LET Elementary Physics — Work, Energy, Heat and WavesSummary

Work is done when a force moves an object through a distance in the direction of that force. The unit is the joule (J), where 1 joule = 1 newton-meter. A person pushing a crate 5 meters with 20 newtons of force does W = 20 × 5 = 100 joules of work. Critically, if there is no movement (distance = 0), no work is done, no matter how much force is applied—this is a common LET test point. Similarly, if a force acts perpendicular to the motion (like gravity on a bag carried horizontally), that force does zero work. For lifting, work against gravity is W = m g h, where m is mass in kilograms, g is gravitational acceleration (9.8 m/s²), and h is height in meters.

Kinetic energy is the energy of motion; a moving object has KE. A 2 kg ball rolling at 3 m/s has KE = ½ (2)(3²) = 9 joules. Potential energy is stored energy of position; a 5 kg object at a height of 10 m has PE = (5)(9.8)(10) = 490 joules. Note that kinetic energy depends on velocity squared: doubling the speed quadruples the kinetic energy. A 1000 kg car at 20 m/s has KE = ½ (1000)(400) = 200,000 joules, which is why high-speed car crashes are so much more destructive than low-speed ones. Other forms of energy include chemical (food, batteries), thermal, electrical, light, sound, nuclear, and elastic (springs).

Energy cannot be created or destroyed; it only transforms from one form to another. In a closed system, total energy remains constant. A falling ball illustrates this perfectly: at the top, it has maximum PE and zero KE; as it falls, PE converts to KE; just before landing, PE is zero and KE is at maximum. Ignoring air resistance, the KE at the bottom equals the starting PE. Setting them equal: ½ m v² = m g h gives v = √(2 g h). Dropping a ball from 5 m yields v = √(2 × 9.8 × 5) = √98 ≈ 9.9 m/s. Everyday examples: a battery converts chemical energy to electrical; a light bulb turns electrical to light and heat; photosynthesis converts light to chemical energy in plants; a hydroelectric dam converts water's PE to electrical energy. In real machines, some energy always becomes heat through friction, so no machine is 100% efficient—but this energy is not lost, only dispersed as low-grade thermal energy.

Power is the rate of doing work or using energy—how fast, not how much. The unit is the watt (W), where 1 watt = 1 joule per second. A motor doing 100 joules in 5 seconds has power P = 100 ÷ 5 = 20 watts. Two students climbing the same staircase (same work) but one takes 6 seconds and the other takes 3 seconds: both do the same work (1500 joules), but the faster student has twice the power (250 watts versus 150 watts). Electrical appliances are rated in watts for exactly this reason: a 1000 W iron consumes energy ten times faster than a 100 W light bulb. Power also relates to energy: Energy = Power × Time, so a 100 W bulb burning for 10 hours uses 100 × 10 = 1000 watt-hours = 1 kilowatt-hour (kWh).

These are constantly confused and the LET specifically tests the difference. Temperature measures the average kinetic energy of particles and is an intensive property (does not depend on the amount of matter). Unit: degrees Celsius (°C), Kelvin (K), or Fahrenheit (°F). Heat is the total thermal energy transferred from one object to another and is an extensive property (depends on the amount of matter). Unit: joule (J) or calorie (cal). A cup of boiling water and a large pot of boiling water are both at 100°C at sea level, but the pot contains far more heat because it has more particles. Heat always flows from a hotter object to a colder one until thermal equilibrium is reached. Water freezes at 0°C (32°F) and boils at 100°C (212°F) at sea level; 0°C = 273 K on the Kelvin scale, which starts at absolute zero.

Heat moves by three distinct mechanisms. Conduction is direct contact transfer; particles vibrate and pass energy without moving overall. A metal spoon heating in hot soup is conduction. Metals are good conductors (aluminum, copper); wood, plastic, and air are insulators, which is why pot handles are wood or plastic. Convection is the movement of heated fluid (liquid or gas) carrying energy along. Boiling water circulates by convection; sea breezes occur because land heats faster than ocean, creating a pressure difference that drives convection. Radiation is the transfer of energy by electromagnetic waves; it needs no medium and is the only mode that works through the vacuum of space. This is how the Sun's warmth reaches Earth. A lit candle radiates heat to nearby objects even in a vacuum chamber.

A wave is a disturbance that transfers energy from one place to another without transferring matter. A cork on water ripples bobs up and down but does not travel across the pond. Two types: transverse waves have particles vibrating perpendicular to the wave's direction (light, water surface waves, a shaken rope); longitudinal waves have particles vibrating parallel to the direction, forming compressions and rarefactions (sound waves). Key properties: wavelength (λ) is the distance between matching points (crest to crest), in meters; frequency (f) is the number of waves per second in hertz (Hz); amplitude is maximum displacement from rest, relating to energy (louder sound, brighter light); period (T) is time for one complete wave, T = 1/f; speed (v) is how fast the wave travels. The wave equation is v = f × λ. If frequency is 2 Hz and wavelength is 3 m, then v = 2 × 3 = 6 m/s. For sound at 340 m/s with frequency 170 Hz, wavelength = 340 ÷ 170 = 2 meters.

Sound is a longitudinal wave produced by vibrations and requires a medium (solid, liquid, or gas) to travel. There is no sound in a vacuum because there are no particles to vibrate and transmit the disturbance. Sound travels fastest in solids (roughly 5000 m/s in steel), slower in liquids (about 1500 m/s in water), and slowest in gases (about 340 m/s in air at room temperature) because tightly packed particles transmit vibrations more efficiently. Pitch depends on frequency: high frequency yields high pitch; low frequency yields low pitch. Loudness depends on amplitude, measured in decibels (dB). Echoes are reflected sound; sonar uses reflected sound pulses to measure distance underwater and in air. The difference in arrival time between lightning (light, instant) and thunder (sound, 340 m/s) estimates storm distance: if thunder arrives 3 seconds after lightning, the storm is 340 × 3 ≈ 1020 meters away.

Light is a transverse electromagnetic wave. Unlike sound, light travels through a vacuum (the only medium it needs is to exist in; it can also travel through transparent materials). Its speed in a vacuum is approximately 3 × 10⁸ m/s (300,000,000 m/s), the fastest speed in the universe. Visible light is one band of the electromagnetic spectrum. In order of increasing frequency: radio → microwave → infrared → visible light → ultraviolet → X-ray → gamma ray. Within visible light, colors separate by wavelength and frequency: red has the longest wavelength and lowest frequency; orange, yellow, green, blue, indigo, violet (ROYGBIV) follows; violet has the shortest wavelength and highest frequency. White light is a mixture of all colors; a prism or raindrops separate it, producing a rainbow. Objects have color because they reflect certain wavelengths and absorb the rest: a leaf looks green because it reflects green light and absorbs other colors. A blue shirt appears blue because it reflects blue wavelengths and absorbs others.

When work is done on an object (like pushing it or lifting it), energy is transferred to it. A worker pushing a crate transfers kinetic energy to the crate; lifting a box transfers gravitational potential energy. Conversely, an object with energy can do work: a compressed spring releases its elastic potential energy to push an object.

On a frictionless roller coaster, as an object descends, PE decreases and KE increases at the same rate, so total mechanical energy stays constant. At the bottom, all starting PE has become KE. This interplay explains swinging pendulums, falling objects, and sliding blocks.

Power = Work / Time = Energy / Time. A high-power appliance transfers or uses energy quickly (high wattage), while a low-power appliance does so slowly. An electric kettle rated at 2000 W heats water much faster than a 500 W heating plate.

Temperature is the average kinetic energy of particles in an object. Heat flows spontaneously from high-temperature (high average particle energy) to low-temperature (low average particle energy) objects until thermal equilibrium (equal temperature) is reached. Heat is the energy transferred; temperature is the property that drives the transfer.

Conduction (direct contact, no bulk movement) predominates in solids; convection (bulk movement of fluid) dominates in liquids and gases; radiation (EM waves, no medium) dominates across vacuums. Real heat transfer often involves all three: a pot on a stove heats by conduction through the base, convection circulates the water, and radiation from the stovetop heats nearby objects.

Sound and light are waves that transport energy from a source to a receiver. A speaker produces sound waves that carry acoustic energy to your ear; the Sun produces light waves that carry electromagnetic energy to Earth. The medium (air for sound) or even vacuum (for light) transmits the disturbance, not the source material itself.

In a given medium, wave speed is constant. If frequency increases, wavelength must decrease proportionally so that their product (speed) remains the same. High-frequency sound (high pitch) has short wavelength; low-frequency sound (low pitch) has long wavelength. This relationship holds for light and all waves.

Sound is a mechanical longitudinal wave that requires a medium (solid, liquid, or gas) to travel and is fastest in solids. Light is an electromagnetic transverse wave that travels fastest in a vacuum (in fact, it cannot travel through all media—some are opaque). This fundamental difference explains why we hear nothing from space but see starlight.

Levers, pulleys, ramps, and screws are simple machines that reduce the force needed to lift or move an object. However, they do not reduce the work (force × distance remains constant): a ramp allows a child to push with less force but over a longer distance, doing the same total work. A pulley system multiplies force but requires pulling more rope. Teachers use this to explain why machines are useful (they make tasks easier by reducing force) but still obey the conservation of energy. Grade 3 pupils investigate this by comparing the effort needed to lift a bucket directly versus using a pulley.

Pupils learn where energy comes from: food (chemical → body heat and motion), batteries (chemical → electrical), solar panels (light → electrical), windmills (kinetic → electrical), and dams (gravitational PE → kinetic → electrical). Teachers emphasizing conservation of energy help pupils understand that we do not create energy but harness and transform it. A lesson on 'Where does the Sun's energy go?' traces light energy from the Sun being absorbed by plants (photosynthesis converts it to chemical energy in food), eaten by animals (chemical → body heat and motion), and eventually released as heat back to space.

Grade 5 pupils often investigate melting, boiling, and freezing. A teacher distinguishing heat from temperature explains why ice at 0°C remains at 0°C even as you add heat during melting—the added thermal energy breaks bonds rather than raising temperature. After melting, further heat raises the water's temperature. Similarly, water at 100°C boiling on a stove does not get hotter no matter how much heat is applied; instead, the heat converts water to steam. These investigations directly demonstrate that heat and temperature are different, preparing pupils for formal science reasoning.

Conduction: a metal spoon in hot soup heats up because of direct contact (conduction). Convection: a heater in a room warms air that rises and circulates, so the room gradually warms. Radiation: you feel the Sun's warmth on your face without touching it, or warmth from a campfire across a distance. A teacher asking 'Why is the pot handle made of wood instead of metal?' develops pupils' understanding of insulators versus conductors. 'Why does hot air rise?' illustrates convection. 'Why do you feel warmth from the Sun even though space is empty?' introduces radiation.

Pupils use tuning forks, rulers struck at different speeds, or drums to hear how frequency affects pitch (high frequency = high pitch, low frequency = low pitch). Loudness investigations show that hitting a drum harder (larger amplitude) produces a louder sound. Pupils might measure the delay between lightning and thunder to estimate storm distance (dividing delay in seconds by 340 m/s). The principle that sound requires a medium can be demonstrated by a bell in a vacuum chamber: it flashes light but produces no sound. This aligns with Grade 4 BEC standard 'Sound and how it travels.'

Using prisms, pupils separate white light into the visible spectrum (ROYGBIV), observing that red bends least and violet bends most. They learn that objects have color because they reflect certain wavelengths: a blue object reflects blue light and absorbs the rest. Pupils investigate that when different colors of light mix, they create new colors (additive color mixing), while pigments mixed together subtract wavelengths. A lesson on shadows and light introduces straight-line propagation: light travels in straight lines until it hits a surface, which is why shadows have sharp edges. This connects to Grade 4 BEC standard 'Properties and uses of light.'

Pupils learn that appliances are rated in watts: a 1000 W electric kettle uses energy ten times faster than a 100 W light bulb. The concept of kilowatt-hours (kWh) helps explain electricity bills: if a 1000 W air conditioner runs for 5 hours, it consumes 1000 × 5 = 5000 watt-hours = 5 kWh. Teachers connect this to environmental responsibility: using less power or using appliances fewer hours reduces energy consumption and electricity costs. This ties to Grade 6 Science standard on 'Energy resources and conservation.'

Drivers' education and safety lessons benefit from understanding that KE = ½ m v², which depends on velocity squared. A car at 60 kph has four times the kinetic energy of the same car at 30 kph, requiring four times the braking distance and resulting in far more severe crashes. This principle justifies school zones and speed limits. A teacher explaining this to Grade 5–6 pupils (age-appropriately) helps them understand why running into traffic at high speed is much more dangerous than at low speed, supporting DepEd's advocacy for child safety (RA 7610: Special Protection of Children Against Child Abuse, Exploitation and Discrimination Act).

Energy conservation and renewable sources (solar, wind, hydro) are central to Grade 5–6 Science standards. Understanding that energy transforms but is not destroyed, and that no process is 100% efficient (some energy always becomes heat), justifies investing in renewable energy and energy-saving technologies. A teacher discussing why solar panels convert sunlight to electricity (light energy → electrical energy) or why dams harness falling water (PE → KE → electrical) grounds abstract physics in real-world sustainability. This aligns with DepEd's environmentally integrated curriculum.