Fall of a meteorite

This activity allows students to understand the propagation of seismic waves and the transfer of energy during an impact. It makes the link between an astronomical phenomenon and its physical measurement.

About 66 million years ago, an asteroid roughly 10 km across struck the Earth near what is now the Yucatan Peninsula, generating seismic waves that were felt worldwide and ultimately contributing to the extinction of the dinosaurs. While we cannot recreate such an event, the fundamental physics of impact, the conversion of kinetic energy into seismic waves, can be studied on a small scale with a smartphone. When a ball drops onto a surface, its gravitational potential energy converts to kinetic energy during the fall, and upon impact, a fraction of this energy is transmitted into the surface as mechanical waves. These waves propagate outward from the impact point, much like the seismic waves from an earthquake or meteorite strike. A smartphone accelerometer, sensitive enough to detect vibrations of just 0.01 m/s², can serve as a miniature seismograph, capturing these waves and allowing students to investigate how impact energy depends on mass and drop height, and how wave amplitude decreases with distance from the impact point.

Learning objectives:

The student simulates the impact of a meteorite by dropping balls of different masses and different heights on various surfaces. By using the FizziQ accelerometer to measure the vibrations generated by these impacts, the student analyzes how the energy of the impact propagates in the form of waves and how the different parameters influence the intensity of the seismic waves.

Level:

Middle school

FizziQ

Author:

Duration (minutes) :

35

What students will do :

- Use the smartphone accelerometer as a seismograph to detect vibrations from ball impacts
- Investigate how the amplitude of the detected vibrations depends on the drop height and mass of the ball
- Verify the relationship between gravitational potential energy (mgh) and the intensity of the impact signal
- Study how vibration amplitude decreases with distance from the impact point
- Understand the principles of seismic wave propagation through solid materials

Scientific concepts:

- Seismic waves
- Energy transfer
- Potential and kinetic energy
- Propagation of mechanical waves
- Impacts and cratering

Sensors:

- Accelerometer (vibration detection, high sampling rate)

What is required:

- Smartphone with the FizziQ application
- Balls of different masses and sizes
- Surfaces of different compositions (wood, metal, sand, etc.)
- Ruler or tape measure to measure the height of fall
- Salad bowl filled with sand

Experimental procedure:

Open FizziQ and select the Accelerometer sensor. Choose the Z-axis (vertical) component and set the sampling rate to the maximum available for best time resolution.
Place the smartphone flat on a hard, smooth surface (a table or hard floor). This surface will transmit the impact vibrations to the sensor.
Choose a starting ball (e.g., a marble or small metal ball). Hold it at a height of 20 cm directly above a spot on the surface, about 30 cm from the smartphone.
Start recording in FizziQ, then drop the ball onto the surface. Let it bounce and settle, then stop recording.
Examine the accelerometer graph. You should see a sharp spike at the moment of impact, followed by smaller oscillations.
Measure the peak amplitude of the impact spike (the maximum acceleration value).
Repeat the drop from the same height three times and calculate the average peak amplitude for reproducibility.
Now repeat the experiment from heights of 10 cm, 30 cm, 40 cm, and 50 cm, recording the peak amplitude for each height.
Plot a graph of peak amplitude versus drop height. The amplitude should increase with height.
Next, keep the drop height constant at 30 cm and use balls of different masses (marble, rubber ball, steel ball). Record the peak amplitude for each.
Finally, investigate distance dependence: drop the same ball from the same height but place the smartphone at distances of 10 cm, 20 cm, 40 cm, and 60 cm from the impact point. Record the amplitude at each distance.
Plot the amplitude versus distance graph. The amplitude should decrease with distance, approximately following an inverse relationship.

Expected results:

Impact amplitudes typically range from 0.05 to 2 m/s² depending on the ball mass, drop height, and distance. The amplitude should increase roughly as the square root of the drop height (since impact velocity v = √(2gh) and the impulse is proportional to v). Heavier balls produce larger amplitudes at the same height. The distance dependence should show a clear decrease, roughly following a 1/r or 1/r² law depending on the surface material and wave type. Hard surfaces (metal, stone) transmit vibrations more efficiently than soft ones (wood, carpet). Students should observe that each impact produces a brief, sharp spike followed by damped oscillations as the surface vibrates freely. The reproducibility between identical drops should be within ±20%, reflecting variations in the exact impact location and angle.

Scientific questions:

- How does the potential energy of the ball relate to the amplitude of the seismic wave detected?
- Why does the vibration amplitude decrease with distance from the impact point?
- What fraction of the ball's kinetic energy is converted into seismic waves? Where does the rest go?
- How does the surface material affect the propagation of impact waves?
- What is the difference between P-waves and S-waves in seismology, and which type is your smartphone detecting?
- How do real seismographs work, and how are they similar to and different from a smartphone accelerometer?

Scientific explanations:

A smartphone's accelerometer can detect very low amplitude vibrations (down to 0.01 m/s²) at a high sampling rate (>100 Hz), making it ideal for this experiment. When a ball falls, its potential energy (Ep = mgh) is converted into kinetic energy (Ec = ½mv²) just before impact.

During the collision, this energy is partially transferred to the surface in the form of mechanical waves which propagate through the medium. These waves are similar to the seismic waves produced during the impact of a meteorite, but on a much smaller scale.

The amplitude of the measured vibrations depends on several factors: the mass of the ball (m), the height of fall (h), and the properties of the material (rigidity, elasticity, density). The formation of craters in sand also follows physical laws established by studies of meteorite impacts: the diameter of the crater is proportional to the cubic root of the impact energy (D ∝E^(1/3)).

This experiment also illustrates why seismometers are used to detect meteorite impacts on other celestial bodies, such as Mars where the Perseverance rover is equipped with similar instruments.

Extension activities:

Frequently asked questions:

Q: I cannot detect any vibration when the ball drops. What should I do?
R: Use a heavier ball (steel or glass marble) and a harder surface. Soft surfaces like carpet absorb vibrations before they reach the smartphone. Also ensure the phone is lying flat directly on the surface, not on a case or stand.

Q: The impact spike is clipped or saturated. Is this a problem?
R: If the amplitude exceeds the sensor range, the peak value will be truncated. Move the smartphone further from the impact point or use a lighter ball to bring the amplitude within the sensor range.

Q: The signal shows many oscillations after the impact. What are these?
R: After the initial impact, the surface vibrates at its natural frequencies (like a drumhead). These damped oscillations are the surface ringing down to equilibrium. The frequency and decay time depend on the surface material and geometry.

Q: How can I measure the speed of wave propagation in the surface?
R: Use two smartphones placed at different distances from the impact point. The time difference between the arrival of the impact spike at each phone, divided by the distance between them, gives the wave speed. Typical values are 3000-5000 m/s in concrete and 1000-2000 m/s in wood.