← Gravitational fields

Rank gravitational forces: the mass rule and the distance rule

Half the grain is unloaded from one of two docked ships, the distance unchanged. The pull between the ships…

Two pairs of crates sit in a warehouse. Pair one holds heavy crates, far apart. Pair two holds light crates, close together. In which pair is the gravitational force stronger? You now hold the tools to answer, most of the time.

Rank gravitational forces with the two factors you know, mass and distance, taken one at a time.

If the masses match, distance decides: the closer pair's force is stronger. If the separations match, mass decides: the heavier pair's force is stronger.

The same two masses at 1 m, 2 m, and 3 m rank by distance alone: the 1 m pair strongest, the 3 m pair weakest.

PAIRMASSESSEPARATIONRANKpair 1same two masses1 mstrongestpair 2same two masses2 mmiddlepair 3same two masses3 mweakest
Equal masses, so distance decides: the closest pair's force is strongest.

Two pairs of identical barrels: pair A's barrels sit 10 m apart, pair B's sit 4 m apart. In which pair is the gravitational force stronger?

Compare one factor at a time:

The masses match: all four barrels are identical.

So distance decides: the closer pair's force is stronger.

Pair B's barrels are closer.

The force within pair B is stronger.

Identical buoy pairs: A 2 m apart, B 5 m, C 8 m. Which pair's pull is strongest?

separation (m)2 mpair A5 mpair B8 mpair C

Two more pairs sit at the same 5 m separation: pair C's barrels hold 60 kg each, pair D's hold 20 kg each. In which pair is the gravitational force stronger?

Compare one factor at a time:

The separations match.

So mass decides: the heavier pair's force is stronger.

Pair C's barrels hold more.

The force within pair C is stronger.

Two pairs of canoes float at the same spacing: one pair empty, one pair fully loaded. Which pair's pull is stronger?

PAIRSPACINGLOAD IN EACH CANOEthe empty pairthe samenonethe loaded pairthe samea full load

When both factors agree, heavier and closer, the verdict is certain. When the factors disagree, heavier but farther, the two rules alone cannot decide.

A disagreement needs the exact law of gravity. The next lesson gives it.

Back to the warehouse: pair one is heavier but farther. The factors disagree, so hold that question for one lesson.

Your turn

Three pairs of identical drums: pair X 3 m apart, pair Y 9 m apart, pair Z 5 m apart. Which order runs strongest pull to weakest?

separation (m)3 mpair X9 mpair Y5 mpair Z

Pair J: two 90 kg crates 8 m apart. Pair K: two 40 kg crates 2 m apart. Each rule says only stronger or weaker — never how much. Using only the mass rule and the distance rule, what can be said?

PAIRMASS OF EACH CRATESEPARATIONpair J90 kg8 mpair K40 kg2 m

Pair L: two 40 kg drums 3 m apart. Pair M: two 90 kg drums 3 m apart. Pair N: two 90 kg drums 6 m apart. Which pair's pull is strongest?

PAIRMASS OF EACH DRUMSEPARATIONpair L40 kg3 mpair M90 kg3 mpair N90 kg6 m