Defying gravity
Question:

First, watch the experiment below:

English version:

Hebrew version:


If you are already familiar with basics of circular motion, skip straight to the question. The theoretical background brought below contains clues for the answer.

In physics, circular motion is a movement of an object along the circumference of a circle or rotation along a circular path. Examples of circular motion include: an artificial satellite orbiting the Earth at constant height, a stone which is tied to a rope and is being swung in circles, a car turning through a curve in a race track, an electron moving perpendicular to a uniform magnetic field and many more.

Since the object's velocity vector is constantly changing direction, the moving object is undergoing acceleration by a centripetal force in the direction of the center of rotation. Without this acceleration, the object would move in a straight line, according to Newton's laws of motion. If we take, for example, a marble rotating inside a cone, the forces acting on the marble are shown below:

N- the normal force- is applied by the surface perpendicularly. The horizontal component of the Normal force is the centripetal force, which keeps the marble rotating around the glass circumference.

Mg- the gravitational force- applied by the earth towards the center of the earth. For more information about the value of g, watch the BLOSSOMS video: http://blossoms.mit.edu/videos/files/english/fingerprinting_gravity_0

 If both the marble  and the cone  are smooth, friction can be neglected.

Question: Would the marble rise to the same height as in the wine glass, when using the coffee cup shown in the video?

6 People tried to answer this question

No, the marble will not rise at all, since the normal force will not oppose gravity due to the wall angle.

No. the marlbe will rise lower, because it wiill lose it's velocity to friction, due to the larger cup diameter.

Yes- the marble will rise to the same height, since the graviational force is similar in both cases.

No, the marble will rise higher- since the coffee cup is shorter, less energy is required for the marble to rise.

That's correct! watch the answer in the video below:

English version:

Hebrew version:

Wrong. The friction deiiferences can be neglected in this case, since both the marble and cup are smooth. Try again! 

Wrong. The gravitational force is indeed similar, but it's not enough to lift the marble. On the contrary- an upwards facing force is required. Try again! 

Wrong. The height of the cup is not related to the energy  stored in the marble. The marble can rise higher in a short cup than in a tall glass- depending on the shape of the cup. Try again! 



Comments:
1.  vainberg uri (13.06.2017 pm 12.36.44)
peer review

The subject is presented very clear and precise. 
The dual-lingual movies are explaining the material well and coherent.
The qustion itself requires understanding of the phenomenon of the normal force
and the need of upward force vector to make the ball climb up.
Good explinations and overall it is a very impresive work.



History of edits
Edited BY: Helen Wolfson Edit Date: 2017-05-22 17:20:59
Created By: Helen Wolfson Edit Date: 2017-05-13 12:15:54
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