• Inertia

    The property of an object because of which it resists to get disturb its state is called inertia. Inertia of an object is measured by its mass. Inertia is directly proportional to the mass. This means inertia increases with increase in mass and decreases with decrease in mass. A heavy object will have more inertia than the lighter one.

    In other words, the natural tendency of an object that resists the change in state of motion or rest of the object is called inertia.Since a heavy object has more inertia, thus it is diffcult to push or pull a heavy box over the ground than the lighter one.


    Examples of Inertia of Rest:

    1. A passenger in a bus jerks backward when the bus starts suddenly because the passenger tends to be in inertia of rest whereas the bus is moved away forcefully.
    2. When a bed sheet is flicked away suddenly dust particles fall away as they tend to be in inertia of rest.
    3. When a branch of a tree carrying a mango is suddenly flicked mango falls off due to inertia of rest.


    Examples of Inertia of Motion:

    1. A passenger in a bus jerks forward when the bus stops suddenly because the passenger tends to be in inertia of motion whereas the bus is stopped forcefully.
    2. An athlete after reaching the finishing point  can not stop suddenly or if he stops suddenly then he falls toppling head down.
    3. A car takes some time and moves through some more distance before coming to rest even after the application of brakes.
    4. A rotating fan continues to do so for some more time even after the current is switched off.
    5. An oscillating simple pendulum bob does not halt at the mean position but continues to move further.
    6. When a car or bus turns around a sharp corner, we tend to fall sideways because of our inertia to continue to move in a straight line.
    7. It is dangerous to jump out of a moving bus because the jumping man’s body is in the state of inertia of motion but the legs are suddenly stopped by the ground and hence he topples down.



    Momentum is the power of motion of an object. The product of velocity and mass is called the momentum. Momentum is denoted by ‘p’. Therefore, Momentum of the object = Mass × Velocity

    or, p = m × v

    Where, p = momentum, m = mass of the object and v = velocity of the object.

    Consider the following explanations to understand the momentum : A person get injured in the case of hitting by a moving object, such as stone, pebbles or anything because of momentum of the object. Even a small bullet is able to kill a person when it is fred from a gun because of its momentum due to great velocity. A person get injured severely when hit by a moving vehicle because of momentum of vehicle due to mass and velocity.


    Momentum, Mass and Velocity

    Since momentum is the product of mass and velocity (p = m × v) of an object. This means momentum is directly proportional to mass and velocity. Momentum increases with increase of either mass or velocity of an object. 

    This means if a lighter and a heavier object is moving with same velocity, then heavier object will have more momentum than the lighter one.

    If a small object is moving with great velocity, it has tremendous momentum. And because of momentum, it can harm an object more severely. For example, a small bullet having a little mass even kills a person when it is fred from a gun.

    Usually, road accidents prove more fatal because of high speed than in slower speed. This happens because vehicles running with high speed have greater momentum compared to a vehicle running with slower speed.


    Momentum of an object which is in the state of rest :

    Let an object with mass ‘m’ is in the rest. Since, object is in rest, therefore, its velocity, v = 0

    Now, we know that,   Momentum = mass × velocity

    or p = m × 0 = 0

    Thus, the momentum of an object in the rest i.e., non-moving, is equal to zero.


    Unit of Momentum :

    SI unit of mass = kg
    SI unit of velocity =

    Since, Momentum (p) = m × v. Therefore, SI unit of momentum is kg m/s.