(22 개정) 중1 과학 시험정리 2학기 #1(벼락치기 인강)

Scientific concepts & discoveries / nature phenomena

Force and power (work / “received force” idea) — kinematics link

• Definition of force (power): when an object is subjected to a force, it causes a change.
• Changes caused by net force:
  • Speed change (accelerate or decelerate)
  • Direction change
  • Shape change (e.g., denting/breaking)
• No net force / force equilibrium:
  • If forces balance so that the net force is 0, there is no change in motion.
  • Constant speed motion can occur with no net force.
• Inclined plane example (gravity component):
  • Going down: speed increases (gravity component aids motion)
  • Going up: speed decreases (gravity component opposes motion)
  • A position where speed is constant corresponds to no power transfer / no net force segment (as described in the lecture)

Representation and combining of forces

• Free-body / force diagrams
  • Use arrows that show magnitude and direction.
  • Emphasize correctly marking the point of application on the object.
• Resultant (net) force:
  • If force directions are the same → add magnitudes
  • If directions are opposite → subtract magnitudes
• Force equilibrium condition:
  • Equal magnitude, opposite direction forces → net force 0
  • Multiple forces can still produce no change if net force is zero.

Types of forces

Gravity (gravitational force / weight)

• Attraction between masses.
• Compared forces from Earth vs Moon on objects.
• Direction: toward the center of Earth (downward in usual contexts).
• Magnitude depends on mass and distance/size of the attracting body (lecture uses Earth ~6× Moon as a memory aid).
• Mass vs gravity (lecture framing):
  • Mass (amount of matter) doesn’t change with location.
  • Gravity/weight changes depending on the source (Earth vs Moon).

Elastic force (spring force)

• Elasticity: force that makes a deformed object return to its original shape.
• Direction: opposite the deformation-causing force direction.
• Magnitude increases as deformation increases (consistent with Hooke-like proportional behavior).
• Clarification: “elastic force” is the force exerted by the elastic object, not the force you apply.

Friction

• Friction is a force that hinders motion at the contact surface.
• Magnitude factors:
  • Larger normal/pressing force → larger friction
  • Rougher contact surfaces → larger friction
• Direction rule:
  • Friction acts opposite the direction of relative motion.
  • The lecture warns against confusing friction with elasticity and notes that friction direction relative to an applied force can vary by situation.
• Can handle equilibrium cases:
  • An object can remain at rest if the applied force magnitude equals friction magnitude (net force = 0).

Buoyancy (upthrust)

• Buoyancy is the upward force from a liquid or gas on an immersed object.
• Compared with gravity:
  • Buoyancy resists gravity (gravity pulls down)
• Three outcome concept:
  • Buoyancy > gravity → object rises
  • Buoyancy < gravity → object sinks
  • Buoyancy = gravity → object stays suspended / in equilibrium
• Key dependence:
  • Buoyancy depends strongly on the displaced volume of fluid, not simply the object’s weight.
• Illustrative examples:
  • A light metal spoon may sink if it displaces little water (small submerged/displaced volume).
  • A heavy basin may float if it displaces enough water due to larger volume.
• Weight–buoyancy “independence” (lecture framing):
  • Object weight and buoyancy are not directly correlated; rather, volume affects displaced water → buoyancy.

Net force and change in state of motion (summary logic)

• If net force = 0:
  • Object remains at rest or moves at constant velocity.
• If net force ≠ 0:
  • Motion changes: speed and/or direction changes.
• Special motion cases (qualitative memory aids):
  • Speed increases/decreases when force aligns/opposes motion
  • Direction changes when force is not aligned/opposed but still has a component that turns
  • Projectile and pendulum motion: both speed and direction change

Gas properties: pressure and gas laws

Pressure (general)

• Concept emphasized as a “stabbing” effect: depends on force distributed over area.
• Higher pressure from:
  • Larger force on the same area
  • Smaller area for the same force

Gas pressure (microscopic interpretation)

• Pressure linked to collisions of gas particles with container walls.
• Factors increasing gas pressure:
  • Higher temperature → particles more energetic → more vigorous collisions
  • More gas amount → more particles → more collisions
  • Smaller container volume → more frequent collisions

Boyle’s Law (pressure–volume inverse relationship at constant temperature)

• As pressure increases, volume decreases, and vice versa.
• Example idea: balloon expansion with reduced pressure aloft (qualitative).
• Extra emphasis: temperature/pressure/volume changes occur with altitude because atmospheric pressure decreases with height.

Charles’s Law (temperature–volume proportional relationship at constant pressure)

• As temperature increases, volume increases.
• Real-life examples: hot air balloon; balloons/bottles behavior with heating/cooling.

Graph caution (temperature at 0°C)

• Lecture warns not to misread gas volume vs temperature graphs as implying volume becomes zero at absolute 0°C; curriculum graphs may start above zero volume due to different references.

Combining ideas

• The relationship depends on whether pressure or temperature is held constant.
• Pressure and volume change in opposite directions when temperature is constant.

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Researchers / sources featured (named)

• Isaac Newton (force unit “newton” named after him)
• Robert Boyle (Boyle’s Law)
• Jacques Charles (Charles’s Law)