The word kinetic is derived from the Greek word “kinesis” meaning motion. When an object is moving, it possess a energy and such energy is called kinetic energy. The object includes very large things, like planets, and very small ones, like atoms. Heavier objects moves faster and have more kinetic energy. Kinetic energy may be best understood by problems that demonstrate how it is transformed to and from other forms of energy.

Kinetic Energy Definition

  • Kinetic energy is one of many types of energy that exist. This is energy generated because something is moving — the faster it's going, the more kinetic energy it has. A person sitting has no kinetic energy, but a person running like a maniac has tremendous kinetic energy: 

  • The energy possessed by a body because of its motion, equal to one half the mass of the body times the square of its speed.

 

 

Kinetic Energy Definition (Cont)

  • The amount of kinetic energy KE of an object in translational motion is equal to one-half the product of its mass m and the square of its velocity v.

  • Kinetic energy is given as follows,

    Kinetic energy = ½(mv2)

 

Problem 1

A herder is herding his sheep into the kraal. A mother sheep and its lamb are both running at 2,7 m·s−1 towards the kraal. The sheep has a mass of 80 kg and the lamb has a mass of 25 kg. Calculate the kinetic energy for each of the sheep and the lamb

Solution

Given data:

  • Velocity or speed of mother sheep and lamp, v =2.7 m·s−1 
  • Mass of the mother sheep = 80 kg
  • Mass of the lamb = 25 kg

To find

  • Kinetic energy of mother sheep
  • Kinetic energy of lamb

 

Formula

  • Kinetic energy of the object = ½*mv2

Calculation

Kinetic energy of mother sheep

K.E

=1/2*(80)*(2.7)2

= 2916 Joules

kinetic energy of lamb

K.E

=1/2*(25)*(2.7)2

= 1913 Joules

 

Note

Even though the sheep and the lamb are running at the same velocity, due to their different masses, they have different amounts of kinetic energy. The sheep has more than the lamb because it has a higher mass.

Problem 2

A rocket of mass 1.5x104 kg accelerates at 220m/s2  for 29s from an initial speed of 5200m/s.

(a) How fast will be rocket be travelling after the 29s?

(b) How much Kinetic Energy has the rocket gained?

Solution:

Given data:

t = time = 29s

a = acceleration = 220m/s2

v = final speed = ?

u = initial speed = 5200m/s

Formula

(a) a=(v-u)/t

v – u = at

v = u + at

 (b) KE = ½(mv2)

 

Calculation

(a) v = u + at

v = 5200 + (220 x 29)

   = 5200 + 6380

so

v = 11580m/s

 

(b) Now KE=½(mv2)

Initial Kinetic Energy =0.5 x (1.5x104) x (5200)2

=2.028 x 1011J

Final Kinetic Energy =0.5 x (1.5x104) x (11580)2

=1.006 x 1012J

Kinetic Energy gained = Final Kinetic Energy- Initial Kinetic Energy

=1.006 x 1012J - 2.028 x 1011J

= 8.032 x 1011J

 

 

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