In your experiment, where a car is connected to a fixed metal disk via a string, the variable that remains constant during the trials typically depends on the setup and conditions of the experiment. Here are some options:
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Mass of the Car and Disk: Assuming that neither the mass of the car nor the mass of the metal disk changes during the trials, this quantity remains constant.
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Tension in the String: If the car is moving with a constant acceleration, the tension in the string may remain constant depending on the forces acting upon the car and the disk.
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Acceleration due to Gravity: If the experiment takes place in a gravitational field and no significant vertical motion is involved, the acceleration due to gravity remains constant.
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Length of the String: If the string does not stretch or change length during the trials, its length remains constant.
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Force applied: If you're applying a constant force on the car, then that force also remains constant throughout the trials.
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Velocity: If the experiment is designed to maintain the velocity of the car at a constant speed (e.g., through control systems), then velocity can be constant.
Without additional context on how the car is being moved (is it under constant force, is it decelerating, etc.), you'll need to clarify how the trials are set up. But generally, the mass of the objects involved or the gravitational acceleration are likely to remain constant throughout the experiment.