Physics: gravitational and electric forces (grade 11) · lesson

Newton's law of gravity

💡 Gravity pulls between any two masses, and it grows with each mass and weakens with distance.

The gravitational force depends on the two masses and the distance between their centers. Double one mass and the force doubles. Triple it and the force triples.

Two balls do pull on each other, but their masses are tiny, so the force is too small to notice. Earth's mass is huge, so its pull is easy to feel.

Gravity forces come in equal and opposite pairs. The Moon pulls on Earth exactly as hard as Earth pulls on the Moon. Earth moves less because it has more mass.

🧠 Let's work it out

Change one thing at a time.
A 12 N force with one mass tripled becomes 12 × 3 = 36 N.
Color, temperature, and speed don't appear in the law, so they don't matter.

What do you think comes next?

gravitational force: the pull between any two masses

The inverse square rule

💡 For both gravity and electric force, the force changes with 1 ÷ distance².

Double the distance and the force becomes 1/2² = 1/4 as large. Triple it and the force becomes 1/9. Halve it and the force becomes 4 times larger.

Coulomb's law for electric force has the same distance rule as gravity. Charge plays the role that mass plays in gravity.

🧠 Let's work it out

Two charges at 2.0 m repel with 10 N.

Move them to 1.0 m: distance halves, so force × 4 = 40 N.
Double one charge: × 2 = 80 N.
Handle each change one at a time.

What do you think comes next?

inverse square: a quantity that changes with 1 divided by the distance squared

🔗 PhET: Coulomb's Law · PhET Interactive Simulations (free) ↗

Electric charges

💡 Like charges repel, opposite charges attract, and extra electrons make an object negative.

Electrons carry negative charge. An object that picks up extra electrons becomes negative. One that loses electrons becomes positive.

Two positive or two negative objects push apart. A positive and a negative object pull together. The force grows with more charge and less distance.

🧠 Let's work it out

Track the electrons.
Rubbing a rod with wool moves electrons onto the rod, so the rod ends up negative.
If one sphere has no charge, there is no electric force at all.

What do you think comes next?

electron: a tiny negative particle that can move between objects

🔗 PhET: Balloons and Static Electricity · PhET Interactive Simulations (free) ↗

Comparing the two forces

💡 For tiny charged particles, the electric force is far, far stronger than gravity.

A proton and an electron have tiny masses, so gravity between them is extremely weak. But they carry real charges, so the electric pull is enormous by comparison.

Gravity wins at large scales because big objects like planets have huge mass and are nearly neutral overall.

🧠 Let's work it out

Ask what the object has a lot of.
Tiny particles have charge but little mass, so electric force wins.
Planets have huge mass but little net charge, so gravity wins.

What do you think comes next?