Work, Energy and Power – Long Answer Questions (CBSE Class 10 Physics)
Medium Level (Application & Explanation)
Q1. Explain the relationship between work and energy with an example from daily life.
Answer:
Work is the transfer of energy by a force acting over a distance.
Energy is the capacity to do work.
When work is done on an object, its energy increases.
Example: When you push a trolley, your muscles do work on it.
That work increases the trolley’s kinetic energy as it speeds up.
If you stop pushing, friction does negative work and reduces its energy.
Q2. State and explain the work–energy theorem. How does it apply to a rolling ball?
Answer:
The work–energy theorem says: Net work on an object equals the change in kinetic energy.
In symbols: W_net = ΔK.
If the net work is positive, the object speeds up.
If the net work is negative, it slows down.
For a rolling ball on a rough floor, friction does negative work.
So the ball’s kinetic energy decreases, and it comes to rest.
Q3. Differentiate between potential energy and kinetic energy with suitable examples.
Answer:
Potential Energy (PE) is stored energy due to position or configuration.
Gravitational PE near Earth is mgh.
Example: Water stored in an overhead tank has PE.
Kinetic Energy (KE) is the energy of motion.
It depends on mass and speed: KE = 1/2 mv².
Example: A moving bicycle has KE that increases as it moves faster.
Q4. What is power? Show how power is related to work and time. Give a practical example.
Answer:
Power (P) is the rate of doing work.
Formula: P = W / t, where W is work and t is time.
Unit of power is watt (W); 1 W = 1 joule per second.
A machine with more power does the same work in less time.
Example: A 1000 W motor lifts water faster than a 500 W motor.
In constant-speed motion, average P = F × v (force times speed).
Q5. Describe the law of conservation of energy with a falling object example.
Answer:
The law of conservation of energy says energy cannot be created or destroyed.
It only changes form, and total energy of an isolated system stays constant.
A stone falling from height h loses potential energy (mgh).
At the same time, it gains kinetic energy.
Ignoring air resistance, PE lost = KE gained.
With air resistance, some energy goes to heat and sound.
High Complexity (Analysis & Scenario-Based)
Q6. A car engine does work to accelerate the car. Explain how energy changes in this process and the role of friction.
Answer:
The engine’s chemical energy converts into mechanical energy.
The engine does work on the car, increasing its kinetic energy.
According to the work–energy theorem, W_net = ΔK.
Friction and air drag do negative work and reduce the net gain in KE.
So, engine work = gain in KE + energy lost to friction and heat.
Better roads and streamlined shapes reduce energy loss and improve performance.
Q7. A worker lifts a box straight up and then carries it horizontally at constant speed. Compare the work done in both parts.
Answer:
When lifting vertically, the worker does positive work against gravity.
Work done = mgh, where h is the vertical height.
This increases the box’s potential energy.
When carrying horizontally at constant speed, no work is done against gravity.
If speed is constant and friction is negligible, net work is zero in the horizontal part.
The worker still uses energy biologically, but in physics, work on the box is not done unless a force causes displacement in its direction.
Q8. Is the work done on an object independent of its mass? Analyse different cases carefully.
Answer:
In general, work (W = F × d) does not show mass directly.
But the force needed may depend on mass.
To lift at constant speed, force = mg, so W = mg × h.
Here, more mass means more work.
To accelerate on a frictionless surface, F = ma, so W = Fd = mad.
For the same a and d, a larger mass needs more work. So, work is not always independent of mass.
Q9. A roller coaster starts from rest at a height. Explain energy changes along the track with and without friction.
Answer:
At the top, it has maximum potential energy (mgh) and zero kinetic energy.
As it goes down, PE converts to KE.
At the lowest point, KE is maximum; speed is highest.
Without friction, total mechanical energy (PE + KE) stays constant.
With friction, some energy becomes heat and sound.
So, at the end, total mechanical energy decreases, and the coaster stops sooner.
Q10. Two machines lift identical loads to the same height, but one takes half the time. Compare the work and power. Discuss efficiency if friction is present.
Answer:
Both lift the same load to the same height.
So both do the same work: W = mgh.
The faster machine takes half the time, so its power is double: P = W/t.
If friction is present inside the machines, extra energy becomes heat.
The machine with less friction is more efficient: more output for the same input.
Efficiency depends on how much energy stays as useful work versus losses.
Q11. A student says “All potential energy converts to kinetic energy.” Evaluate this claim for a sliding block on a rough surface.
Answer:
On a rough surface, friction acts opposite motion.
Friction does negative work on the block.
So, not all potential energy becomes kinetic energy.
A part converts to heat and sometimes sound.
Only on a frictionless surface would PE convert fully into KE.
Thus, the claim is true only under ideal conditions.
Q12. A constant force pushes a cart along a straight path. Explain how you would calculate the power at any instant and what factors affect it.
Answer:
Instantaneous power is P = F × v at that moment.
If force (F) is constant but speed (v) increases, power increases.
If the cart moves faster, more power is needed to maintain the same force.
If friction grows with speed, extra force is needed, raising power demand.
Over a time interval, average power is P_avg = W/t.
So, power depends on both force and speed, and on energy losses like friction.
