Gravity Challenge: Do Feathers and Steel Balls Really Fall at the Same Speed?
I’m Ken Kuwako, your science trainer. Every day is an experiment.
“If you drop two objects with different weights at the same time, which one hits the ground first?” — how would you answer?
Many people might say the heavier one falls faster. But in reality, that’s not the case. There’s a simple tool you can use at home to see this surprising phenomenon for yourself: it’s called the BeeSpi.
What is BeeSpi?
Do you have a BeeSpi?
Have you ever heard of BeeSpi?
BeeSpi is a simple, U-shaped speed measuring device. Inside the frame are two sensors, and when an object passes through the opening, it measures its speed.
Originally, it was designed to measure the speed of toys like marbles, but thanks to its convenience, it eventually found its way into science experiments. It’s even exported worldwide. One of its biggest advantages is that, despite being a speedometer, it’s very affordable.
BeeSpi V (Simple Speed Measuring Device)
You can get one for around 3,000 yen, and science versions can measure speed not only in km/h but also in m/s (meters per second), which is very useful. It’s often used in experiments like building roller coasters to study potential and kinetic energy. Even more impressively, if you use two BeeSpi devices, you can measure acceleration.
That means you can actually calculate the famous “acceleration due to gravity” yourself!
This experiment is easy enough to try at home, so give it a go with your kids. You’ll be amazed to see that both light and heavy objects experience the same acceleration.
What You’ll Need
Here’s what to prepare:
Two BeeSpi devices, a stand, clips for the stand, a measuring tape, a mat (or even a paper cup), and beads or marbles.
These are common science experiment tools, but as long as you can secure the BeeSpi devices, you can improvise. The mat can be something simple like a cut piece from a 100-yen shop item. A paper cup works just fine too.
Procedure
① Attach the two BeeSpi devices to a stand and place a mat (or paper cup) underneath.
② Drop a marble and record the speeds measured by the upper and lower devices. If the marble hits the BeeSpi, try again.
③ Once successful, measure the distance between the two BeeSpi devices using a ruler or measuring tape.
④ Take the two recorded speeds (V₀ and V) and the distance (x), and plug them into the equation of uniformly accelerated motion:
V² − V₀² = 2ax
Then calculate the acceleration a using a calculator. (We’ll explain the principle shortly.)
Example Results
Sample results
The Principle: Why Can We Calculate Gravity?
Let’s look at the theory behind measuring gravitational acceleration with a simple speedometer.
By eliminating time t from two equations of uniformly accelerated motion (the velocity equation and the position equation), we obtain the following:
x − x₀ represents the distance traveled.
From this equation, if you measure v, v₀, x, and x₀, you can determine the acceleration a between those two points. Since falling motion is uniformly accelerated, measuring speed at just two positions allows you to calculate gravitational acceleration.
For example, drop a ball five times from a height of 60.0 cm and measure its speed at 40.0 cm and 20.0 cm. Here’s an example from an actual experiment:
Height [m] Speed [m/s] (average of 5 trials)
0.400 1.93
0.200 2.74
Plugging these values into the formula gives 9.46 m/s² — very close to the theoretical value of 9.8 m/s². Try calculating it with your own data!
What is Uniformly Accelerated Motion?
The formula used here is one of the equations of uniformly accelerated motion. This type of motion refers to movement in a straight line where acceleration (the rate of change of speed) remains constant.
Free fall is a classic example. Ignoring air resistance, objects accelerate downward at about 9.8 m/s² every second.
The Fascinating World of Gravity
In this experiment, we used marbles, but try using objects with different weights, like beads. You’ll find that the acceleration remains nearly the same, close to 9.8 m/s². That’s the beauty of gravitational acceleration.
This fact was discovered over 400 years ago by Galileo Galilei. His finding that “heavy and light objects fall at the same rate” overturned common belief at the time and eventually led to Newton’s law of universal gravitation.
Interestingly, gravitational acceleration varies slightly depending on where you are on Earth. It’s a bit stronger at the poles than at the equator. That’s why rocket launch sites are built as close to the equator as possible — even a small difference in gravity can affect launch efficiency.
On other celestial bodies, gravity is completely different. On the Moon, for example, it’s only about 1.6 m/s² — roughly one-sixth of Earth’s gravity. That’s why astronauts can bounce so lightly there.
Even something as simple as gravity connects to the shape of our planet and the vastness of space. That’s where the true wonder of science lies. Try changing the drop height or the objects you use, and see what happens!
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Let’s bring the wonders of science closer to everyday life! I share fun, easy-to-follow experiments you can try at home, along with helpful tips. Feel free to explore more!
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