Grade 8 Science

Grade 8 Science · State Standards

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Motion, Speed, and Acceleration

Speed describes distance traveled per unit time. Velocity includes direction. Acceleration describes a change in velocity, so an object can accelerate by changing speed or direction.

Example

A car moving around a curve can accelerate even when its speed remains steady, because its direction changes.

Try it

Compare constant speed in a straight line with constant speed around a curve. Explain the difference in acceleration.

Forces, Mass, and Motion

The effect of an unbalanced force depends on an object’s mass. Compare situations while stating what is held constant. Force diagrams and motion data can support an explanation.

Example

With other conditions similar, the same push changes the motion of a light cart more than a much heavier cart.

Try it

Plan a comparison of two carts. Explain what force and motion measurements would make the comparison useful.

Atoms Are Conserved in Reactions

Chemical reactions rearrange atoms. The types and numbers of atoms are conserved in a closed system. A particle model can account for the atoms before and after a reaction.

Example

A reaction model should not create an extra oxygen atom or make a hydrogen atom vanish. All atoms must be accounted for.

Try it

Inspect a particle model before and after a reaction. Explain how you would check conservation.

Kinetic and Potential Energy

Kinetic energy depends on motion. Potential energy depends on the arrangement of a system, such as an object’s position relative to Earth. Track transfers as the arrangement and motion change.

Example

A raised ball and Earth form a system with gravitational potential energy. As the ball falls, its motion changes and energy transfers.

Try it

Draw an energy model for a falling ball. Include what changes before it reaches the ground.

Waves and Electromagnetic Radiation

Wave properties help explain how energy and information travel. Mechanical waves require a medium. Electromagnetic radiation can travel through empty space, which helps explain how sunlight reaches Earth.

Example

Sound cannot travel through empty space in the same way light can. The two require different models of transmission.

Try it

Compare sound traveling through air with light traveling from the Sun. Identify a difference in their paths.

Genes, Traits, and Variation

Genes carry information that can influence traits. Changes in genetic information may affect proteins and traits, but effects vary. Use a model that distinguishes genes, proteins, traits, and environmental influences.

Example

A change in a gene may alter a protein’s function. Some changes have little observable effect, while others matter more.

Try it

Draw a model connecting genetic information with a trait. Add one environmental influence.

Evidence for Evolutionary Relationships

Scientists compare fossils, anatomy, development, and molecular evidence to study relationships among organisms. Similarities must be interpreted in context. Several lines of evidence strengthen an explanation.

Example

Similar bone arrangements can suggest common ancestry even when limbs serve different jobs. Other evidence can help test that explanation.

Try it

Describe two types of evidence you would compare when studying related organisms.

Selection and Changing Populations

Selection can change the frequency of inherited traits over generations. A population changes; an individual does not acquire a needed inherited trait simply because the environment demands it.

Example

A population may already contain individuals with different resistance traits. A changed environment can affect which individuals leave more offspring.

Try it

Explain a population change without saying the organisms decided to change their inherited traits.

Gravity and Space Systems

Gravity acts between objects with mass. In space systems, it contributes to the motion of planets, moons, and other bodies. Scale models must distinguish distances, sizes, and the limits of a drawing.

Example

Earth and the Moon interact through gravity. A small classroom model may show their arrangement without representing both size and distance accurately.

Try it

Explain one limitation of a model of the Earth–Moon system. Suggest how to label it clearly.

Interpreting Earth’s History

Rock layers, fossils, and geologic relationships provide evidence of Earth’s history. Relative age describes an order, not an exact number of years. Disturbances can complicate the record.

Example

In an undisturbed sequence, lower sedimentary layers are generally older than layers above them. A fault cutting layers records a later event.

Try it

Use a rock-layer drawing to order three events. State the evidence for your order.

Human Impacts and Resource Decisions

Resource decisions can affect Earth systems and communities. Compare benefits, costs, and environmental effects using evidence. A useful proposal explains both its expected effect and how that effect would be measured.

Example

A town comparing water-management designs may consider water saved, energy use, cost, and effects on habitats.

Try it

Choose two criteria for a resource decision. Explain what data would allow a fair comparison.

Designing and Critiquing Investigations

An investigation needs a clear question, suitable measurements, and attention to variables. Repeated trials help reveal variability. A conclusion should describe what the data supports and what remains uncertain.

Example

Two tests may give slightly different results. Recording both, rather than selecting only the preferred result, gives a more honest account of the evidence.

Try it

Outline an investigation. Identify the variable you change, the measurement you collect, and one possible source of uncertainty.