Hi, I am new to this app and am doing a lab regarding the centripetal acceleration of a bicycle wheel corresponding to different radii on the wheel. I have mounted the bike on a treadmill to obtain a constant speed, however, I keep getting sin curves for the centripetal acceleration when it should be constant. I have attached my phone to the spike of the bike vertically so that the phone is parallel to the spike, however, I'm not sure how I can obtain a constant acceleration.

01-14-2021, 04:16 PM (This post was last modified: 01-14-2021, 04:17 PM by Sebastian Staacks.)

(01-14-2021, 03:10 PM)solid Wrote: Mount your wheel horizontally .. Row accelerometer measures \vec{a} - \vec{g}.

That would be my guess, too. However, the centripetal acceleration experiment does not use the raw sensor but the accelerometer "without g" (aka linear acceleration). This is supposed to subtract the gravitational contribution and in theory it should not show this behavior. In practice, the device cannot do this perfectly and it will depend on the phone how well it can compensate the force of gravity on the sensor. Some device do this near perfectly while others have trouble in specific situations. If you start the measurement while the wheel is already spinning, please try starting the experiment while the wheel is at rest as some devices take a reference vector when starting which fails if it is already spinning.

Besides that phyphox tries to help a bit by averaging the acceleration over 0.5s, but as this time window will shift over the oscillation without perfectly matching a period, there will still be an oscillation as a result. it might be interesting to see what the acceleration looks like when recorded with the simple "acceleration without g" configuration as this will not average the data.

Probably the bike "jumps" a little on the treadmill due to some asymmetry of the wheel. The oscillation period corresponds well to the rotation speed (1.3 rad/s): 2*%pi / 1.3 rad/s * 9 rotations = 43.5 s ..

In addition to the oscillations there is a strange increase of the average acceleration with time.

01-16-2021, 01:03 PM (This post was last modified: 01-19-2021, 01:52 PM by Roland Van Kerschaver.)

(01-15-2021, 02:45 AM)solid Wrote: Probably the bike "jumps" a little on the treadmill due to some asymmetry of the wheel. The oscillation period corresponds well to the rotation speed (1.3 rad/s): 2*%pi / 1.3 rad/s * 9 rotations = 43.5 s ..

In addition to the oscillations there is a strange increase of the average acceleration with time.

Wheel
Raw data
Accelerator x
Gyroscope z

Other interesting examples are most experiments that measure centripetal acceleration by rotating a phone (for example in a salad spinner or on a record player) and experiments in which the phone is placed in a tire or a roll to measure its velocity.

Rotating on a record player = rotating on a horizontal bike wheel