How does a cup move on its own when pouring hot water into it?

  • Context: High School 
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SUMMARY

The phenomenon of a cup moving on its own when hot water is poured into it can be attributed to several factors, including the motion of the liquid inside the cup, reduced friction due to heated air, and the presence of spilled water on the surface. Participants in the discussion noted that the cup may have experienced a "planing" effect on the water, allowing it to glide with minimal resistance. Additionally, the pressure difference created by the heated air venting from under the cup can generate sufficient force to initiate movement, as demonstrated by calculations estimating a force of up to 26N at elevated temperatures.

PREREQUISITES
  • Understanding of fluid dynamics, particularly the behavior of liquids in motion.
  • Familiarity with thermodynamics, specifically the effects of temperature on air pressure.
  • Basic knowledge of friction and its role in motion on surfaces.
  • Experience with experimental physics, including the ability to conduct practical tests and calculations.
NEXT STEPS
  • Research the principles of fluid dynamics, focusing on sloshing and planing effects.
  • Explore thermodynamic concepts related to gas laws and pressure differentials.
  • Investigate the impact of surface tension and friction on the movement of objects on liquids.
  • Conduct experiments to observe and measure the effects of heated air on the movement of lightweight objects on smooth surfaces.
USEFUL FOR

This discussion is beneficial for physicists, engineers, and anyone interested in understanding the mechanics behind fluid motion and the interaction of heat with materials. It is particularly relevant for those conducting experiments in fluid dynamics or thermodynamics.

  • #31
You use the same principal in oil bearings. A small oil pump pressurizes the oil and the load floats on an almost frictionless film of oil.
You can push a 200ton telescope around with your hand.
 
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  • #32
Here is the phenomena in action:



This always happens at a certain asian restaurant we visit, and takes everyone by surprise.

I finally was able to reproduce it by putting in very hot tea and letting it sit for a while. Its a good party trick.
 
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