Work, Energy and Power — Long Answer Questions (Class 9 Physics)

Medium Level (Application & Explanation)

Answer:

Work is done when a force causes a displacement of an object in the direction of the force. Mathematically, W = F × d (when force is constant and along displacement). Energy is the ability to do work.
When you push a stalled car, your muscles apply a force and the car moves a distance, so you do work on the car.
The work you do transfers energy to the car in the form of kinetic energy, making it move.
If the car gains speed, its kinetic energy increases; if it is pushed uphill, the work increases its gravitational potential energy.
Thus, work is the means by which energy is transferred from you to the car.

Answer:

Potential energy (PE) is stored energy due to position. Kinetic energy (KE) is energy of motion.
In a pendulum, at the highest point the bob is momentarily at rest. It has maximum PE (relative to lowest point) and zero KE.
As it swings down, PE decreases and is converted into KE; at the lowest point the bob has maximum KE and minimum PE.
On the upward swing, KE converts back into PE until the motion stops momentarily at the next peak.
This continuous exchange illustrates conservation of mechanical energy (ignoring friction): PE + KE = constant.

Answer:

When lifting an object of mass m by height h against gravity, the force you must overcome is weight = mg (g is acceleration due to gravity).
Work done W = force × displacement = mg × h. This work is stored as gravitational potential energy (PE). So PE = mgh.
Units are joules (N·m).
PE depends on mass because a heavier object (larger m) requires more force to lift the same height, so more work is needed.
Therefore greater mass → greater mgh → greater stored potential energy for the same height and g.

Answer:

Power (P) is the rate of doing work, P = W / t. Work done in lifting = change in gravitational potential energy = mgh.
For m = 20 kg, h = 2 m, g = 9.8 m/s²: W = mgh = 20 × 9.8 × 2 = 392 J.
Time t = 5 s, so P = 392 / 5 = 78.4 W.
Thus the student delivers an average power of 78.4 watts while lifting the box.
Power tells how fast energy is transferred; the same work done in less time requires more power.

Answer:

The law of conservation of energy says total energy in an isolated system remains constant. For a frictionless roller coaster, mechanical energy (PE + KE) is conserved.
At the top of a hill the coaster has maximum PE (mgh) and low KE if slow. As it descends, PE decreases and KE increases, so the coaster speeds up.
At the lowest point PE is minimum and KE is maximum. As it climbs the next hill, KE converts back into PE and the coaster slows.
Ignoring friction and air resistance, the coaster would return to the initial height — demonstrating energy conversion without loss.

Answer:

The claim is misleading because whether mass appears in work depends on the situation. Work W = F × d; if force depends on mass, mass may remain in the expression. For example lifting vertically W = mgh, so W ∝ m.
Mass cancels only in special cases. Example: if you apply a force that gives the same acceleration a to two masses over the same distance, then W = Fd = ma × d, so W ∝ m and does not cancel. If comparing required force to overcome inertia for same acceleration, heavier mass needs larger force and thus more work for same distance.
Mass cancels when dealing with formulas where acceleration depends inversely on mass under constant force, e.g., kinetic energy gained under constant force and same distance gives KE = Fd independent of mass in description, but the velocity reached differs.
Always check the force, distance, and context before concluding mass independence.

Answer:

Initially the box at top has gravitational potential energy relative to bottom: PE = mgh where h = 5 sin30° = 2.5 m. So PE = 10 × 9.8 × 2.5 = 245 J.
As it slides down, PE decreases and is partly converted into kinetic energy (KE), increasing the box’s speed.
Some energy is also converted into thermal energy because of work done by friction against motion. Friction does negative work, removing mechanical energy and turning it into heat.
At the bottom, KE = initial PE − work done by friction. If frictional work equals 45 J, then KE at bottom would be 200 J.
Thus friction reduces the final KE and produces heat; mechanical energy is not conserved but total energy is conserved.

Answer:

The work–energy theorem states that net work done on an object equals its change in kinetic energy, W_net = ΔKE = (1/2)m(v_f² − v_i²).
For m = 1000 kg, v_i = 10 m/s, v_f = 20 m/s: ΔKE = 0.5 × 1000 × (20² − 10²) = 500 × (400 − 100) = 500 × 300 = 150000 J.
So the net work done on the car is 150 kJ.
This work could come from the engine overcoming resistances and providing the net force to increase speed. The theorem links forces and motion through energy directly.

Answer:

Work done against gravity is the same for both because work depends on vertical displacement: W = mgh = 30 × 9.8 × 1.5 = 441 J. So both do 441 J of work (ignoring friction).
Force needed differs: A must apply an upward force roughly equal to the weight (≈294 N) over 1.5 m. B applies a smaller force along the ramp (~weight × sinθ) over 4.5 m. The ramp reduces the required force but increases distance.
Effort feels less for B because the push force is smaller, though the work (energy used) is the same. If ramp has friction, B must do more work than A to overcome it.
This shows how simple machines trade force for distance without changing the ideal work.

Answer:

Efficiency = (useful output energy / input energy) × 100%. It measures how well a device converts input energy into useful work.
Here useful output = 2000 J, input = 2500 J. Efficiency = (2000 / 2500) × 100% = 0.8 × 100% = 80%.
The remaining 20% (500 J) is lost mainly as heat due to friction, sound, vibration, or internal electrical losses.
Improving efficiency means reducing these losses by better lubrication, insulation, or design, so a larger fraction of input energy becomes useful work.
Efficiency cannot exceed 100% because of the law of conservation of energy.