Free fall is motion under gravity alone. On Earth the acceleration is about 9.8 m/s², downward.
Free fall Motion under gravity alone, with no other force acting.
Step 1: Something to Notice
Watch first. The explanation comes later.
David Scott dropped a hammer and a falcon feather on the Moon in 1971. They hit the surface together.
Why does that work there and not in a classroom?
Step 2: Find Out
Isolate the acceleration and note that it does not depend on what is falling.
Step 1 — Predict
A hammer and a feather are dropped together on the Moon. What happens?
Choose what you think will happen. You cannot see the experiment until you do — guessing first is what makes it worth watching.
Step 3: So Here Is Why
Now the explanation, after you have seen it happen.
Mass cancels out
A heavier object is pulled harder and resists proportionally more. The two effects cancel exactly.
Air resistance is a separate force
It depends on speed, area and shape — not on mass. That is what separates a feather from a hammer.
Terminal velocity
Resistance grows with speed until it balances the weight. The net force is then zero and the speed stops rising.
Terminal velocity The steady speed at which air resistance balances weight.
And free fall includes going up
A ball thrown upward is in free fall the whole time, decelerating at the same 9.8 m/s².
Mass does not appear in the answer
All objects fall with the same acceleration regardless of mass. A heavier object is pulled harder and resists acceleration proportionally more, and the two effects cancel exactly.
Air resistance is the everyday complication
A feather falls slowly because of air, not because it is light. In a vacuum a feather and a hammer land together, which was demonstrated on the Moon precisely to make the point.
Step 4: A Common Mistake
Lots of people think
“Heavier objects fall faster.”
Step 5: Why a Skydiver Has Two Terminal Velocities
The same idea somewhere new.
Falling flat and spread out, a skydiver reaches about 55 m/s. Pulling into a dive reduces the cross-sectional area, so air resistance falls, and the terminal velocity rises to around 90 m/s. Opening the parachute increases the area enormously and the terminal velocity drops to about 5 m/s — the same mass throughout, and three different terminal velocities set entirely by area and shape.
Step 6: Think It Through
Practice makes it stick.
Hammer and Feather
Problem 1 of 2
Why do a hammer and a feather land together on the Moon?
Three Terminal Velocities
Problem 2 of 2
A skydiver has terminal velocities of 55, 90 and 5 m/s at different times. What changed?
Falling
1 of 5
Do heavier objects fall faster in free fall?
2 of 5
What does air resistance depend on?
3 of 5
How fast is a stone falling 3 s after being dropped?
4 of 5
What is happening at terminal velocity?
5 of 5
Is a ball thrown upward in free fall?
Step 7: Quick Check
Show what you know.
Question 1 of 1
Do heavier objects fall faster?
Step 8: Explain It in Writing
Claim, evidence, then reasoning.
The question
Explain why a hammer and a feather fall together on the Moon and not on Earth.
Fill in all three boxes. The reasoning box is the one that matters most.
What You Found Out
- Free fall is motion under gravity alone, at about 9.8 m/s² on Earth.
- Mass cancels: the extra pull is matched by extra inertia.
- Air resistance depends on speed, area and shape, not mass.
- At terminal velocity resistance balances weight and the speed stops rising.