PhysicsLAB
Video LAB: A Gravitron
This lab is based on a video entitled Einstein on the Graviton. It was originally part of the Direct Measurement Video Project hosted at the Science Education Research Center [SERC] at Carleton College. The project was partially funded by a Science Foundtation Grant [#1245268] awarded in September 2013. The copyright for this video now belongs to Pivot Interactives which requires users to pay a fee to access their content. Peter Bohacek and Matt Vonk are its founders and current directors.

The following lab implementation was designed for use in my Honors Physics I class and only represents one method of analyzing the data provided in the video.
 
This video represents a classic first year physics problem in which the centripetal force in a rotating vertical cylinder is provided by the normal force. This scenario models a common amusement park ride called the The Gravitron.
 
The video begins with an Einstein "action figure" (doll) pinned against a vertical wall on a rotating platform. During the video, the platform's rate of rotation is decreased and the doll eventually slides down the wall. You will collect data during Phase I of the video that will allow you to determine the coefficient of static friction between the doll and the wall.
 
A similar situation is modeled by this physlet called The Gravitron. You might like to preview it before starting your analysis of the video.
 
Now watch the video several times to become familiar with the experimental scenario. Be aware that the first part of the video does not have any measurement information. The scales and frame numbers are provided approximately 60% of the way through. Notice that a protractor tool is superimposed over the video once the experimental apparatus is viewed from above. Each section of the protractor represents a 30º arc.
 
Phase I - Einstein pinned to the top of the wall
 
What is the radius to the front of the inner wall against which the Einstein doll was pinned? State your answer in meters. 

At what frame rate is the video viewed? 

With respect to frames+137:
 
 - on which frame number does Einstein return to the same right hand position as in frames#137? 

- on which frame number does Einstein complete a second complete revolution once again reaching the same right-hand position as in frames#137? 

On average, how much time was required for one complete revolution during the first part of the video when Einstein was pinned to the wall? 

What was the rotational frequency of the platform during the first part of the video? 

Calculate the magnitude of Einstein’s tangential velocity. 

What centripetal acceleration did Einstein experience in the first part of the video? 

 
Phase III - Einstein's feet are resting on the rotating platform
 
During the second section, or Phase II of the video, the platform begins to slow down allowing Einstein to slide down the wall. For the purposes of our analysis we are going to assume that the platform once again rotates at a constant speed once Einstein’s feet reach the platform. This final section will be called Phase III
 
On which frame number do Einstein’s feet first touch the rotating platform? 

On which frame number does Einstein complete one revolution to return to that same position on the protractor as your answer to the previous question? 

Unfortunately, the video does not last long enough for Einstein to complete another complete revolution at this lower speed. On the final frames#826 what fraction of the second revolution was completed? 

Using your values from the previous three questions, calculate the average rotational frequency in Phase III of the video? 

What was the rotational period of the gravitron during Phase III of the video? 

On average, what tangential velocity did Einstein experience in Phase III of the video? 

Conclusions 
 
Which of the following freebody diagrams correctly displays the forces acting on Einstein in frames#137 during Phase I of the video? (The doll is instantaneously located on the right side of the frame at 0º facing the camera's position at the center.)
 
 
In Phase I of the video, while Einstein was pinned to the wall, calculate the minimum coefficient of static friction between Einstein and the supporting wall. Be care to correctly identify the source of the normal force. 

In Phase II of the video, that is, the transition between Phase I when Einstein was pinned to the wall and Phase III in which he was "standing" on the rotating platform, Einstein slides down the wall 5.8 cm. If this transition took 38 frames, determine the vertical acceleration Einstein experienced during his slide. 

Which of the following freebody diagrams correctly displays the forces acting on Einstein in frames#521 in Phase II while he was sliding down the wall? (The doll is instantaneously located at the top of the frame facing the camera's position at the center.)
 
 
 
While Einstein was sliding downwards, what was the average tangential velocity provided by the rotating platform? 

What centripetal acceleration was Einstein experiencing while he slid down the wall? 

Solve for the coefficient of kinetic friction between Einstein and the wall. Be care to correctly identify the source of the normal force. 

Why did reducing the graviton angular velocity result in Einstein sliding down the wall? Do not state that the coefficient of static friction was greater than the coefficient of kinetic friction. Base your answer on the rotational speed of the platform.
 

Why was it never necessary to know the value of for the mass of the Einstein doll?