Astronaut and wringer

This experiment allows students to understand the concepts of centripetal acceleration and g-force in a concrete way. It establishes a tangible link between theoretical physics and a real application in the space domain.

Before astronauts can fly to space, they must endure one of the most grueling tests in their training: the human centrifuge. This massive spinning machine subjects the body to forces several times that of normal gravity, simulating the extreme accelerations experienced during rocket launch and atmospheric reentry. The same physical principle that pushes you into your car seat during rapid acceleration, centripetal force, governs the operation of both billion-dollar NASA training centrifuges and the humble salad spinner in your kitchen. In both cases, an object moving in a circular path requires a constant inward force, and the resulting acceleration increases with both the speed of rotation and the radius of the circle. Fighter pilots experience up to 9g during sharp turns, while astronauts may endure sustained accelerations of 3-4g during launch. But how do these forces compare to those generated by everyday rotating devices? In this experiment, students place a smartphone inside a salad spinner to measure the centripetal acceleration directly, then evaluate whether a human could withstand the same forces.

Activity overview:

The student transforms a salad spinner into a miniature juicer by placing their protected smartphone inside, then measures the maximum centripetal acceleration generated during rotation. By comparing this value to the limits of human tolerance, the student evaluates whether an astronaut could withstand this acceleration. It then calculates the rotational speed in revolutions per minute and analyzes how the size of the basket influences the acceleration.

Level:

High school

FizziQ

Author:

Duration (minutes) :

35

What students will do :

- Measure the centripetal acceleration inside a rotating salad spinner using the smartphone accelerometer
- Relate the measured acceleration to the rotational speed and radius of rotation
- Compare the measured g-forces with human physiological tolerance limits
- Verify the relationship a = ω²r between centripetal acceleration, angular velocity, and radius
- Understand the concept of g-force and its effects on the human body

Scientific concepts:

- Centripetal acceleration, G force and its physiological effects, Uniform circular motion, Relationship between radius, angular velocity and acceleration, Physiology and limits of the human body

Sensors:

- Accelerometer (total acceleration magnitude or single-axis)

Material needed:

- Smartphone with the FizziQ application, Salad spinner, Protection for the smartphone, Stopwatch (optional), Tape measure to measure the radius of the spinner

Experimental procedure:

Measure the inner radius of the salad spinner basket using a tape measure. Record this value in meters.
Open FizziQ on your smartphone and select the Accelerometer sensor. Choose the absolute acceleration measurement.
Protect your smartphone by wrapping it in a soft cloth or placing it in a padded case. Ensure it fits securely inside the spinner basket.
Place the smartphone inside the salad spinner basket with the screen visible through the transparent lid (if possible). Position it so the sensitive axis is oriented radially (pointing toward the center of rotation).
Set the FizziQ recording frequency to the maximum available rate for best time resolution.
Start recording in FizziQ, then begin spinning the salad spinner steadily and as fast as possible.
Maintain a constant spinning speed for at least 5 seconds, then stop. Stop the FizziQ recording.
Examine the acceleration graph. Identify the plateau region where the spinning speed was approximately constant and read the maximum acceleration value.
Convert the measured acceleration to g-force units by dividing by 9.81 m/s². Record the number of g's.
Calculate the angular velocity ω from the measured acceleration using ω = √(a/r), where r is the basket radius.
Convert ω to revolutions per minute (RPM): RPM = ω × 60 / (2π).
Compare the measured g-force with human tolerance thresholds: 2-3g (blood pooling in legs), 4-6g (risk of blackout for untrained persons), 9g+ (limit for trained fighter pilots with g-suits).
Discuss whether an astronaut could survive a ride in this salad spinner, and calculate how large the spinner would need to be to reach dangerous g-forces at the same rotation speed.

Expected results:

Typical salad spinners can generate accelerations of 5-15 m/s² (0.5-1.5g) depending on their size and the vigor of spinning. A spinner with a 10 cm radius spun at 3 revolutions per second would produce a centripetal acceleration of about 3.6g. Students should find that the measured acceleration is well within human tolerance for brief exposure, though sustained exposure at even 2-3g would be uncomfortable. The calculation of angular velocity from the measured acceleration and radius provides a useful cross-check: if the student counts rotations during a timed interval, the independently measured RPM should approximately match the calculated value. Measurement noise and vibrations from the spinning mechanism may cause the acceleration graph to show oscillations around the mean value, typically ±10-20% of the plateau value.

Scientific questions:

- How does the centripetal acceleration change if you double the rotational speed while keeping the radius constant?
- Why do larger centrifuges produce higher g-forces at the same rotational speed?
- What physiological effects would an astronaut experience at 3g? At 6g? At 9g?
- Why do fighter pilots wear special g-suits, and how do these suits work?
- What is the difference between centripetal acceleration and centrifugal force?
- How could you modify this experiment to verify the relationship a = ω²r more precisely?

Scientific explanations:

Centripetal acceleration is the force that keeps an object on a circular path. It is directed towards the center of the circle and is calculated according to the formula a = v²/r = ω²r, where v is the tangential speed, r the radius of rotation and ω the angular speed.

In this experiment, the smartphone's accelerometer measures this acceleration in units of g (1g = 9.81 m/s²). Astronaut training centrifuges use this same principle to simulate the forces supported during takeoff or atmospheric reentry.

The physiological effects vary depending on the intensity: at 2-3g, blood accumulates in the legs; at 4-6g, an untrained person may lose consciousness; above 9g, even trained pilots risk blackout. The relationship between radius and centripetal acceleration is particularly important: for the same angular speed, the acceleration increases proportionally to the radius.

This is why professional juicers usually have large radii (4-8 meters).

Extension activities:

Frequently asked questions:

Q: The acceleration reading keeps changing during spinning. Which value should I use?
R: Look for the region of the graph where the acceleration is most stable (the plateau). Take the average value during this steady-state period. Fluctuations are normal and result from slight variations in spinning speed and vibrations.

Q: Can this experiment damage my smartphone?
R: The accelerations in a salad spinner (typically under 2g) are well within the tolerance of modern smartphones, which are designed to withstand drops generating 100g or more. However, always protect the phone with padding to prevent scratches or impact damage if it shifts inside the spinner.

Q: My calculated RPM seems unreasonably high or low. What could be wrong?
R: Double-check your radius measurement (use the distance from the center of the spinner to the smartphone's accelerometer location, not the outer edge) and ensure you are using consistent units (meters, not centimeters). Also verify that you are reading acceleration in m/s², not in g units.

Q: Why does the acceleration not return exactly to 9.81 m/s² when the spinner stops?
R: The absolute acceleration includes gravity (1g = 9.81 m/s²). When stationary, it should read approximately 9.81 m/s². Small deviations (±0.1 m/s²) are due to sensor calibration. During spinning, the total acceleration combines gravity and centripetal acceleration vectorially.