Question

A bead of mass 400 g is moving along a straight line under a force delivering constant power 1.2 W. If the bead is initially at rest, the speed attained after 6 seconds is (in \( \text{ms}^{-1} \))

• A. 5
• B. 4
• C. 6
• D. 3

Hint:

For variable force and constant power: \( \frac{1}{2} mv^2 = P t \)

Answer 1

The Correct Option is C

Power \( P = Fv \Rightarrow F = \frac{P}{v} \)
Use work-energy theorem: \( \frac{1}{2} mv^2 = P t \)
Given: \( m = 0.4\, \text{kg}, P = 1.2 \, \text{W}, t = 6 \, \text{s} \)
\[ \frac{1}{2} \cdot 0.4 \cdot v^2 = 1.2 \cdot 6 \Rightarrow 0.2 v^2 = 7.2 \Rightarrow v^2 = 36 \Rightarrow v = 6 \, \text{ms}^{-1} \]