Parabolic flight
This activity allows students to understand the phenomenon of weightlessness by simulating a parabolic flight with their smartphone. It develops the understanding of free fall movement and the effects of gravity.
Astronauts on the International Space Station float weightlessly not because they have escaped Earth's gravity, which is still about 90% of its surface value at orbital altitude, but because they are in continuous free fall, orbiting around the planet. The same weightless sensation can be briefly experienced on Earth through parabolic flight: an aircraft climbs steeply, then cuts its engines and follows a ballistic parabolic trajectory for about 20-25 seconds, during which everything inside experiences microgravity. NASA uses modified KC-135 aircraft (nicknamed the Vomit Comet) for astronaut training, while companies like Zero-G offer commercial flights. Remarkably, you can recreate this experience on a small scale in your classroom by simply tossing a smartphone into the air above a soft landing. During the brief airborne phase, the accelerometer reads zero, not because gravity disappears, but because the phone and its sensor are falling together. This beautifully illustrates Einstein's equivalence principle and the true meaning of weightlessness.
Learning objectives:
The student uses the FizziQ accelerometer to record the acceleration during a parabolic motion. By gently throwing his smartphone above a mattress and analyzing the acceleration data, the student observes that during the free flight phase the measured acceleration is close to zero despite the constant presence of gravity, which allows us to understand the principle of apparent weightlessness.
Level:
High school
FizziQ
Author:
Duration (minutes) :
25
What students will do :
- Record and analyze the acceleration during a brief parabolic flight of a smartphone
- Observe that the accelerometer reads approximately zero during free fall despite gravity still acting
- Identify the three phases of motion: launch, free flight (weightlessness), and landing impact
- Understand why free fall produces apparent weightlessness
- Connect the experiment to real parabolic flights and the physics of orbital motion
Scientific concepts:
- Free fall
- Weightlessness
- Inertial frames of reference
- Parabolic movement
- Equivalence principle
Sensors:
- Accelerometer (absolute acceleration)
What is required:
- Smartphone with the FizziQ application
- A mattress or soft surface to securely hold the smartphone
- A clear space
- FizziQ experience notebook
Experimental procedure:
Prepare a thick mattress, cushion, or foam pad on the floor to safely catch the smartphone.
Open FizziQ and select the Accelerometer sensor with absolute acceleration. Set the sampling rate to maximum.
Place the smartphone in a protective padded sleeve (a thick sock with padding works well).
Hold the protected phone at waist height directly above the mattress. Start recording in FizziQ.
Wait 2 seconds (to record the resting phase), then gently toss the phone upward (about 30-50 cm height) so it lands on the mattress.
Let the phone settle, then stop recording after 2 more seconds.
Examine the acceleration graph. Identify three distinct phases: the resting phase (≈ 9.81 m/s²), the free flight phase (≈ 0 m/s²), and the impact spike (>> 9.81 m/s²).
Measure the duration of the free flight phase (the time the acceleration is near zero). For a 30 cm toss height, this should be about 0.5 seconds.
Calculate the expected free-flight duration using t = 2 × √(2h/g) for a toss of height h. Compare with the measured duration.
Repeat the experiment with different toss heights: 20 cm, 40 cm, and 60 cm. Record the free-flight duration for each.
Plot the free-flight duration versus toss height and verify the expected relationship.
Discuss: during the free flight, does gravity disappear? Why does the accelerometer read zero even though the phone is clearly decelerating (going up) and then accelerating (coming down)?
Expected results:
During the free-flight phase, the absolute acceleration should drop to near 0 m/s² (typically 0.1-0.5 m/s² due to air resistance and phone rotation). For a 30 cm toss, the total airborne time should be about 0.49 seconds; for 60 cm, about 0.70 seconds. The resting phase before and after should show approximately 9.81 m/s². The impact spike can reach 20-100 m/s² depending on the cushioning. Students should observe that the free flight duration increases as the square root of the toss height. The key conceptual insight is that the accelerometer reads zero not because gravity is absent but because both the phone and its sensor are accelerating at exactly g, so there is no relative acceleration between them.
Scientific questions:
- Why does the accelerometer read approximately 9.81 m/s² when the phone is at rest on a table?
- Why does it read zero during free fall, even though gravity is clearly still acting on the phone?
- How does this experiment relate to the weightlessness experienced by astronauts on the ISS?
- What is Einstein's equivalence principle, and how does this experiment illustrate it?
- If you could throw the phone high enough and fast enough horizontally, what trajectory would it follow?
- How do Zero-G parabolic flight aircraft create 25 seconds of weightlessness?
Scientific explanations:
The parabolic flight experiment simulates on a small scale the phenomenon used by "Zero-G" aircraft to create temporary weightlessness conditions. This phenomenon is based on a fundamental principle of physics: an object in free fall does not "feel" gravity.
A smartphone's accelerometer measures not gravity directly, but the reaction force that the mount exerts on the device. When resting on a table, it detects an upward acceleration of approximately 9.8 m/s², corresponding to the normal force opposing the weight.
When the smartphone is thrown into the air, it enters into free fall under the exclusive influence of gravity. In this non-inertial frame of reference (the smartphone itself), all objects appear to be floating, because they experience exactly the same acceleration.
This is why the accelerometer indicates a value close to zero during the free flight phase. It's not that gravity has disappeared, but rather that its effects can no longer be measured by an accelerometer free-falling with it.
This phenomenon illustrates Einstein's principle of equivalence, the foundation of general relativity: it is impossible to locally distinguish between a gravitational effect and an equivalent acceleration in the opposite direction. Planes carrying out parabolic flights for astronaut training or scientific experiments follow precisely this trajectory: they climb sharply nose-up, then reduce the thrust of the engines to follow exactly the natural trajectory of a body in free fall for 20-25 seconds, creating perfect weightlessness inside.
Likewise, the International Space Station is constantly in free fall around the Earth, hence the state of permanent weightlessness of its occupants, despite a gravity still equal to approximately 90% of that on the earth's surface at this altitude (400 km).
Extension activities:
- Why does the accelerometer read approximately 9.81 m/s² when the phone is at rest on a table?
- Why does it read zero during free fall, even though gravity is clearly still acting on the phone?
- How does this experiment relate to the weightlessness experienced by astronauts on the ISS?
- What is Einstein's equivalence principle, and how does this experiment illustrate it?
- If you could throw the phone high enough and fast enough horizontally, what trajectory would it follow?
- How do Zero-G parabolic flight aircraft create 25 seconds of weightlessness?
Frequently asked questions:
Q: The free-flight acceleration is not exactly zero. Is something wrong?
R: Air resistance, phone rotation, and sensor noise all contribute small non-zero readings during free fall. Values of 0.1-0.5 m/s² are normal. The key observation is that it drops dramatically from 9.81 to near zero.
Q: Is it safe to throw my smartphone?
R: With proper padding and a soft landing surface, the risk is minimal. The impact from a 50 cm toss onto a mattress produces accelerations well within the phone's tolerance. Always use padding.
Q: The free-flight duration seems shorter than expected. Why?
R: You may be identifying the start and end of free fall too conservatively. The transition from rest to free fall is very abrupt; look for the sharp drop in acceleration as the precise start time.
Q: How is this different from dropping the phone?
R: A toss includes both upward and downward motion, doubling the free-flight time compared to a simple drop from the same height. The physics is identical during both phases: the phone is in free fall the entire time it is airborne.