# Experiment: Roll

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Roll
Experiment Roll
Category Mechanics
Used sensors Gyroscope

The experiment "roll" aims at tracking the speed of a paper roll. The phone is placed in this roll and uses its gyroscope to determine the angular velocity. With the roll's radius (entered by the user), it can calculate the speed of the roll, which is usually observed through the remote interface.

This setup is great to demonstrate the constant acceleration on an inclined plane.

This experiment is no longer available on iOS due to a ruling by Apple.

## Requirements

• A roll of some sort is needed, large enough to hold the phone. A typical example is a poster roll which has been cut to a reasonable length or a part of a rainwater pipe. End caps are not required for the experiment to work, but they add to fun factor as they conceal the phone and give the feel of the roll being an individual object.
• Additionally, some padding is needed to hold the phone in place. This can be some paper or packaging materials.
• As the phone is not accessible within the pipe, you certainly want to remote control it in this experiment. So a second device for the remote interface is usually required as well.

## Setup

The phone is placed in the roll with some padding to hold it in the center. It should not be loose and rotate about it y axis (the axis along the longer side of the screen). Make sure to open the roll experiment in the app and enable the remote access first.

## Analysis

The gyroscope directly measures the angular velocity in radians per second. So this experiment simply multiplies this by the roll's radius to get the tangential velocity of the rotation, which is the speed of the roll itself.

## Problems and resolutions

• The measured speed seems to oscillate: This is usually caused by an asymmetric distribution of weight within the roll. It actually wobbles instead of rolling smoothly. In most cases this is caused by the phone not being centered properly within the roll, so make sure that your padding holds the phone in place exactly in the center of the roll.