Question

A boy of mass 50 kg runs with a speed of 4 m/s. His kinetic energy is-

• A. 40 J
• B. 4 J
• C. 400 J
• D. 0.4 J

Hint:

Kinetic energy depends on the square of velocity. If velocity doubles, kinetic energy becomes four times.

Answer 1

The Correct Option is C

Step 1: Note what is given.
Mass of the boy is $m = 50$ kg. His speed is $v = 4$ m/s.

Step 2: Recall the kinetic energy formula.
Kinetic energy is the energy of a moving body. \[ KE = \frac{1}{2} m v^2 \]

Step 3: Square the speed first.
\[ v^2 = 4^2 = 16 \]

Step 4: Multiply mass by this.
\[ m v^2 = 50 \times 16 = 800 \]

Step 5: Take half of it.
\[ KE = \frac{1}{2} \times 800 = 400 \]

Step 6: State the answer with unit.
Energy is measured in joules. \[ \boxed{400 \text{ J}} \]