A body is pulled over a distance of 500m by a force of 20N. If the power developed is o.4 kW, calculate the time interval during which the force is applied

Analyze the question We're given the distance (d = 500 m), the force (F = 20 N) and the power (P = 0.4 kW). We need the time (t) during which the force acts.

Concept

• Work done: W = F × d
• Power: P = W / t, so t = W / P

Solution

1. Convert power to watts: P = 0.4 kW = 0.4 × 1000 = 400 W

2. Calculate the work done: W = F × d = 20 × 500 = 10,000 J

3. Calculate the time: t = W / P = 10,000 / 400 = 25 s

Answer: The time interval is 25.0 s, which is option C.

Tip: Always convert kW to W before you start (1 kW = 1000 W). Forgetting this is the most common slip in power questions. You can also quickly check your answer with P = W/t: 10,000 J ÷ 25 s = 400 W ✓.