6.4 Kinetic energy
As noted earlier, if an object of mass m has velocity v, its kinetic energy K is
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 e NEETprep Audio Note (English):
nergy of the wind. Table 6.2 lists the kinetic energies for various objects.
Table 6.2 Typical kinetic energies (K)
Example 6.4 In a ballistics demonstration a police officer fires a bullet of mass 50.0 g with speed 200 m s-1 (see Table 6.2) 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 of the bullet is mv2/2 = 1000 J. It has a final kinetic energy of 0.1×1000 = 100 J. If vf is the emergent speed of the bullet,
= 63.2 m s–1
The speed is reduced by approximately 68% (not 90%). NEETprep Audio Note (English):