Work, Energy, and Power – Long Answer Questions (CBSE Class 10 Science – Physics)
Medium Level (Application & Explanation)
Q1. Define scientific work. Explain with two daily-life examples why some tasks feel tiring but are “no work” in science.
Answer:
- In science, Work is done when a force causes displacement in its direction.
- If there is no movement, the scientific work is zero.
- Example 1: Studying for hours feels tiring, but there is no displacement of an object. So, no scientific work.
- Example 2: Pushing a huge rock that does not move. You feel tired, but no work is done because there is no displacement.
- The body may use energy, but science counts work only when an object actually moves.
- Thus, effort without displacement is not scientific work.
Q2. Why is holding a box above your head not considered work, but lifting it up is considered work?
Answer:
- Work = Force × Displacement in the same direction.
- When you simply hold a box still, there is no displacement. So, no work is done on the box.
- When you lift the box upwards, there is displacement in the direction of your upward force.
- Hence, lifting the box counts as scientific work.
- You may feel tired in both cases, but science looks only at movement caused by force.
- So, holding is zero work, lifting is positive work.
Q3. Explain why climbing stairs is considered scientific work. What is the role of gravity here?
Answer:
- While climbing stairs, your body moves upward.
- You apply an upward force and there is upward displacement.
- This matches the scientific rule for work: force and displacement in the same direction.
- You are working against gravity, which pulls you downward.
- Gaining height means you have done work on your body.
- Therefore, climbing is scientific work because there is displacement against gravity.
Q4. Compare “walking with groceries” and “standing with groceries.” In which case is scientific work done and why?
Answer:
- In science, work needs force and displacement.
- When you are standing with groceries, the bag does not move. So, no work is done on it.
- When you are walking, there is displacement as you move forward.
- Thus, walking while carrying groceries counts as work in the simple school-level view.
- You are applying force and there is movement of the object with you.
- Hence, walking involves scientific work, but standing does not.
Q5. A car engine is running, but the car is not moving. Explain why no work is done on the car according to science.
Answer:
- Scientific work requires displacement of the object.
- If the car remains stationary, the car’s displacement is zero.
- So, the work done on the car is zero, even if the engine is running.
- The burning fuel may produce heat and sound, but the car’s position does not change.
- Hence, there is energy use but no scientific work on the car.
- Work on an object counts only when it moves due to an applied force.
High Complexity (Analysis & Scenario-Based)
Q6. A student pushes a wall for 5 minutes. It does not move. Later, the wall cracks and shifts slightly. Discuss when scientific work is done in this situation.
Answer:
- For the first 5 minutes, there is no displacement of the wall.
- So, the work done on the wall during that time is zero.
- When the wall finally cracks and shifts, there is displacement.
- Only at that moment, if your force causes that movement, scientific work is done.
- Work depends on displacement in the force’s direction, not on effort or time spent.
- Thus, long effort without movement is no work, but any actual shift due to force is work.
Q7. You throw a ball vertically upward and then catch it. Analyze when work is done and by whom during its motion.
Answer:
- When you throw the ball, your hand applies force and the ball moves upward.
- Here, you do work on the ball because there is displacement in the force’s direction.
- After release, gravity acts downward while the ball moves up and then down.
- During its upward journey, gravity causes the ball to slow down; gravity is doing work on it.
- While coming down, gravity still acts downward and there is downward displacement.
- When you catch it, your hand applies force to stop it, and work is done to bring it to rest.
Q8. Evaluate the following activities for scientific work: (a) Holding a suitcase at a bus stop, (b) Climbing stairs with a suitcase, (c) Pushing a trolley that rolls forward. Justify your answers.
Answer:
- (a) Holding a suitcase: No displacement, so no work is done on the suitcase.
- (b) Climbing stairs with a suitcase: There is upward displacement against gravity. So, work is done.
- (c) Pushing a trolley that moves: There is force and forward movement. Hence, work is done.
- The key is always force + displacement in the same direction.
- Feeling tired does not prove scientific work.
- Only actual movement caused by force counts as work.
Q9. A clock is ticking on a wall. Using the scientific meaning of work, explain whether work is being done on the clock and why.
Answer:
- Scientific work needs displacement of the object being considered.
- The clock as a whole is stationary on the wall.
- So, no displacement of the clock means no work is done on the clock itself.
- The ticking sound or internal parts moving do not move the clock as a whole.
- Therefore, according to the simple definition, scientific work on the clock is zero.
- Movement inside does not count unless the object itself moves.
Q10. A person walks 200 m while carrying groceries, then stands still for 5 minutes holding the bag, and finally places it on a table without lifting it higher. Identify where scientific work is done and explain.
Answer:
- While walking 200 m, there is displacement with applied force, so work is done.
- While standing still for 5 minutes, there is no displacement. So, no work is done on the bag.
- When placing it on a table at the same height, if there is no upward movement, there is no work in lifting.
- The key steps with movement count; the steps without movement do not.
- Science focuses on force causing displacement.
- Thus, only the walking phase clearly involves scientific work here.