Home
Class 12
PHYSICS
Define mechanical energy and law of cons...

Define mechanical energy and law of conservation of mechanical energy. Prove conversion of gravitational potential energy in to kinetic energy by the help of law of conservation of mechanical energy. Also draw graph between energy and height.

Text Solution

Verified by Experts

Law of Conservation of Mechanical Energy : The total mechanical energy of a system (E) is defined as the sum of the kinetic energy (K) and the potential energy (U) of the system, i.e., E = K + U According to the law of conservation of mechanical energy:
The total mechanical energy of a system remains constant if the only force that does work is a conservative force.
Let us consider a body of mass m placed at A as shown in
h = AB = height of the body above the ground
x = distance of any point C from A
g = acceleration due to gravity at the place
`v_1` = velocity of the body at C
v = velocity of the body at B, a point just above the ground. The velocity at the point A is zero.
At the point A

Potential energy at A, i.e., `U_1 = mgh`
Kinetic energy at A, i.e., `K_1 = 0`
Total mechanical energy at A, i.e., `E_1 = mgh + 0 = mgh … (1)`
At the point C
As the body covers a distance x when it moves from A to C and `v_1` is the velocity at C, then from
`v^2 - v_0^2 = 2` we get : `v_1^2 = 0 = 2 gx` (As `v = v_1, v_0 = 0, a =g, s = x)` or `v_1^2 = 2 gx`
Kinetic energy at C, i.e., `" "K_2 = 1/2 mv_1^2 = 1/2 m xx 2 gx = mgx`
Potential energy at C, i.e., `" "U_2 = mg (h – x)`
Total mechanical energy at C, i.e., `" "E_2 = mgx + mg(h – x) = mgh …(2)`
At the point B
From `v_2 - v_0^2 = 2 as, v^2 -0 = 2gh ("as" v = v, v_0 = 0 , a = g, s = h)`
or, `v^2 = 2gh`
Kinetic energy at B, i.e., `" "K_3 = 1/2 mv^2 = 1/2 m xx 2 gh = mgh`
Potential energy at B, i.e., `U_3 = 0`
Total mechanical energy at B, i.e., `E_3 = mgh + 0 = mgh …(3)`
From eqns. (1), (2) and (3), `E_1 = E_2 = E_3`
Clearly, total mechanical energy of the body at the points A, B and C (also at any other point) is the same. Thus, we find the total mechanical energy throughout the free fall is conserved. As the body falls down, the potential energy goes on decreasing whereas the kinetic energy goes on increasing. The variation of kinetic energy (K) and potential energy (U) and the constancy of the mechanical energy (E) have been represented graphically.
.
Doubtnut Promotions Banner Mobile Dark
|

Topper's Solved these Questions

  • ENERGY & MOMENTUM

    VMC MODULES ENGLISH|Exercise Level - 1|87 Videos
  • ENERGY & MOMENTUM

    VMC MODULES ENGLISH|Exercise Level - 1 PARAGRAPH QUESTIONS|3 Videos
  • ENERGY & MOMENTUM

    VMC MODULES ENGLISH|Exercise LEVEL - 0 -SHORT ANSWER TYPE|22 Videos
  • ELECTROSTATICS

    VMC MODULES ENGLISH|Exercise JEE Advanced (Archive)|89 Videos
  • GASEOUS STATE & THERMODYNAMICS

    VMC MODULES ENGLISH|Exercise JEE ADVANCED (ARCHIVE )|111 Videos

Similar Questions

Explore conceptually related problems

Mechanical energy and kinetic energy

Potential energy is converted into kinetic energy by

Knowledge Check

  • The usable form of mechanical energy is:

    A
    Elastic potential energy
    B
    Kinetic energy
    C
    Gravitational potential energy
    D
    None of the given options.
  • The two basic forms of mechanical energy are:

    A
    PE and KE
    B
    Sound and Light
    C
    Heat and tidal
    D
    Solar and wind
  • Similar Questions

    Explore conceptually related problems

    Define the law of conservation of energy.

    What is the law of conservation of energy?

    Give an example of mechanical energy

    Fuels like coal and wood contain mechanical energy.

    State the principle of conservation of energy.

    Does Lenz's law violate the law of conservation of energy?