As noted earlier, if an object of mass m has velocity v, its kinetic energy K is
K = $\frac{1}{2}m \mathbf{v} .\mathbf{v}=\frac{1}{2}mv^2$
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 6.2 lists the kinetic energies for various objects.
 |
| Fig.1: Typical kinetic energies (K) |
Example 1
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
mv$^{-2}$/2 = 1000 J.
It has a final kinetic energy of
0.1×1000 = 100 J.
If v$_f$ is the emergent speed of the bullet,
$\frac{1}{2}mv_f^2$ = 100 J
$v_f$ = $\sqrt{\frac{2 \times 100 \ J}{0.05 \ kg}}$ = 63.2 m.s$^{–1}$
The speed is reduced by approximately 68% (not 90%).
Post a Comment for "Kinetic Energy"