Two cars are parked side by side. Does each car pull on the other?
Some gravitational forces are mighty, and some are feeble. The force between Earth and the Moon swings the Moon through a full circle every month. The force between two books cannot even slide them together. What makes one force strong and another weak?
The first factor is mass. Here is one pair of objects while only a mass changes.
Two anchored boats float a fixed distance apart. A gravitational force acts between them.
One boat takes on a load of cargo. Its mass increases; nothing else changes. The force between the boats is now stronger.
The cargo is lifted off again. The force returns to what it was.
The same works through either object: load the other boat, and the force strengthens again.
That is why replacing a satellite with a more massive one increases the gravitational force Earth exerts on it.
A boat drops anchor near a loaded freighter. The freighter unloads half its containers, the distance unchanged. What happens to the force between the two vessels?
Write down what changed:
The freighter's mass decreases. The other mass and the distance do not change.
Apply the rule:
Decreasing either mass decreases the force.
The force between the boat and the freighter decreases.
Crates are stacked onto one of two parked trailers, the gap unchanged. The pull between the trailers…
The same boat then takes on a load of gravel. What happens to the force now?
Write down what changed:
The boat's mass increases. The distance is still unchanged.
Apply the rule:
Increasing either mass increases the force.
The force between the two vessels increases.
Which change increases the gravitational pull between two neighboring water tanks?
Two pairs of tugboats float with the same spacing. Pair X's tugs ride empty; pair Y's tugs carry full loads. Which is correct?
Earth pulls on a docked tanker. Overnight the tanker unloads its oil and stays at the same dock. Compare Earth's pull on the tanker before and after.