What is Potential Energy? | What is Kinetic Energy? | Strain Energy

Topic: What is Kinetic Energy? | What is Potential Energy? | Strain Energy

Before going into the topic, we need to know few things before.

[You can skip these definitions and scroll down to directly get into the subject]

What is Energy?

“The capacity to do work,” said as Energy

What is Work?

When a body undergoes displacement due to some external force, it is said to be work done on the body.

Work Done = Force applied on the body (F) x Displacement (x)

The displacement can be either linear or angular. The above formula is for linear displacement. Let’s consider when a couple or a torque(T) acting upon the body, causes an angular displacement of θ about an axis. Then,

Work done = Torque (T) x Angular displacement (θ)

SI units for work done is N-m (1N-m = 1J (joule))

Some related definition of work is power, Efficiency.

What is Power?

Power is defined as the amount of work done per a unit time

Power (P) = Work done/Time taken

Let’s say a torque of T is transmitting thru a shaft with an angular speed of ω then,

Power (P) = Torque (T) x Angular speed (ω)

                    Power (P) = 2πNT/60     (∴Angular speed (ω) = 2πN/60)

SI units of power are watt (W) = 1joule (J) = 1N-m

Now let’s get back to the subject

The energy can exist in many forms like mechanical energy, electrical energy, heat energy, chemical energy light energy…etc. We will talk about the mechanical energy only here.

This mechanical energy is equivalent to work done on a body when we change the position or velocity of the body. There is mainly three type of Energies that we must to talk about.

• Potential energy
• Kinetic Energy
• Strain Energy

What is Potential Energy?

“The energy within the body for doing work by virtue of its position is known as Potential Energy”

Potential Energy Example: Raise a body to a certain amount of height and it will gain a energy to reach the ground when it is released. This energy is called Potential energy. Let’s say the weight of the body is “W”, Height of the object which the body is falling from is “h”

Potential Energy (P.E) = Weight (W) x Height (h)

Potential Energy (P.E) = Mass (m) x Acceleration due to gravity (g) x Height (h)

(∴ Weight (W) =  Mass(m) x Acceleration due to gravity(g))

P.E = mgh

SI units for Potential Energy

SI units for Potential Energy are N-m

= m.g.h              [∴ m in kilogram;   g in metres/sec²;   h in metre]

= kg x m/s² x m

= kg-m/s²-m         [∴ 1N = kg-m/s²]

=N-m

What is Kinetic Energy?

“The energy within the body for doing work by virtue of its mass and Velocity of motion is known as Kinetic Energy”

Kinetic energy Example: A force (F) acts on a body of mass m attain a velocity v from a rest position in time t, the s is the distance travelled by the body. The work is done on the body is

Work Done = Force x Distance travel by the body

As we said above ↑ the mechanical energy equivalent to work done

∴ Kinetic Energy (K.E) = Force x distance travelled by the body

Where

Force = Mass x Acceleration

Distance travelled by the body(s) = v²/2a

[∴ from this eqn (v²–u²) = 2as, and body starts from rest u = 0]

Kinetic Energy (K.E) = mv²/2

This is for linear motion of the body

For the angular motion, Kinetic Energy (K.E) = Inertia x (Angular Velocity)²

Kinetic Energy (K.E) = Iω²/2

When body subjected to both linear and Angular motion then,

Kinetic Energy (K.E) = mv²/2 + Iω²/2

a statement about the energy is that,

“The energy neither be created nor destroyed, though it can be transformed from one form to another form”

This statement is known as “Law of Conservation of Energy”

SI units for Kinetic Energy

SI units for Kinetic Energy are N-m

= mv²/2         [∴ Mass in Kilogram;  Velocity in Metre/sec]

= kg x (m/s) ²

= kg-m/s²-m    [∴ 1N = kg-m/s²]

=N-m

What is Strain Energy?

“When an elastic body is subjected to deformation, the Potential Energy will be stored in the elastic body.” this energy is known as Strain energy.

Strain Energy Example: When a mechanical spring subjected to a compression force, then a deformation occurs in the length of the spring. Once the force removed on the body and the body will get its original shape with the help of strain energy stored in the spring.

Strain Energy = Work done = Weight (W) x Deformation distance in the spring(x)/2

Strain Energy = (W.x)/2

                        Strain Energy = (s.x²)/2        [∴ W = s.x]

 

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