Grade 10 Physics – Work, Energy & Power

Question-and-answer flashcards covering definitions, formulas, examples, calculations, and conceptual explanations for Grade 10 work, energy, and power topics.

What is the definition of work in physics?

Work is the transfer of energy that occurs when a force acts on an object and causes displacement in the direction of the force.

Give two everyday examples in which work is done.

Lifting a heavy box and pushing a shopping cart.

Under what condition is the work done by a force zero even when the force is applied?

When the force is perpendicular to the direction of motion (no component along the displacement).

Calculate the work done when a 20 N force pushes a box 3 m.

W = Fd = 20 N × 3 m = 60 J.

State one key difference between kinetic energy and gravitational potential energy.

Kinetic energy is energy of motion (e.g., a moving car), whereas gravitational potential energy is energy due to height above the ground (e.g., a ball held in the air).

What is the formula for gravitational potential energy (GPE)?

GPE = mgh, where m = mass, g = 9.81 m/s², h = height.

Find the GPE of a 2 kg ball held 5 m above the ground.

GPE = 2 kg × 9.81 m/s² × 5 m = 98.1 J.

Identify the type of energy stored in a stretched rubber band and what it becomes when released.

Elastic potential energy, which converts to kinetic energy when the band is released.

Write the kinetic energy (KE) formula.

KE = ½ mv².

Calculate the kinetic energy of a 3 kg object moving at 4 m/s.

KE = ½ × 3 kg × (4 m/s)² = 24 J.

State the work–energy theorem in words.

The net work done on an object equals the change in its kinetic energy.

Define mechanical energy and name two systems that store it.

Mechanical energy is the sum of kinetic and potential energies in a system; examples include airplanes and windmills.

Describe a real-life example where gravitational potential energy is converted to kinetic energy.

When a ball is dropped from a height, its potential energy converts to kinetic energy as it falls.

How much work is done to accelerate a 1,000 kg car from rest to 20 m/s?

W = ΔKE = ½ × 1,000 kg × (20 m/s)² = 200,000 J.

Explain the energy changes when a stone is thrown vertically upward.

Its initial kinetic energy converts to gravitational potential energy until it stops; on the way down, potential converts back to kinetic.

Differentiate between work and power.

Work is energy transferred; power is the rate at which that transfer occurs.

Compute the power output when 600 J of work is done in 20 s.

P = W/t = 600 J / 20 s = 30 W (note: the notes mistakenly multiplied).

Define power and state its SI unit.

Power is the rate of doing work or transferring energy; its SI unit is the watt (W).

Give two household appliances with high power ratings and their implication.

Washing machines and air-conditioners; a higher power rating means more energy consumed per second, affecting cost and the environment.

State the principle of conservation of energy using a pendulum.

The total energy remains constant: as a pendulum swings, kinetic energy converts to potential energy and back again without loss (ideal case).

Identify one energy transformation in a hydroelectric dam.

Gravitational potential energy of water is converted to electrical energy.

How is energy conserved in an ideal (frictionless) roller-coaster?

Potential energy converts to kinetic on descent and back to potential on ascent while total mechanical energy stays constant.

Give one example each of a conservative and a non-conservative force.

Conservative: gravity (path-independent work). Non-conservative: friction (work depends on path length).

When a book slides to a stop on a rough table, which force does work and where does the energy go?

Friction does the work, converting kinetic energy into thermal energy and sound.

What distinguishes a conservative force from a non-conservative force?

A conservative force stores energy within the system (no dissipation); a non-conservative force dissipates energy as heat, sound, etc.

Describe the force-distance graph for stretching a spring.

A straight line through the origin, indicating force is proportional to extension (Hooke’s law).

What physical quantity equals the area under a force-distance graph?

Work done (in joules).

Calculate the work required to stretch a spring (k = 200 N/m) by 0.1 m.

W = ½ kx² = ½ × 200 N/m × (0.1 m)² = 1 J.

How much work is done lifting a 10 kg mass 2 m using a pulley?

W = mgh = 10 kg × 9.81 m/s² × 2 m = 196.2 J.

Explain one energy transformation when cycling uphill.

Chemical energy in muscles converts to mechanical energy that becomes gravitational potential energy as the bicycle gains height.

Calculate the efficiency of a machine that outputs 600 J from 1,000 J input.

Efficiency = (600 J / 1,000 J) × 100 % = 60 %.

What does 100 % efficiency mean, and is it realistic?

All input energy converts to useful work; it is unrealistic because unavoidable losses (friction, heat, sound) always occur.

A 500 W motor lifts 20 kg by 10 m in 8 s. Was it powerful enough?

Required power = (mgh)/t = (20 kg × 9.81 m/s² × 10 m)/8 s ≈ 245 W; the 500 W motor is more than powerful enough.

Describe energy conservation when a child slides down a frictionless slide.

Gravitational potential energy at the top converts entirely into kinetic energy at the bottom.

If a ball dropped from 10 m rebounds to 8 m, where did the “lost” energy go?

It transformed into heat and sound during the impact with the ground.

How does stretching or compressing a spring store energy? Give an example.

Work done on the spring is stored as elastic potential energy, e.g., in a toy nerf gun plunger or a click pen spring.

Describe the energy transformations in pole vaulting.

Runner’s kinetic energy → elastic potential in the bending pole → gravitational potential as the athlete rises.

How do simple machines like levers reduce effort?

They change the magnitude or direction of the applied force, allowing the same work with less force over a greater distance.

How does a bicycle’s design improve mechanical efficiency?

Gears and chains reduce energy losses and transfer more energy from the rider’s legs to forward motion.

Explain how elevators apply energy principles for efficiency.

Counterweights balance the cabin’s mass, reducing the motor’s work and conserving energy.

List the energy changes for a skier descending a slope.

Chemical energy (body) → gravitational potential energy at height → kinetic energy while descending, with some energy lost as heat and sound.

Calculate the total mechanical energy of a 5 kg object moving at 2 m/s and 3 m high.

KE = ½ × 5 × 2² = 10 J; PE = 5 × 9.81 × 3 ≈ 147.15 J; total ≈ 157.15 J.

At what point in a pendulum’s swing is its kinetic energy greatest?

At the lowest point of the swing.

Compute the efficiency of a car engine that delivers 40,000 J movement and loses 10,000 J as heat.

Efficiency = (40,000 J / 50,000 J) × 100 % = 80 %.

How do car brakes illustrate the work-energy principle?

Brakes do negative work on the car, converting its kinetic energy into thermal energy, stopping the vehicle.

How do friction and air resistance affect mechanical systems?

They decrease efficiency by converting useful mechanical energy into heat and sound.

Why can’t a roller-coaster reach a higher point than its start without extra energy?

Energy losses to friction and air resistance mean total mechanical energy available is insufficient to reach a greater height.

Describe energy conversions in a mass oscillating on a spring.

Elastic potential ↔ kinetic ↔ gravitational potential, cycling as the mass moves up and down.

When a hammer drives a nail, what energy transformation occurs?

The hammer’s kinetic energy becomes work on the nail and heat in the wood and nail.

Find the power of a motor lifting 50 kg by 4 m in 10 s.

Work = mgh = 50 kg × 9.81 m/s² × 4 m = 1,962 J; Power = 1,962 J / 10 s = 196.2 W.

Give two benefits of using energy-efficient household appliances.

They lower energy bills and reduce environmental impact by wasting less energy.