- Mechanical advantage
- The factor by which a simple machine multiplies your input force — output force ÷ input force. A machine never gives free work: force saved is paid back in distance.
- Fixed pulley
- A pulley anchored in place. It only changes the direction of the force (pull down to raise up) and has a mechanical advantage of 1 — you still pull the full weight.
- Movable pulley
- A pulley that travels with the load on two rope strands, so each strand carries half. Mechanical advantage of 2 — you pull half the weight but twice the rope.
- Block and tackle
- A system of fixed and movable pulleys. Its mechanical advantage equals the number of rope strands directly supporting the movable block.
- Pulley mechanical advantage rule
- Count the rope strands that support the load; that number is the mechanical advantage. Effort = load ÷ number of supporting strands.
- Lever
- A rigid bar that pivots on a fulcrum. Effort × effort-arm = load × load-arm, so a longer effort arm multiplies your force.
- Fulcrum
- The fixed pivot point a lever turns on.
- Law of the lever
- Effort × effort-arm = load × load-arm. To balance a lever, the weight-times-distance must be equal on both sides of the fulcrum.
- Class 1 lever
- Fulcrum in the middle, between effort and load. Examples: crowbar, seesaw, scissors, pliers. Can multiply force or just change direction.
- Class 2 lever
- Load in the middle, between fulcrum and effort. Always multiplies force (MA > 1). Examples: wheelbarrow, nutcracker, bottle opener.
- Class 3 lever
- Effort in the middle, between fulcrum and load. Trades force for speed and range (MA < 1). Examples: tweezers, fishing rod, the human forearm.
- Lever mechanical advantage
- Effort arm length ÷ load arm length. A 24-inch effort arm over a 6-inch load arm gives a mechanical advantage of 4.
- Gear ratio
- The driven gear's teeth divided by the driver gear's teeth. A higher ratio means the driven gear turns slower with more torque.
- Meshed gear direction
- Two externally meshed gears always turn in opposite directions — if one turns clockwise, the other turns counterclockwise.
- Idler gear
- A gear placed between the driver and driven gear. It reverses the direction of rotation but does NOT change the overall gear ratio.
- Larger vs smaller gear
- The larger gear turns slower but with more torque; the smaller gear turns faster with less torque.
- Belt-driven pulleys
- An open (uncrossed) belt makes both pulleys turn the same direction; a crossed (figure-eight) belt reverses the driven pulley. The smaller pulley turns faster.
- Wheel and axle
- A simple machine in which a large wheel turns a smaller axle (or vice versa). Mechanical advantage = wheel radius ÷ axle radius.
- Inclined plane
- A ramp that reduces the force to raise a load by spreading the work over a longer distance. Mechanical advantage = slope length ÷ vertical height.
- Wedge
- A moving inclined plane (two ramps back to back). It converts a forward push into a strong sideways splitting or gripping force. Example: an axe, a doorstop.
- Screw
- An inclined plane wrapped around a cylinder. Its pitch is the distance it advances along its axis in one full turn.
- Worm-and-wheel drive
- A worm gear meshing a toothed wheel. It produces a large speed reduction and often self-locks, resisting being driven backward.
- Torque
- A turning or twisting force, equal to the applied force times the distance from the pivot (the lever arm). A longer wrench makes more torque for the same push.
- Inertia
- An object's resistance to a change in its motion. It increases with mass, so a heavier object is harder to start, stop, or turn.
- Newton's first law
- An object stays at rest, or keeps moving in a straight line at constant speed, unless an unbalanced (net) outside force acts on it. The law of inertia.
- Centripetal force
- The inward force that holds an object on a circular path, pointing toward the center. Remove it and the object flies off straight along the tangent.
- Centrifugal effect
- The apparent outward shove felt in a turn. It is really inertia resisting the change in direction, not a real outward force.
- Resolving a force
- Splitting one angled force into two perpendicular parts — a horizontal component and a vertical component — that together produce the same effect.
- Resultant of two forces
- When two forces act at an angle, they add as vectors into one resultant that points between them and can exceed either single force.
- Friction
- A resisting force that acts opposite to the direction of motion (or attempted motion) between surfaces in contact.
- Flywheel
- A heavy spinning wheel that stores rotational energy. Its rotational inertia smooths out an engine's speed fluctuations between power strokes.
- Counterweight
- A weight added to a crane or elevator to balance the load, so the motor or operator only has to move the difference, reducing required force.
- Pascal's principle
- Pressure applied to a confined fluid is transmitted equally and undiminished in all directions throughout the fluid.
- Pressure
- Force per unit area: P = F ÷ A. The same force over a smaller area gives a higher pressure.
- Hydraulic force
- Force = pressure × area. A larger output piston area produces a larger force at the same pressure — how a hydraulic jack multiplies force.
- Hydraulic mechanical advantage
- Output piston area ÷ input piston area. A 6-times-larger output piston multiplies the input force by 6.
- Pressure and depth
- Fluid pressure increases with depth because the deeper fluid supports the weight of all the fluid above it. It depends on depth, not on the tank's width.
- Buoyant force
- The upward force a fluid exerts on a submerged object, equal to the weight of the fluid the object displaces (Archimedes' principle).
- Float, sink, or suspend
- If buoyant force > weight, the object floats; if buoyant force < weight, it sinks; if they are equal, it stays suspended at depth.
- Why a steel hull floats
- Its hollow shape displaces enough water for the buoyant force to equal its weight, while a solid steel block displaces too little water and sinks.
- Liquids are nearly incompressible
- Squeezing a confined liquid barely changes its volume — which is exactly why hydraulic systems transmit pressure instead of just squashing down.
- Check valve
- A valve that lets fluid flow in only one direction and blocks backflow. Forward pressure opens it; reverse pressure seats it shut.
- Acceleration of gravity
- Gravity gives every object the same downward acceleration regardless of mass, so without air resistance a heavy and a light object fall and land together.
- Terminal velocity
- The steady falling speed reached when air resistance grows to balance gravity; net force is then zero and the object stops accelerating.
- Velocity at the top of a throw
- An object thrown straight up has zero velocity at its highest point, even though gravity still acts on it and pulls it back down.
- Conduction
- Heat transfer through direct contact, as a metal spoon's handle warms in hot soup. Metals conduct heat well; foam and air are poor conductors.
- Convection
- Heat carried in a moving fluid: warm, less-dense fluid rises and cooler, denser fluid sinks, circulating heat through air or water.
- Radiation
- Heat carried as infrared energy through space without a medium, as a campfire warms your skin from across a yard.
- Insulator (heat)
- A material that slows heat flow, such as foam or trapped air. It keeps coffee warm by resisting conduction.
- Thermal expansion
- Most materials expand when heated and contract when cooled — why gaps are left in steel rail and a hot lid loosens on a glass jar.
- Conductor vs insulator (electricity)
- A conductor (copper) lets current flow with little resistance; an insulator (rubber, plastic) resists current flow and prevents shocks.
- Series circuit
- A circuit with one current path. If one bulb burns out, the circuit breaks and every bulb goes dark.
- Parallel circuit
- A circuit where each branch has its own path. If one bulb fails, the others keep working.
- Fuse
- A safety device that melts and breaks a circuit when the current gets too high, cutting power before the wiring overheats.
- Center of gravity
- The single point where an object's weight can be treated as acting. An object balances when supported directly under it.
- Stability rule
- An object stays upright while its center of gravity is above its base of support; it tips when the center of gravity passes beyond the base's edge.
- Low center of gravity + wide base
- The most stable arrangement. Race cars ride low and tractors use a wide wheelbase to resist tipping.
- Simple machine
- A basic device that changes the size or direction of a force to make work easier. The six classic types are the lever, pulley, wheel and axle, inclined plane, wedge, and screw.
- First-class lever examples
- Seesaw, crowbar, scissors, pliers, and a claw hammer pulling a nail — all have the fulcrum sitting between the effort and the load.
- Second-class lever examples
- Wheelbarrow, bottle opener, nutcracker, and a hand-truck dolly — the load sits between the fulcrum and the effort, always giving a mechanical advantage greater than 1.
- Third-class lever examples
- Tweezers, tongs, a broom, a fishing rod, and the human forearm — the effort is applied between the fulcrum and the load, trading force for greater speed and range of motion.
- Effort arm
- The distance from the fulcrum to the point where the effort (input force) is applied on a lever. A longer effort arm gives more mechanical advantage.
- Load arm
- The distance from the fulcrum to the load (resistance) on a lever. A shorter load arm relative to the effort arm increases mechanical advantage.
- Balancing a seesaw
- A lighter person sits farther from the pivot and a heavier person sits closer, so that weight × distance is equal on both sides of the fulcrum.
- Compound pulley
- A pulley system combining fixed and movable pulleys to multiply force. Mechanical advantage equals the number of rope sections supporting the load.
- Two-pulley system
- One fixed pulley to change direction plus one movable pulley to share the load. Two supporting strands give a mechanical advantage of 2, halving the effort.
- Trade-off of a pulley system
- A pulley with a mechanical advantage of 4 cuts your pulling force to one-quarter, but you must pull four times as much rope to raise the load the same height.
- Inclined plane mechanical advantage
- MA = ramp length ÷ vertical height. A 12-foot ramp rising 3 feet has a mechanical advantage of 4, so the push needed is one-quarter of the load's weight.
- Longer ramp effect
- A longer, gentler ramp reduces the force needed to raise a load, but the load must travel a greater distance — the same total work is spread out.
- Screw pitch
- The distance a screw advances in one full turn. A finer pitch (threads closer together) gives more mechanical advantage but requires more turns.
- Screw thread spacing
- Threads spaced closer together let a screw move less per turn, multiplying force more — like a tighter inclined plane wrapped around the shaft.
- Wedge mechanical advantage
- MA = length of the slope ÷ width of the thick end. A long, thin wedge splits with far more force than a short, fat one.
- Steering wheel as a wheel and axle
- The large steering wheel turns a thin steering column, multiplying the driver's force. The bigger the wheel relative to the axle, the greater the mechanical advantage.
- Doorknob as a wheel and axle
- The wide knob is the wheel and the latch spindle is the axle. Turning the large knob produces enough force to turn the small spindle and retract the latch.
- Gear train
- A series of meshed gears that transmits motion and torque. Each external mesh reverses rotation direction, and the overall ratio depends only on the first and last gears.
- Driver gear
- The gear connected to the power source that drives the rest of the train. Its tooth count compared to the driven gear sets the gear ratio.
- Driven gear
- The output gear turned by the driver. A larger driven gear turns slower with more torque; a smaller driven gear turns faster with less torque.
- Speed-reducing gears
- A small gear driving a large gear reduces output speed but increases torque — useful for heavy lifting or slow, powerful motion.
- Speed-increasing gears
- A large gear driving a small gear raises output speed but lowers torque — useful where fast rotation matters more than force.
- Gear ratio example
- A 12-tooth driver turning a 36-tooth driven gear gives a 3:1 ratio: the big gear turns one-third as fast but with three times the torque.
- Bevel gears
- Gears with angled teeth that transmit rotation between shafts meeting at an angle, usually 90 degrees — as in a hand drill or a car's differential.
- Rack and pinion
- A round pinion gear meshing a flat toothed bar (the rack). It converts rotation into straight-line motion, as in steering systems.
- Chain and sprocket drive
- A chain links two toothed sprockets so they always turn the same direction. The smaller sprocket spins faster, like the gears on a bicycle.
- Larger pulley on a belt
- On a belt drive, the larger pulley turns slower than the smaller one. Speed is inversely proportional to pulley diameter.
- Cam
- A rotating oval or lobed part that pushes a follower up and down, converting rotary motion into repeated linear motion — as in an engine's valve lifters.
- Crank and slider
- A linkage that turns rotary motion into back-and-forth straight-line motion, like the crankshaft and piston in an engine.
- Ratchet and pawl
- A toothed wheel and a catch that lets motion go one way only and locks against the other — found in winches, socket wrenches, and tie-down straps.
- Efficiency of a machine
- Useful output work ÷ input work, always less than 100% because friction wastes some energy as heat. No real machine is perfectly efficient.
- Conservation of work in machines
- A machine cannot create energy. What you gain in force you give back in distance, so input work always equals output work plus friction losses.
- Velocity ratio
- The distance the effort moves divided by the distance the load moves. For an ideal machine it equals the mechanical advantage.
- Jackscrew
- A screw geared to lift heavy loads, like a car jack. Its fine thread pitch gives huge mechanical advantage, turning a small crank force into a large lift.
- Lug wrench leverage
- A longer lug wrench applies more torque to a bolt for the same hand force, because torque equals force times the distance from the pivot.
- Pry bar
- A first-class lever used to lift or move heavy objects. A longer bar and a closer fulcrum to the load both increase the lifting force.
- Newton's second law
- The acceleration of an object equals the net force divided by its mass: a = F ÷ m. More force gives more acceleration; more mass gives less.
- Newton's third law
- For every action there is an equal and opposite reaction. Forces always come in pairs acting on two different objects.
- Force
- A push or pull that can start, stop, speed up, slow down, or change the direction of motion. Measured in pounds or newtons.
- Net force
- The single force left after adding all forces on an object. If it is zero the object stays at rest or moves at constant velocity; if not, the object accelerates.
- Balanced forces
- Forces that cancel out to a net force of zero. The object's motion does not change — it stays still or keeps moving steadily.
- Unbalanced forces
- Forces that do not cancel, leaving a net force. They cause an object to accelerate in the direction of the net force.
- Equilibrium
- The state in which all forces and all torques on an object are balanced, so it neither accelerates nor rotates.
- Static friction
- The resisting force that keeps a stationary object from sliding. It rises to match the push until the object finally breaks free and moves.
- Kinetic friction
- The friction between surfaces already sliding past each other. It is usually a bit less than the static friction needed to start the motion.
- Rolling friction
- The small resistance a wheel or ball meets as it rolls. It is far less than sliding friction, which is why wheels and bearings ease motion.
- Reducing friction
- Lubricants, smoother surfaces, wheels, rollers, and ball bearings all cut friction, letting machines run easier and waste less energy as heat.
- Friction and weight
- Friction increases when an object presses harder on a surface, so a heavier load is harder to slide. It does not depend much on contact area.
- Momentum
- An object's mass times its velocity. A heavy or fast-moving object has large momentum and is hard to stop or deflect.
- Conservation of momentum
- In a collision with no outside forces, the total momentum before equals the total momentum after — momentum is transferred, not lost.
- Impulse
- A force applied over a time interval that changes momentum. A longer contact time, as in airbags, lowers the peak force during a stop.
- Hooke's law
- The force a spring exerts is proportional to how far it is stretched or compressed: F = k × x. Stretch it twice as far and it pulls back twice as hard.
- Spring constant
- The stiffness rating (k) of a spring — the force needed per unit of stretch. A high spring constant means a stiff, hard-to-deform spring.
- Acceleration
- The rate at which velocity changes over time. Speeding up, slowing down, or turning all count as acceleration because velocity includes direction.
- Velocity
- Speed in a given direction. Two objects at the same speed but heading different ways have different velocities.
- Speed
- How fast an object moves, found as distance traveled ÷ time taken. It has size but no direction.
- Weight versus mass
- Mass is the amount of matter in an object and never changes; weight is the pull of gravity on that mass and varies with location, such as on the Moon.
- Action-reaction pair
- When you push on a wall, the wall pushes back on you with equal force. A rocket pushes gas down and the gas pushes the rocket up.
- Centripetal force source
- Whatever keeps an object curving — tension in a string, friction on tires, or gravity on a satellite — all pull inward toward the center of the circle.
- Torque and wrench length
- A longer wrench produces more torque for the same hand force, because torque = force × lever-arm length. Doubling the length doubles the turning effect.
- Net torque
- The combined turning effect of all torques on an object. When clockwise and counterclockwise torques are equal, the object does not rotate.
- Tension
- The pulling force carried along a rope, cable, or chain. It is the same throughout an ideal rope and acts away from the object at each end.
- Compression force
- A squeezing force that pushes the ends of an object toward each other, as a column carries the weight pressing down on it.
- Normal force
- The support force a surface pushes back with, perpendicular to the surface. It balances the part of an object's weight pressing into the surface.
- Free fall
- Motion under gravity alone, with no air resistance. Every object accelerates downward at the same rate regardless of weight.
- Projectile motion
- The curved path of a thrown object. Its horizontal motion stays steady while gravity pulls it down, producing an arc.
- Pendulum
- A weight swinging on a string or rod. Its swing time depends on length, not on the weight — a longer pendulum swings more slowly.
- Vibration and resonance
- Pushing a swing in time with its natural rhythm makes it swing higher. Matching a force to an object's natural frequency builds large vibrations.
- Drag (air resistance)
- The backward force air exerts on a moving object. It grows with speed and with frontal area, which is why streamlined shapes move easier.
- Lever arm
- The perpendicular distance from a pivot to the line of an applied force. The longer it is, the more torque that force produces.
- Stable versus unstable equilibrium
- In stable equilibrium an object returns to its position after a small nudge; in unstable equilibrium a small nudge makes it tip or roll away.
- Banking a curve
- Tilting a road or track inward lets part of the surface's push supply centripetal force, helping vehicles turn without relying only on tire friction.
- Gravity
- The attractive force pulling objects toward Earth's center, giving weight and an acceleration of about 9.8 meters per second squared near the surface.
- Applied force
- Any push or pull a person or machine directly exerts on an object, such as pushing a cart or pulling a rope.
- Mass and acceleration
- For the same force, a heavier object accelerates less than a lighter one because acceleration equals force divided by mass.
- Hydraulics
- The use of confined liquids to transmit and multiply force. Because liquids barely compress, pressure applied at one piston appears fully at another.
- Hydraulic press
- A device where a small force on a small piston creates pressure that acts on a large piston, producing a much larger output force.
- Hydraulic trade-off
- The large output piston of a hydraulic system makes more force but moves a shorter distance, so the small input piston must travel much farther.
- Archimedes' principle
- An object in a fluid is pushed up by a buoyant force equal to the weight of the fluid it displaces.
- Displacement
- The volume of fluid pushed aside by a submerged object. More displaced fluid means a greater buoyant force lifting the object.
- Bernoulli's principle
- In a moving fluid, faster flow means lower pressure. Fast air over a curved wing lowers the pressure on top and helps create lift.
- Venturi effect
- When fluid speeds up through a narrowed pipe, its pressure drops. This pressure dip is used in carburetors and spray nozzles.
- Atmospheric pressure
- The weight of the air column pressing on everything at Earth's surface, about 14.7 pounds per square inch at sea level.
- Siphon
- A tube that carries liquid up over a barrier and down to a lower level. Once started, the heavier downward column keeps the flow going by itself.
- Suction pump
- A pump that lowers pressure above a liquid so atmospheric pressure pushes the liquid up the pipe. It can lift water only about 34 feet at most.
- Force pump
- A pump that pushes liquid up with a piston instead of relying on suction, letting it move fluid to far greater heights than a suction pump can.
- Density
- Mass per unit volume. An object less dense than a fluid floats in it; an object more dense sinks.
- Specific gravity
- The ratio of a material's density to water's density. A value below 1 means it floats on water; above 1 means it sinks.
- Pressure equals force over area
- P = F ÷ A. A sharp knife or a thin heel concentrates force on a tiny area, producing very high pressure.
- Pressure in all directions
- A confined fluid pushes equally on every wall of its container, not just downward — pressure acts in all directions at a given depth.
- Gauge pressure
- Pressure measured above atmospheric pressure, the way a tire gauge reads. Absolute pressure adds the surrounding atmospheric pressure to it.
- Compressed air (pneumatics)
- Using pressurized air to transmit force or do work. Unlike liquids, air compresses, so pneumatic systems are springier than hydraulic ones.
- Flow rate and pipe size
- For a steady flow, narrowing a pipe makes the fluid speed up and widening it slows the fluid down, since the same volume must pass each second.
- Viscosity
- A fluid's resistance to flowing. Honey is highly viscous and pours slowly; water has low viscosity and flows freely.
- Surface tension
- The pull along a liquid's surface that makes it act like a stretched skin, letting small insects rest on water and droplets bead up.
- Hydrostatic pressure
- The pressure a still fluid exerts due to gravity. It grows with depth and with the fluid's density, independent of the container's shape.
- Pressure relief valve
- A valve that opens to release fluid when pressure climbs too high, protecting tanks and pipes from bursting.
- Water tower pressure
- A water tower stores water high up so gravity creates pressure throughout the system. The higher the tank, the greater the water pressure below.
- Plimsoll line
- The marking on a ship's hull showing the safe loading depth. A heavier load makes the ship ride lower until buoyancy again balances the weight.
- Why ice floats
- Water expands and becomes less dense when it freezes, so ice is lighter than the liquid water it displaces and floats on top.
- Work
- Force applied through a distance: W = force × distance. No distance moved means no work done, no matter how hard you push.
- Power
- The rate of doing work: power = work ÷ time. Doing the same work faster requires more power.
- Energy
- The capacity to do work. It comes in forms like kinetic, potential, heat, and electrical, and can change form but is never destroyed.
- Kinetic energy
- The energy of a moving object, equal to one-half mass times velocity squared. Doubling the speed quadruples the kinetic energy.
- Potential energy
- Stored energy due to position or condition, such as a raised weight or a stretched spring, ready to be released as motion.
- Gravitational potential energy
- The energy an object has from its height: PE = weight × height. Lifting a load higher stores more energy in it.
- Conservation of energy
- Energy is never created or destroyed, only changed from one form to another. A falling object turns potential energy into kinetic energy.
- Energy conversion in a pendulum
- At the top of its swing a pendulum has maximum potential energy and zero motion; at the bottom it has maximum kinetic energy and least height.
- Horsepower
- A unit of power equal to 550 foot-pounds of work per second, or about 746 watts. It rates how fast an engine or motor can do work.
- Ohm's law
- Voltage = current × resistance, or V = I × R. For a fixed voltage, higher resistance means lower current.
- Voltage
- The electrical pressure that pushes current through a circuit, measured in volts. It is the driving force behind the flow of electrons.
- Current
- The flow of electric charge through a conductor, measured in amperes. More voltage or less resistance increases the current.
- Resistance
- The opposition to current flow, measured in ohms. Thin or long wires and poor conductors have higher resistance.
- Series circuit resistance
- In a series circuit, resistances add up, so total resistance rises and the same current flows through every component.
- Parallel circuit current
- In a parallel circuit, each branch gets the full voltage and current splits among the branches, so total resistance is lower than any one branch.
- Electrical power
- Power = voltage × current (P = V × I). A device drawing more current at a given voltage uses more power, measured in watts.
- Battery
- A device that stores chemical energy and converts it into electrical energy, providing the voltage that drives current through a circuit.
- Circuit breaker
- A resettable switch that trips open when current is too high, protecting wiring. Unlike a fuse, it can be switched back on after the fault clears.
- Short circuit
- An unintended low-resistance path that lets a large current flow, which can overheat wires and trip a breaker or blow a fuse.
- Grounding
- Connecting a device to the earth or a common return so stray current flows safely away instead of through a person, preventing shocks.
- Magnetism
- The force from magnets and moving charges. Like poles repel, opposite poles attract, and every magnet has both a north and a south pole.
- Magnetic poles
- The two ends of a magnet where its force is strongest. North attracts south, but two norths or two souths push apart.
- Electromagnet
- A coil of wire that becomes magnetic when current flows through it. More turns or more current makes it stronger, and cutting the power turns it off.
- Electric motor
- A device that turns electrical energy into rotating motion using the force between magnetic fields and current-carrying coils.
- Generator
- A device that turns motion into electricity by spinning a coil within a magnetic field, the reverse of a motor.
- Transformer
- A device that raises or lowers AC voltage using two coils. More turns on the output coil raise voltage; fewer turns lower it.
- Direct current (DC)
- Electric current that flows steadily in one direction, as supplied by batteries.
- Alternating current (AC)
- Electric current that reverses direction many times each second, the form delivered by household wall outlets.
- Switch
- A device that opens or closes a circuit. An open switch breaks the path and stops current; a closed switch completes it and lets current flow.
- Heat versus temperature
- Temperature measures how hot something is; heat is the energy that flows from a hotter object to a cooler one until they reach the same temperature.
- Thermal conductor
- A material that lets heat pass through easily, such as metal. It feels cold to the touch because it quickly draws heat from your hand.
- Bimetallic strip
- Two bonded metals that expand at different rates, bending when heated. It is used in thermostats to open or close a circuit at a set temperature.
- Expansion gap
- A deliberate gap left in bridges, rails, and pipes so the material can expand in heat and contract in cold without buckling or cracking.
- Friction produces heat
- Rubbing surfaces converts mechanical energy into heat, which is why brakes warm up and why machines need lubrication to limit heat buildup.
- Kinetic energy and braking
- A faster vehicle carries far more kinetic energy, so it needs a much longer distance to stop, because energy rises with the square of speed.
- Energy stored in a spring
- A compressed or stretched spring holds elastic potential energy that is released as motion when the spring returns to its natural shape.
- Heat transfer direction
- Heat always flows naturally from a warmer object to a cooler one, never the other way without help such as a refrigerator.
- Conservation of energy in a machine
- A machine cannot output more energy than it takes in; some input energy always becomes waste heat, so efficiency is always below 100%.
- Lever and work
- A lever multiplies force but not work — the effort end moves a longer distance so that effort × its distance equals load × its shorter distance.
- Watt
- The unit of power equal to one joule of work per second. A 100-watt bulb uses energy at twice the rate of a 50-watt bulb.
- Joule
- The standard unit of work and energy, equal to one newton of force acting through one meter.
- Insulated wire
- A conductor wrapped in a non-conducting cover like rubber or plastic, so current stays in the wire and does not shock or short to other wires.
- Static electricity
- A build-up of electric charge on a surface, often from rubbing, that can jump as a spark when it finds a path to discharge.
- Watt-hour
- A unit of electrical energy equal to one watt of power used for one hour. Power companies bill in kilowatt-hours, a thousand of these.
- Heat capacity of water
- Water absorbs a great deal of heat for a small temperature rise, which is why it is used as an engine coolant and warms and cools slowly.