LET Elementary Physics — Work, Energy, Heat and WavesRevision Notes

Formulas

Exam Tips

• If the LET item says 'a student pushes a wall with great effort but it does not move' – the answer is ZERO joules of work.
• Any time you see 'lifted' or 'raised,' use W = mgh.
• Any time a force moves something along a flat surface, use W = Fd.
• Watch for trick questions: 'A satellite orbits Earth at constant speed' – gravity acts perpendicular to motion, so it does zero work on the satellite.

Key Points

• In physics, WORK is done only when a force causes displacement in the direction of that force. It is NOT the same as everyday 'effort.'
• Formula: W = F × d (Work = Force × distance). The unit is the JOULE (J), where 1 J = 1 Newton-meter (N·m).
• If there is NO movement, work done = 0 J, regardless of how much force is applied (e.g., pushing against a concrete wall).
• If the force and motion are perpendicular (at 90°), no work is done by that force (e.g., carrying a bag horizontally – gravity does no work on it).
• Lifting an object against gravity: W = mgh, where g ≈ 9.8 m/s².
• Work is a SCALAR quantity (it has magnitude only, no direction).
• Philippine classroom connection: A janitor pushing a mop across the classroom floor is doing work; a student holding a heavy bag motionless is NOT doing physics work.

Definitions

Section Title

Work

Common Mistakes

• Confusing everyday 'work' with physics work – in physics, holding something heavy but stationary = ZERO work.
• Forgetting that force must be in the SAME DIRECTION as motion. Carrying a bag while walking horizontally: gravity does no work on the bag.
• Using mass instead of weight. Weight = mg; when lifting, F = mg, not just m.
• Not converting units before computing (e.g., cm to m, grams to kg).

Formulas

Exam Tips

• When a ball is at the HIGHEST point of its trajectory, KE is minimum (possibly zero) and PE is maximum.
• When a ball is at the LOWEST point (just before hitting the ground), PE is minimum and KE is maximum.
• The LET often asks: 'What happens to KE when speed doubles?' Answer: KE becomes FOUR TIMES larger (quadruples).
• Match energy forms to real-life objects: a stretched rubber band = elastic PE; a running child = KE; a mango on a high branch = gravitational PE.

Key Points

• Energy is the CAPACITY to do work. It is measured in JOULES (J), the same unit as work.
• KINETIC ENERGY (KE): Energy of motion. Formula: KE = ½mv². Mass is in kg, velocity in m/s.
• POTENTIAL ENERGY (PE): Stored energy due to position or condition. Gravitational PE = mgh.
• DOUBLING the speed QUADRUPLES the kinetic energy because velocity is squared. This explains why fast-moving vehicles cause far more damage in accidents.
• Other forms of energy: Chemical (food, batteries, fuel), Thermal/Heat, Electrical, Light (Radiant), Sound, Nuclear, Elastic (stretched rubber band or spring).
• Energy transformations are everywhere: a battery converts chemical to electrical; a bulb converts electrical to light and heat; photosynthesis converts light to chemical energy in plants.
• The LET often uses the context of a swinging pendulum or a ball rolling down a ramp to test KE and PE transformations.

Definitions

Section Title

Energy: Kinetic and Potential

Common Mistakes

• Using mass in grams instead of kilograms in KE = ½mv². Always convert: 1 kg = 1000 g.
• Forgetting to SQUARE the velocity in the KE formula – the most common calculation error.
• Treating all forms of potential energy as the same; the LET distinguishes gravitational PE (mgh) from elastic PE.
• Thinking energy is 'used up' rather than transformed from one form to another.

Formulas

Exam Tips

• When the LET asks about a swinging pendulum: at the highest point = maximum PE; at the lowest (equilibrium) = maximum KE; it keeps transforming PE ↔ KE.
• For a ball dropped from height h, use v = √(2gh) to find its speed at the bottom. No mass needed.
• 'Energy is wasted as heat due to friction' = a correct and acceptable LET answer for why machines are not 100% efficient.
• Trace energy chains in sequence: Eating rice → chemical energy → body moves → kinetic energy → friction with floor → heat.

Key Points

• The LAW OF CONSERVATION OF ENERGY states: Energy cannot be created or destroyed; it can only be TRANSFORMED from one form to another. The total energy in a closed system remains constant.
• Classic example: A falling ball. At the top – maximum PE, zero KE. As it falls – PE decreases, KE increases. At the bottom (just before impact) – PE is zero, KE is maximum. Ignoring air resistance, KE at bottom = PE at top.
• Setting KE = PE allows you to find the speed of a falling object: v = √(2gh).
• In REAL machines, friction converts some mechanical energy into heat (thermal energy). This is why no machine is 100% efficient. The 'lost' energy is NOT destroyed – it becomes low-grade heat.
• Energy chain in a hydroelectric dam: Gravitational PE of water → Kinetic energy of falling water → Electrical energy (generator) → Light/Heat (in homes).
• Photosynthesis is the reverse of combustion: Light energy → Chemical energy (glucose) stored in plants.
• The LET connects this topic to environmental science: energy efficiency and responsible energy use in schools.

Definitions

Section Title

Conservation of Energy

Common Mistakes

• Saying energy is 'lost' to friction – energy is never lost; it is CONVERTED to heat (thermal energy).
• Thinking that a 100% efficient machine is possible – it is not, because friction always converts some energy to heat.
• Not canceling mass (m) in the equation mgh = ½mv² before solving for v. Mass cancels: gh = ½v², so v = √(2gh).
• Mixing up energy transformation sequences – always trace from INPUT form to OUTPUT form step by step.

Formulas

Exam Tips

• If the LET says 'Student A climbs the stairs in 5 s, Student B in 10 s' – Student A has TWICE the power (same work, half the time).
• The classic LET formula check: Power (W) = Work (J) ÷ Time (s). Memorize the unit: Watt = Joule per second.
• For lifting problems: First get Work = mgh, then get Power = Work ÷ time.
• Higher wattage = more energy consumed per second = higher electricity bill for the same usage time.

Key Points

• POWER is the RATE of doing work or using energy. It tells us HOW FAST work is done, not HOW MUCH work is done.
• Formula: P = W/t (Power = Work divided by Time). Unit: WATT (W), where 1 W = 1 J/s.
• Two people can do the SAME amount of work but have DIFFERENT power if one finishes faster. The faster person has GREATER power.
• Electrical appliances are rated in watts: a 1000 W electric iron uses energy 10 times faster than a 100 W light bulb.
• The kilowatt (kW) = 1000 W, and kilowatt-hour (kWh) is the unit used in your electric bill. 1 kWh = 3,600,000 J.
• Human power: An average person can sustain about 100 W of power output. Olympic athletes can produce much more for short bursts.
• Philippine context: Electric bills from Meralco or local electric cooperatives are measured in kWh – this is energy, NOT power.

Definitions

Section Title

Power

Common Mistakes

• Confusing power (W) with energy (J). Power is the RATE; energy is the AMOUNT.
• Using minutes or hours instead of SECONDS when computing power. Always convert time to seconds.
• Thinking that more powerful means more work was done. Two students climbing the same stairs do the same work, but the faster one has more power.
• Forgetting to first compute work (W = Fd or W = mgh) before dividing by time to get power.

Formulas

Exam Tips

• LET classic question: 'A teaspoon and a pot of boiling water at 100°C – which has more heat?' Answer: the POT, because it has more mass and more particles releasing thermal energy.
• Heat flows from HIGH temperature to LOW temperature – this is the direction of natural heat transfer (Second Law of Thermodynamics, simplified).
• Normal body temperature: 37°C. This may appear in health/science integration questions.
• Remember: Temperature = HOW HOT (intensive). Heat = HOW MUCH thermal energy transferred (extensive).

Key Points

• TEMPERATURE measures the AVERAGE kinetic energy of particles in a substance (how fast particles move). It tells us HOW HOT or COLD a substance is.
• HEAT is thermal energy that is TRANSFERRED from a hotter object to a cooler object. It is the total thermal energy in transit.
• Key distinction: A cup and a pot of boiling water are at the SAME TEMPERATURE (100°C), but the pot contains FAR MORE HEAT because it has more particles.
• Heat ALWAYS flows from a region of HIGHER temperature to LOWER temperature until THERMAL EQUILIBRIUM is reached (both at the same temperature).
• Temperature scales: Celsius (°C), Fahrenheit (°F), Kelvin (K). Water freezes at 0°C / 32°F and boils at 100°C / 212°F at sea level.
• Kelvin scale: 0 K = Absolute Zero (the lowest possible temperature, where particle motion theoretically stops). Conversion: K = °C + 273.
• Units: Temperature is in °C, °F, or K (intensive property – does not depend on amount). Heat is in Joules or Calories (extensive property – depends on amount of matter).
• Temperature is an INTENSIVE property (independent of the amount of substance); heat is an EXTENSIVE property (depends on the amount).

Definitions

Section Title

Heat versus Temperature

Common Mistakes

• Saying 'the pot has a higher temperature than the cup when both are boiling' – WRONG. They have the SAME temperature (100°C) but different amounts of heat.
• Confusing the units: temperature is in °C/K/°F; heat is in JOULES (or calories). They are not interchangeable.
• Saying heat flows from cold to hot – WRONG. Heat always flows from HOT to COLD.
• Forgetting to add 273 (not 237!) when converting Celsius to Kelvin.

Exam Tips

• Only ONE mode needs no medium: RADIATION. Memorize this. The Sun heats Earth by radiation through the vacuum of space.
• Metal = CONDUCTOR. Wood, plastic, rubber = INSULATORS. The LET loves to ask why pot handles are made of wood.
• Sea breeze and land breeze = CONVECTION. Weather and ocean currents = CONVECTION.
• If the LET asks 'how does heat from the Sun reach Earth?' – the answer is always RADIATION (not conduction or convection, because space is a vacuum).

Key Points

• Heat transfers through three mechanisms: CONDUCTION, CONVECTION, and RADIATION.
• CONDUCTION: Transfer of heat through DIRECT CONTACT between particles. The particles vibrate and pass energy to neighbors without the particles themselves moving from place to place. Requires a MEDIUM (solid, liquid, gas – but best in solids).
• CONVECTION: Transfer of heat through the MOVEMENT OF FLUIDS (liquids or gases). Hot fluid rises (less dense), cool fluid sinks (denser), creating a convection current. Requires a FLUID MEDIUM.
• RADIATION: Transfer of heat through ELECTROMAGNETIC WAVES. Does NOT need a medium – it can travel through a vacuum. This is how the Sun's energy reaches Earth across the vacuum of space.
• CONDUCTORS are materials that transfer heat quickly (metals like iron, copper, aluminum). INSULATORS transfer heat slowly (wood, plastic, cloth, air, styrofoam).
• Why pot handles are made of wood or plastic: they are INSULATORS that protect the cook's hand from conducted heat.
• Convection explains land and sea breezes: During the day, land heats up faster, air above it rises, sea air rushes in (sea breeze). At night, the reverse occurs (land breeze).
• Global wind patterns, ocean currents, and even the movement of magma in Earth's mantle are large-scale convection.

Definitions

Section Title

Heat Transfer Modes

Common Mistakes

• Saying convection works in solids – WRONG. Convection requires FLUIDS (liquids and gases only). Solids transfer heat by conduction.
• Saying radiation needs a medium – WRONG. Radiation is the ONLY mode that works without matter (through vacuum).
• Confusing conduction (particles vibrate in place and pass energy) with convection (particles physically move and carry energy).
• Forgetting that air is a poor conductor (good insulator) – this is why styrofoam and wool (which trap air) are effective insulators.

Formulas

Exam Tips

• Memorize: Sound = LONGITUDINAL; Light = TRANSVERSE. These are almost guaranteed LET test points.
• v = f × λ is the most important wave formula. If you know two of the three quantities, you can find the third.
• Larger AMPLITUDE → more energy → LOUDER sound or BRIGHTER light.
• Higher FREQUENCY → higher PITCH (sound) or towards VIOLET/BLUE end of spectrum (light).
• Period (T) and frequency (f) are always reciprocals: T = 1/f. If f = 2 Hz, T = 0.5 s.

Key Points

• A WAVE is a disturbance that transfers ENERGY from one place to another WITHOUT transferring matter. A floating coconut shell bobs up and down in water waves but does not travel across the ocean with the wave.
• TRANSVERSE WAVE: Particles vibrate PERPENDICULAR (at 90°) to the direction the wave travels. Examples: light, water surface waves, a shaken rope.
• LONGITUDINAL WAVE: Particles vibrate PARALLEL (in the same direction) to the wave's travel. They create alternating areas of COMPRESSION (crowded particles) and RAREFACTION (spread-out particles). Example: sound waves.
• KEY WAVE PROPERTIES: Wavelength (λ), Frequency (f), Amplitude (A), Period (T), Speed (v).
• WAVELENGTH (λ): Distance between two identical points in a wave (e.g., crest to crest, or compression to compression). Unit: meters (m).
• FREQUENCY (f): Number of complete waves (cycles) per second. Unit: HERTZ (Hz). 1 Hz = 1 wave per second.
• AMPLITUDE (A): Maximum displacement of a particle from its rest position. Related to ENERGY carried by the wave (larger amplitude = more energy = louder sound or brighter light).
• PERIOD (T): Time for ONE complete wave cycle. T = 1/f (Period and frequency are reciprocals).
• WAVE SPEED equation: v = f × λ (Speed = Frequency × Wavelength).
• Increasing frequency while speed is constant → wavelength DECREASES (they are inversely proportional).

Definitions

Section Title

Waves and Their Properties

Common Mistakes

• Confusing transverse and longitudinal: SOUND = longitudinal; LIGHT = transverse. This is a very common error.
• Thinking waves transfer matter – waves transfer ENERGY, not matter (the medium particles vibrate but do not move with the wave).
• Mixing up amplitude and wavelength in diagrams. Amplitude is the HEIGHT of the wave; wavelength is the HORIZONTAL LENGTH of one cycle.
• Using v = f + λ instead of v = f × λ (it is multiplication, not addition).
• Confusing period (time per wave) and frequency (waves per second). They are RECIPROCALS: T = 1/f.

Formulas

Exam Tips

• Sound cannot travel in vacuum → no sound in space. LET fact: 'In space, no one can hear you scream.'
• Speed ranking: Solid > Liquid > Gas. Sound is FASTEST in solids.
• Pitch → Frequency. Loudness → Amplitude. These two pairs are non-negotiable LET knowledge.
• For echo distance problems: d = (v × t) ÷ 2. The '÷ 2' accounts for the round trip.

Key Points

• Sound is a LONGITUDINAL MECHANICAL wave produced by vibrations. It requires a MEDIUM to travel – it CANNOT travel through a vacuum (empty space).
• Sound travels fastest in SOLIDS, slower in LIQUIDS, and slowest in GASES. The closer and more tightly packed the particles, the faster sound travels.
• In air at room temperature (~25°C), sound travels at approximately 340 m/s (about 344 m/s at 20°C).
• PITCH depends on FREQUENCY: High frequency = High pitch. A soprano voice has a higher frequency than a bass voice.
• LOUDNESS depends on AMPLITUDE: Higher amplitude = Louder sound. Measured in DECIBELS (dB).
• ECHO: The reflection of sound off a surface. Used by bats (echolocation), ships (SONAR), and ultrasound medical imaging.
• ULTRASOUND: Sound with frequency above 20,000 Hz (above human hearing range). Used in prenatal scans, cleaning devices, and sonar.
• Human hearing range: approximately 20 Hz to 20,000 Hz.
• Philippine connection: Decibel levels matter for child protection – RA 7610 indirectly connects to protecting children from environmental hazards including harmful noise levels in schools.

Definitions

Section Title

Sound

Common Mistakes

• Saying sound travels fastest in air – WRONG. Sound travels FASTEST in SOLIDS (e.g., steel), slower in water, and slowest in air.
• Thinking sound can travel in space – WRONG. Space is a vacuum; sound needs a MEDIUM.
• Confusing pitch (frequency) with loudness (amplitude). Plucking a guitar string HARDER changes loudness; plucking a SHORTER or TIGHTER string changes pitch.
• Not dividing by 2 when calculating distance to a sound source using echo (sound travels TO the surface and BACK).

Formulas

Exam Tips

• EM Spectrum order (increasing frequency): Radio, Microwave, Infrared, VISIBLE, Ultraviolet, X-ray, Gamma. Mnemonic: 'Raging Martians Invade Venus Using X-ray Guns.'
• ROYGBIV: Red (longest wavelength, lowest frequency in visible) → Violet (shortest wavelength, highest frequency in visible).
• Speed of light ≈ 3 × 10⁸ m/s. Speed of sound ≈ 340 m/s. Light is about 1,000,000 times faster than sound.
• Lightning before thunder = light faster than sound. This is a classic real-world application the LET uses.
• A red object absorbs all colors EXCEPT red, which it reflects back to your eyes.

Key Points

• Light is a TRANSVERSE ELECTROMAGNETIC wave. Unlike sound, it does NOT need a medium – it can travel through vacuum. Speed in a vacuum: ≈ 300,000,000 m/s or 3 × 10⁸ m/s (the fastest speed in the universe).
• VISIBLE LIGHT is just one band in the ELECTROMAGNETIC SPECTRUM. In order of INCREASING frequency (and decreasing wavelength): Radio → Microwave → Infrared → Visible Light → Ultraviolet → X-ray → Gamma Ray.
• VISIBLE SPECTRUM (in order of decreasing wavelength / increasing frequency): Red, Orange, Yellow, Green, Blue, Indigo, Violet (ROYGBIV). Red = longest wavelength, lowest frequency. Violet = shortest wavelength, highest frequency.
• WHITE LIGHT is a MIXTURE of all visible colors. A prism or raindrops separate white light into its component colors (DISPERSION), producing a RAINBOW.
• Objects appear colored because they REFLECT certain wavelengths and ABSORB the rest. A red shirt reflects red light and absorbs all other colors.
• Opaque black objects ABSORB all light (reflect none); white objects REFLECT all light (absorb none). This is why dark-colored clothing feels hotter in sunlight.
• Light travels MUCH faster than sound (3 × 10⁸ m/s vs. 340 m/s). This is why lightning is SEEN before its THUNDER is HEARD.
• Phenomena: REFLECTION (bouncing off surfaces), REFRACTION (bending when passing between media), DIFFRACTION (bending around edges), ABSORPTION.
• The LET may test the approximate time for sunlight to reach Earth: about 8 minutes (distance ÷ speed of light).

Definitions

Section Title

Light

Common Mistakes

• Saying light is a longitudinal wave – WRONG. Light is a TRANSVERSE electromagnetic wave.
• Confusing which end of the visible spectrum has higher frequency: VIOLET (not red) has the HIGHEST frequency and SHORTEST wavelength.
• Saying light needs a medium – WRONG. Light is an EM wave and needs NO medium (travels freely through vacuum).
• Confusing the color an object REFLECTS with the color it ABSORBS. A green leaf REFLECTS green, ABSORBS other colors.