Gravity and the Pull That Shapes Our World

The Phenomenon: A Ball and a Feather

Now imagine something even stranger. In 1971, astronaut David Scott stood on the surface of the Moon and dropped a hammer and a feather side by side. On the Moon — where there is no air — both objects hit the ground at exactly the same time. Scientists have observed this result again and again: without air getting in the way, every object falls at the same rate, no matter how heavy or light it is.

Something is clearly pulling objects downward, both on Earth and on the Moon. But if the pull treats all objects equally, why do a basketball and a feather behave so differently here on Earth? What invisible force is responsible — and how can we build an argument to explain what is happening?

• What force do you think is pulling both objects downward?
• If gravity pulls on everything, why does the feather fall more slowly on Earth but not on the Moon?
• What evidence would you need to build an argument about how gravity affects objects near Earth?

What Scientists Know About Gravity

For thousands of years, humans have noticed that everything falls down — never up or sideways on its own. Scientists have studied this behavior carefully and developed an explanation for it. The invisible force responsible is called gravity, and it is one of the fundamental forces in our universe. Understanding gravity allows scientists to explain everything from why raindrops fall to why planets orbit the Sun.

Let's Investigate: Building an Argument from Evidence

Investigation: The Drop Test

Question: Does the mass of an object affect how fast gravity pulls it to the ground?

Materials you would need:

• A heavy ball (like a baseball or tennis ball filled with sand)
• A light ball (like a ping-pong ball or foam ball) of similar size
• A flat sheet of paper
• The same sheet of paper crumpled into a tight ball
• A ruler or measuring tape
• A stopwatch (optional)

Procedure:

• Hold the heavy ball and the light ball at the same height (about 1.5 meters). Drop them at the same time. Observe which hits the ground first. Repeat 3 times.
• Hold the flat sheet of paper and the crumpled paper ball at the same height. Drop them at the same time. Observe which hits the ground first. Repeat 3 times.
• Hold the heavy ball and the flat sheet of paper at the same height. Drop them together and observe.
• Record all observations in a data table.

What you would observe: In Trial 1, both balls land at nearly the same time. In Trial 2, the crumpled paper hits the ground much sooner than the flat paper. In Trial 3, the ball hits the ground far before the flat paper. This evidence shows that gravity pulls all objects equally, but air resistance slows objects down — especially those with a larger surface area, like a flat sheet of paper.

This investigation gives us the evidence we need to build a scientific argument. The Science and Engineering Practice we are using here is called "Engaging in Argument from Evidence." Scientists do not simply state what they believe — they make a claim, support it with evidence from investigations, and then explain their reasoning for why the evidence supports the claim.

What We Discovered: Gravity's Effect on Objects Near Earth

Our investigation produced some important evidence. When we dropped two balls of different masses from the same height, they landed at essentially the same time. This tells us that gravity pulls on all objects equally, regardless of how heavy or light they are. The force of gravity near Earth's surface accelerates all freely falling objects at the same rate — about 9.8 m/s².

But when we introduced a flat sheet of paper into the experiment, something changed. The paper floated slowly while the ball dropped quickly. Did gravity stop working on the paper? Not at all. Gravity was still pulling the paper downward with the same acceleration. The difference was caused by air resistance — a force that pushes against an object as it moves through the air. The flat paper had a large surface area, so air resistance had a big effect. The crumpled paper had less surface area exposed to the air, so it fell faster. The heavy ball pushed through the air easily because its weight overwhelmed the small amount of air resistance acting on it.

This is exactly what happened in our anchoring phenomenon. The basketball fell quickly because air resistance barely slowed it down. The feather fell slowly because its shape and light weight meant air resistance had a huge effect compared to the pull of gravity. On the Moon, where there is no atmosphere and therefore no air resistance, both the hammer and the feather fell at the same rate — proving that gravity treats all objects equally.

Building Our Scientific Argument

Using the evidence from our investigation, we can now construct a scientific argument with three parts:

Patterns and Connections: Cause and Effect

The Crosscutting Concept at the heart of this lesson is Cause and Effect. Scientists look for cause-and-effect relationships to explain why things happen. In our investigation, we identified two causes that determine how an object falls near Earth: the cause of downward motion is gravity, and the cause of slower falling is air resistance. The effect we observe — how quickly and in what path an object falls — depends on the interaction between these two forces.

This same pattern of cause and effect appears across many areas of science. Whenever you see something happen in the natural world, you can ask: What caused this? What forces or factors are interacting to produce the effect I observe? Scientists design investigations specifically to test cause-and-effect relationships by changing one variable at a time (the cause) and measuring what changes (the effect).

Real-World Connections and Engineering

Understanding how gravity and air resistance interact has led to incredible engineering achievements. Engineers use their knowledge of these forces to design everything from parachutes to spacecraft. When engineers design a solution to a problem, they must consider the cause-and-effect relationships between forces — the same relationships we investigated in this lesson.

Think about: What shape would create the most air resistance? What materials would be lightweight but strong? How could you test your design to make sure it works? You could build and test small prototypes using paper, plastic bags, string, and small weights — then compare which design slows the fall the most. Each redesign uses evidence from your tests, just like real engineers do.

Key Vocabulary Review

Practice: Test Your Understanding

What's Next?