GED Science — Projectile motion, gravity, and air resistance
The scenario
Two people stand at the edge of a high cliff. At the exact same moment:
Person A drops a rock straight down (vertical drop)
Person B throws a rock horizontally outward (projectile)
According to Galileo's principle: both rocks should hit the ground at the same time — because gravity pulls both downward at the same rate regardless of horizontal motion.
The GED question asks: "Which uncontrolled part of this investigation can prevent the rocks from hitting the ground at the same time?"
The experiment diagram
The dropped rock falls straight down. The thrown rock travels outward — but both fall the same vertical distance in the same time.
Gravity
Pulls both rocks downward at 9.8 m/s². Same force on both — so vertical fall time is identical.
Horizontal motion
The thrown rock moves sideways but this does NOT affect how fast it falls. The two motions are independent.
Controlled variables
Height of cliff, mass of rocks, gravity — these are the same for both rocks.
Uncontrolled variable
Air resistance — the thrown rock travels farther and faster, so it encounters more air drag. This can delay its landing.
Key question: If gravity pulls both rocks at the same rate and mass doesn't matter (Galileo proved this), what factor could still make them land at different times in a real experiment?
Interactive cliff simulator
Adjust air resistance and throw speed, then press Launch to see both rocks fall. Watch how air resistance affects the thrown rock's landing time.
15 m/s
45 m
—
Dropped rock land time
—
Thrown rock land time
What to observe
With air resistance OFF: both rocks land at the exact same time — Galileo's principle holds
With air resistance ON: the thrown rock lands slightly later — air drag on its horizontal path slows its overall descent
A faster throw speed increases the effect of air resistance even more
Cliff height affects how long each rock is in the air, but not whether they land together
What is air resistance?
Air resistance (also called drag) is a friction force that acts on any object moving through air. It pushes back against the direction of motion.
Key facts:
Air resistance increases with speed — faster objects experience more drag
Air resistance increases with surface area — wider objects experience more drag
Air resistance acts in the opposite direction of motion
It is not constant — it changes as the object's speed changes
Why does this matter for our experiment? The dropped rock only moves vertically — so air resistance only acts downward against its fall. The thrown rock moves both horizontally AND vertically — so air resistance acts in both directions. This gives the thrown rock more total drag, slightly slowing its descent.
Controlled vs. uncontrolled variables
Variable
Dropped rock
Thrown rock
Controlled?
Height of cliff
Same
Same
✓ Yes
Gravity (9.8 m/s²)
Same
Same
✓ Yes
Mass of rock
Same
Same
✓ Yes
Release time
Same
Same
✓ Yes
Air resistance
Low (vertical only)
Higher (horizontal + vertical)
✗ NOT controlled
Throw strength
Zero
Variable
✗ NOT controlled
Uncontrolled variable = air resistance. Because the thrown rock moves horizontally at speed, it encounters much more air drag than the dropped rock. This is the one factor that could prevent them from landing at the same time in a real-world (non-vacuum) experiment.
Why NOT gravity, mass, or throw strength?
NOT
Gravity — Gravity is a controlled variable. It acts equally on both rocks (9.8 m/s² downward). Galileo proved gravity accelerates all objects at the same rate regardless of mass. Gravity would make them land together, not apart.
NOT
Mass of the rocks — Mass is a controlled variable (the problem states same height, same setup). Also, Galileo's experiments showed that mass does not affect free-fall time — a heavy and a light rock fall at the same rate.
NOT
Strength of the person — Throw strength affects horizontal distance and speed, but horizontal motion is independent of vertical fall in ideal physics. Throw strength alone (without air resistance) would not change landing time.
YES
Air resistance — This is the uncontrolled variable. The thrown rock travels faster and farther, so it faces more air drag. In a real experiment (not a vacuum), this could delay the thrown rock's landing and break the simultaneous-landing result.
Full answer breakdown
"Which uncontrolled part of this investigation can prevent the rocks from hitting the ground at the same time?"
WRONG
Gravity Gravity is NOT uncontrolled — it is constant for both rocks. It acts downward at 9.8 m/s² on both. Gravity is the reason Galileo predicted they would land together, not why they might land apart.
WRONG
Mass of the rocks Mass does not affect free-fall time. Galileo's famous experiment proved this — a cannonball and a musket ball dropped from the same height land at the same time. Mass is also controlled in the experiment setup.
CORRECT
Air resistance The thrown rock moves horizontally at speed — creating much more air resistance than the dropped rock (which only moves vertically). Air resistance is uncontrolled because it depends on the rock's speed and shape, which can vary. In real-world conditions (not a vacuum), this drag could delay the thrown rock's landing.
WRONG
Strength of the person In ideal physics, throw strength only affects how far the rock travels horizontally — not how fast it falls vertically. Horizontal and vertical motion are independent. Throw strength alone would not prevent simultaneous landing.
GED strategy — "uncontrolled variable" questions
Step 1: Identify what the experiment is trying to show (Galileo: gravity affects all objects equally)
Step 2: List which variables are the same for both objects (mass, height, gravity, release time)
Step 3: Ask — which variable is NOT kept equal between the two objects?
Step 4: Air resistance is uncontrolled because the thrown rock moves faster/farther → more drag
Step 5: Choose the variable that could actually interfere with the predicted result
Remember: "Uncontrolled" means the variable is not kept equal between the two situations being compared. Air resistance on the thrown rock is much higher than on the dropped rock — making it the only uncontrolled factor that could change the outcome.
