Gravity and Coulomb's Law
Aligned to HS-PS2-4 — Next Generation Science Standards.
What this lesson teaches
Two laws share the same inverse-square shape. Gravity: F = G·m₁·m₂/r². Electric force: F = k·q₁·q₂/r². Both weaken with the square of the distance, but gravity is always attractive while electric force can attract or repel depending on charge signs.
Worked examples
Example 1
Two 1 C charges 1 m apart feel F = k·q₁·q₂/r² = (9x10⁹)(1)(1)/(1²) = 9x10⁹ N of repulsion. Move them to 3 m apart and distance triples, so the force drops by 3² = 9 times, to 1x10⁹ N.
Example 2
Gravity and Coulomb share the same inverse-square shape, but gravity only attracts while charge does both.
Example 3
Gravity is fantastically weaker. A fridge magnet lifts a paperclip against the pull of the entire Earth.
Practice questions
- Write both the gravitation and Coulomb equations and label the constant in each.
- If the distance between two charges is halved, by what factor does the electric force change?
- Earth pulls the Moon and two electrons repel each other. Explain which force law applies to each and one key difference between them.
Watch the lesson
Every lesson comes with a video taught in English and Spanish — the same video the QR code in the printed workbook opens.
▶ Watch this lessonEn español
La Gravedad y la Ley de Coulomb
Dos leyes comparten la misma forma de cuadrado inverso. Gravedad: F = G·m₁·m₂/r². Fuerza eléctrica: F = k·q₁·q₂/r². Ambas se debilitan con el cuadrado de la distancia, pero la gravedad siempre es de atracción, mientras que la fuerza eléctrica puede atraer o repeler según el signo de las cargas.
Ejemplo 1: Dos cargas de 1 C separadas 1 m sienten F = k·q₁·q₂/r² = (9x10⁹)(1)(1)/(1²) = 9x10⁹ N de repulsión. Al separarlas a 3 m, la distancia se triplica, así que la fuerza baja 3² = 9 veces, hasta 1x10⁹ N.
Ejemplo 2: La gravedad y Coulomb comparten la misma forma de inverso del cuadrado, pero la gravedad solo atrae mientras que la carga hace las dos cosas.
Ejemplo 3: La gravedad es muchísimo más débil. Un imán de refrigerador levanta un clip contra el jalón de toda la Tierra.
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