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Whirlygig: Quantitative Testing Experiment
Aim
To test whether Newton's second law can be applied to circular motion.
Prior Knowledge
- Kinematics and dynamics of circular motion at constant speed.
- Newton's Laws.
- Components of a force.
Description of the Experiment
In the experiment David makes three black stoppers go in a circle. Use the first frame of the movie to perform neccesary measurements, estimations and calculations to predict the period of revolution of the stoppers (do not watch the video yet). After you made the prediction, measure the period by counting the number of frames during which the stoppers make a full circle. Decide whether your prediction matched the outcome of the experiment. If not, think of how you can revise the model that you used to make a prediction or the additional assumptions to have a better match.
Addtional Information
The three black stoppers that are whirling in a circle have a combined mass of 58.7g. The length of David's arm from elbow to wrist is about 30 cm.
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| Youtube movies can be stepped frame by frame using the , and . keys on your keyboard. If you want to download the movie to your computer, right-click or control-click HERE. |
Questions
- Draw a free-body diagram for the stoppers when they rotate around the tube.
- Why is the string tilted at an angle?
- Use the free-body diagram to write Newton's second law in component form for the stoppers.
- Use Newton's second law and the measurements of the angles and lengths (when needed) to predict the period of revolution of the stoppers. Think of the assumptions you are making.
- What are experimental uncertainties in your prediction? What instrument gives you the largest uncertainty? Rewrite your prediction taking the experimental uncertainties into account.
- Then measure the time for one revolution. What is the experimental uncertainty in your measurement? Did time matched the prediction? If not, do you need to revise the assumptions?
- What is your judgment about the laws of physics that you used to make the prediction? Do they work here?
