an unbalanced force is applied on the object, there will be a change either in its speed or in the direction of its motion.
- Thus, to accelerate the motion of an object, an unbalanced force is required.
- And the change in its speed (or in the direction of motion) would continue as long as this unbalanced force is applied.
- An object remains in a state of rest or of uniform motion in a straight line unless compelled to change that state by an applied force.
- The tendency of undisturbed objects to stay at rest or to keep moving with the same velocity is called inertia.
- The first law of motion is also known as the law of inertia.
- Inertia is the natural tendency of an object to resist a change in its state of motion or of rest.
- The mass of an object is a measure of its inertia.
- The train has more inertia than the cart.
- Clearly, heavier or more massive objects offer larger inertia.
- Quantitatively, the inertia of an object is measured by its mass.
- The impact produced by the objects depends on their mass and velocity.
- The 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 force.
- The third law of motion states that when one object exerts a force on another object, the second object instantaneously exerts a force back on the first.
- These two forces are always equal in magnitude but opposite in direction.
- These forces act on different objects and never on the same object.
- It is important to note that even though the action and reaction forces are always equal in magnitude, these forces may not produce accelerations of equal magnitudes.
- This is because each force acts on a different object that may have a different mass.
- To every action there is an equal and opposite reaction.
- The sum of momenta of the two objects before collision is equal to the sum of momenta after the collision provided there is no external unbalanced force acting on them. This is known as the law of conservation of momentum.