NYS Grade 8 Science cheat sheet

Density is mass per unit volume: d = m/V. A solid object sinks in a fluid when its density is greater than the fluid's density and floats when it is lower.

Average speed = total distance / total time. Acceleration = change in velocity / time and is measured in m/s^2; a change in direction is also an acceleration.

Newton's second law: net force = mass x acceleration. Under the same net force, doubling the mass halves the acceleration; doubling the force doubles it.

Balanced forces produce zero net force, so an object stays at rest or keeps a constant velocity. Unbalanced forces always change an object's motion.

Kinetic energy = 0.5 x mass x speed^2, so doubling speed quadruples kinetic energy. Gravitational potential energy increases with mass and height.

In an atom, protons are positive and sit in the nucleus, neutrons are neutral, and electrons are negative and occupy the space around the nucleus. Atomic number = number of protons.

In a chemical reaction, existing bonds break and new bonds form, but atoms are never created or destroyed, so total mass is conserved. Evidence of a reaction includes gas production, a precipitate, a color change, or a temperature change.

Wave speed = frequency x wavelength. Frequency is measured in hertz (waves per second). Light travels about 300,000 km/s in a vacuum, far faster than sound.

Thermal energy moves from warmer objects to cooler ones until temperatures equalize, by conduction (direct contact), convection (currents in fluids), or radiation (no medium required).

Cell theory states that all living organisms are made of one or more cells and that new cells arise from existing cells.

Plant cells have a cell wall and chloroplasts; animal cells lack both. Both cell types have a cell membrane, nucleus, mitochondria, and cytoplasm.

Photosynthesis occurs in chloroplasts: carbon dioxide + water + light energy yields glucose + oxygen, storing energy in chemical bonds.

Cellular respiration occurs in mitochondria: glucose + oxygen yields carbon dioxide + water + released energy (ATP). It is essentially the reverse of photosynthesis.

Mitosis produces two daughter cells with the same chromosome number as the parent and is used for growth and repair. Meiosis produces gametes with half the chromosome number.

Traits are coded by DNA: genes are sections of DNA that code for proteins, and proteins produce observable traits. A mutation may alter a trait and is heritable only if it occurs in reproductive cells.

Natural selection requires variation, heritable traits, and unequal survival or reproduction. It changes the traits of a population over generations, not an individual within its lifetime.

In a food web, arrows show the direction of energy flow. Only about 10% of energy passes to the next trophic level, the rest being lost mainly as heat, so food chains are short.

Body systems interact: the respiratory system supplies oxygen, the circulatory system delivers it with glucose to cells, and mitochondria use both for cellular respiration.

Earth's layers from outside in: crust (thin, rocky), mantle (hot rock that flows very slowly), outer core (liquid iron and nickel), inner core (solid iron and nickel).

At a divergent boundary, plates move apart and magma rises to form new crust, creating mid-ocean ridges and rift valleys.

At a convergent boundary, plates collide; the denser oceanic plate subducts, forming trenches, volcanoes, and earthquakes. Continental collision builds mountains.

At a transform boundary, plates slide past each other, producing earthquakes with little or no volcanic activity.

Rock cycle: igneous rock forms when magma or lava cools; sedimentary rock forms when sediments are compacted and cemented; metamorphic rock forms from heat and pressure.

Weathering breaks rock into smaller pieces, erosion transports the sediment, and deposition lays it down; together these reshape landforms over time.

Earth rotates on its axis once about every 24 hours, causing day and night, and revolves around the Sun once about every 365.25 days.

Earth's axis is tilted about 23.5 degrees, so sunlight strikes each hemisphere at different angles through the year. This tilt, not distance from the Sun, causes the seasons.

Water cycle: the Sun's energy drives evaporation and transpiration, then condensation forms clouds, precipitation returns water, and runoff or infiltration collects it again.

Before designing, define the problem: identify who needs what, and state the goal in measurable terms that can be tested.

Criteria are the requirements a successful design must meet. Constraints are the limits on a design, such as budget, time, available materials, size, and safety rules.

Generate several possible solutions, then compare each against the criteria and constraints instead of committing to the first idea.

A prototype is a working model built to be tested. Test it under conditions as close to the real use as possible and record quantitative data.

A fair test changes one independent variable at a time, holds all other variables constant, and measures the resulting dependent variable.

Run multiple trials and use averages or ranges rather than a single measurement, which reduces the effect of random error and shows reliability.

Iteration is the cycle of testing, analyzing failure, revising, and retesting. Use data to target the largest failure mode first.

Every design involves trade-offs: improving one criterion, such as strength, often makes another worse, such as mass or cost.

A design that does not meet its criteria has failed; analyze why and decide whether to revise the design or redefine the problem.