Lesson 5 of 13
In Progress

Kinetic Energy

As noted earlier, if an object of mass m has velocity v, its kinetic energy K is

K = 1/2 m v.v = 1/2mv2 ……………………………………………………………………(5)

Kinetic Energy is a scalar quantity. The kinetic energy of an object is a measure of the work an object can do by the virtue of its motion. This notion has been intuitively known for a long time. The kinetic energy of a fast flowing stream has been used to grind corn. Sailing ships employ the kinetic energy of the wind. Table 2 lists the kinetic energies for various objects.

Table-2 Typical kinetic energies (K)

ObjectMass (kg)speed (ms-1)K(J)
Car200256.3 × 105
Running athlete70103.5 × 103
Bullet5×10-2200103
Stone dropped from 10 m114102
Rain drop at terminal speed3.5 × 10-591.4×10-3
Air molecule≅ 10-26500≅ 10-21

Example 4
In a ballistics demonstration a police officer fires bullet of mass 50.0 g with speed 200 m s-1 (See Table 2) on soft plywood of thickness 2.00 cm. The bullet emerges with only 10% of its initial kinetic energy. What is the emergent speed of the bullet?

Answer

The initial kinetic energy 1/2 mv2 = 1000 J. It has a final kinetic energy of 0.1 × 1000 = 100 J. If vf is the emergent speed of the bullet,

1/2 m(v2)f = 100 J

vf = √(2 × 100 J)/0.05kg

= 63.2 ms-1

The speed is reduced by approximately 68% (not 90%)