Case Study:

During a high-intensity training session, a long-distance runner maintains a constant force of 200 N while running on a straight track. For the first 10 minutes, the athlete covers 1500 m steadily.

To understand how this movement translates into physics, it helps to first grasp the different types of work done - whether positive, negative, or zero - and how they apply to motion.

However, in the next 10 minutes, despite applying the same force, the athlete slows down and covers only 1000 m, reporting sudden fatigue and increased effort.

The coach observes that although the work done by the athlete depends on force and displacement, the rate of doing work (power) varies significantly due to changes in speed.

This is essentially why we rely on mechanical help; machines optimize power to save us the very effort and time this athlete is struggling with

Initially, the athlete’s higher speed results in greater power output, but as fatigue sets in, speed decreases, reducing power output even if the applied force remains constant.

The situation raises an important question: If the force applied is unchanged, why does the athlete feel more exhausted? The coach hypothesizes that the body’s efficiency and energy conversion rate decrease over time, affecting the athlete’s ability to sustain high power levels.

At its core, this is a breakdown in how the body converts stored energy into movement. You can explore the mechanics of this in our guide on kinetic vs potential energy.

This scenario highlights the relationship between work done, power, and rate of doing work, and how human performance depends not just on total work but also on how quickly it is done.

Ultimately, the athlete's fatigue is a practical demonstration of the law of conservation of energy, where energy isn't lost, but simply transformed into forms like heat.

CASE-BASED QUESTIONS

MCQ

Q1. The work done by the athlete in the first 10 minutes is:
(a) 200 J
(b) 300,000 J
(c) 1500 J
(d) 2000 J

Q2. Why does the athlete feel more fatigued in the second phase?
(a) Work done is zero
(b) Force applied increases
(c) Power output decreases due to reduced speed
(d) Displacement becomes zero

Assertion - Reason

Q3. Assertion (A): Power depends on both work done and time taken.
Reason (R): Even if work done is constant, reducing time increases power.

(a) Both A and R are true, and R is correct explanation of A
(b) Both A and R are true, but R is not correct explanation
(c) A is true, R is false
(d) A is false, R is true

Application-Based

Q4. Calculate the power output of the athlete in the first 10 minutes.

Application-Based

Q5. Compare the power output in the first and second phase. Which is higher and why?

Data/Logic-Based Question

Q6. If the athlete wants to maintain the same power in the second phase despite fatigue, what must change?
(a) Increase displacement
(b) Increase speed
(c) Increase time
(d) Decrease force

ANSWER KEY WITH EXPLANATION

A1. (b) - 300,000 J
Work = Force × Displacement = 200 × 1500 = 300,000 J.

A2. (c) - Power output decreases due to reduced speed
Power depends on speed; lower speed reduces rate of doing work, increasing fatigue perception.

A3. (a) - Both A and R are true, and R is correct explanation of A
Power = Work/Time; less time means higher power for same work.

A4. Power = Work / Time = 300,000 J / 600 s = 500 W
Power is the rate of doing work.

A5. First phase has higher power because displacement (and speed) is greater in the same time. Lower speed reduces power in second phase.

A6. (b) -  Increase speed
Power depends on how fast work is done; increasing speed increases power.

HOTS EXTENSION QUESTIONS

1. If two athletes perform the same amount of work, but one completes it in half the time, how does their power output compare and how would fatigue differ?

2. Suggest a strategy for athletes to optimize power output without increasing force significantly. Explain using physics principles.