If the net work done by external force on a partical is zero which is the following statement about the partical must be true?

Options

A. Its velocity is constant
B. Its velocity is decreased
C. Its velocity is unchanged
D. Its speed is unchanged

Answer

D

Explanation

The work - energy theorm states that
`W_("net") = Delta K = K_(f) - K_(i)` Thus `W_("net") = 0` then `K_(f) - K_(i) or (1)/(2) mv_(f)^(2) - (1)/(2) mv_(i)^(2)`, which leads to the conclusion that the speed is unchanged `(v_(f) - v_(i))`. The velocity
of the partical involves both magnitude (speed) and direction. The work energy theorem shows that the magnitude or speed is unchanged when `W_("net") = 0` but makes no statement about the direction of the velocity.