Balanced and unbalanced · 8:11
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Unit 06 · Forces & Motion arc · 7th grade
Nothing moves
for free.
Every change in motion has a force with its name on it. Learn to read the pushes and pulls — balanced, unbalanced, and the one that never sleeps — and by the end of this page, the Moon will make perfect sense.
pushes & pulls
6 manipulatives
12 flashcards
1 feather
apollo 15 · the moon · 1971 — they landed together
The phenomenon · replay history
The Hammer and the Feather
1971, Apollo 15. Astronaut David Scott holds a geology hammer in one hand and a falcon feather in the other — and lets go. They fall together and hit the dust at the same time. Replay it. Then switch to Earth and watch the air interfere.
On the Moon: same time, every time. Gravity doesn't pick favorites — air does.
Back on Earth the feather flutters down late, and everyone concludes "heavy things fall faster." Wrong suspect. Gravity pulls the hammer and the feather identically — it's the air resistance that argues with the feather. The Moon has no air to argue. Galileo guessed it in 1589; Scott proved it on camera in 1971, with a falcon feather from the Air Force Academy mascot.
Watch · Khan Academy
The tug-of-war, settled by Khan Academy
Exact page sources: oNgo9bbDi7Q and atogJ2qw9Ko. Saved caption tracks are recorded in units/forces/math-practice-source-record.md; the legacy freelance gravity pick is not used.
Read · Eleo explains
The beetle's commentary track
Companion articles will land only after their exact video transcripts and investigation hand-offs are ready.
Sketch · sketchnotes
Draw the force story
The exact captions are saved. Printable sheets stay pending until complete transcript analysis packages pass the sketchnote source gate.
Gravitational forces · 3:40
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Investigate · evidence first
Commit to a force diagram, then test it
A standalone investigation is being designed around the page's existing tug-of-war, gravity, and drop models.
Investigation in the works
The page labs remain usable now; a print-first prediction and evidence record lands here after review.
in the worksThe vocabulary · pushes with names
A force is a push or a pull
Measured in newtons (N) — 1 N is about the weight of a small apple, which is exactly the kind of unit you get when your name is Newton. Two families: forces that need to touch, and forces that work from a distance.
Contact forces
Friction — argues with sliding. Grip for feet, brake for skids.
Normal force — the desk pushes up on the book. Yes, the desk pushes.
Air resistance — friction from air. The feather's nemesis.
Applied force — your hand, your foot, the beetle's six legs on the tile.
Non-contact forces
Gravity — the headliner. Pulls everything toward everything, no touching required.
Magnetism — a cameo this unit; the fridge door knows more.
Static electricity — balloon-on-hair magic. Also a cameo.
The giveaway — they work through empty space. The Moon pulls the ocean from 384,000 km away.
Manipulative 01 · file the forces
Pick a force, then file it in the right family.
Contact
needs to touch
Non-contact
works from a distance
0 / 8 filed
Manipulative 02 · the lab
Tug of War: the net decides
Add up every push and pull — opposite directions cancel — and what's left is the net force. Zero net: nothing changes (stability!). Nonzero net: motion changes, and mass decides how fast it changes. That's MS-PS2-2 in one sentence. Now pull.
← left winscenterright wins →
team left
4 N
the load
team right
4 N
balanced at 4 N each — nothing changes. stability is a force story too.
The one that never sleeps
Gravity: everything pulls everything
Gravity is attraction between all masses — more mass pulls harder, more distance pulls weaker. Earth pulls you down; you pull Earth up. Earth wins, because Earth is enormous. Your mass is the stuff you're made of (kg, same everywhere). Your weight is gravity's pull on that stuff (N, changes planet to planet).
mass stays the same on every world. watch the weight and the drop change.
the moon
1.6 N/kg
67 N
hammer drop: 1.37 s
earth
9.8 N/kg
412 N
hammer drop: 0.55 s
jupiter
24.8 N/kg
1,042 N
hammer drop: 0.35 s
Friction: the arguer
Slide a book and it stops — friction spent its motion. The beetle's six feet do the opposite job: friction as grip. Same force, different mood.
Air resistance: the invisible cousin
Friction from air — too weak to notice until you're a feather, a parachute, or a crumpled vs flat sheet of paper. (Try the crumple test at your desk. It's the Moon experiment, cheap edition.)
Why don't you fall up?
You pull Earth with the same force Earth pulls you — the universe's most lopsided tie. Earth is just too massive to notice. The floor is where the meeting happens.
Standards: MS-PS2-2 — the change in an object's motion depends on the net force and the object's mass · MS-PS2-4 — gravitational interactions are attractive and depend on the masses involved (NGSS Lead States, 2013). DCIs: PS2.A Forces and Motion, PS2.B Types of Interactions. CCCs: Stability and Change (balanced forces), Systems and System Models (everything pulling everything).
Myth-busters · tap to flip
Unlearn these three things
Manipulative 03 · Spaced repetition
Say it like a scientist
Twelve words. Flip the card, grade yourself honestly, and the deck schedules your next review — hard words come back sooner. Come back tomorrow and it remembers you. (It lives in your browser, no account.)
–due now
–locked in
–seen so far
flip the card, then grade yourself
Practice · Math skills
Tug-of-war arithmetic
Right is positive, left is negative, and the net force is just the sum. Then first contact with F = ma, and your weight on four worlds. Print the sheet, show your work, check the key.
Exit ticket · 5 questions
Before you leave the Moon
No grade, no stakes — just proof the forces balanced. Answer all five to earn the stamp.
question 1 of 5