Question

An old remedy for separating two nested drinking glasses stuck together is to run water at one temperature into the inner glass and then run water at a different temperature over the surface of the outer glass. Which water should be hot and which cold?

Short answer

Run hot water on the outer glass and cold water in the inner glass to separate them.

Step-by-step solution

Understand the Problem

Two glasses are stuck together—one inside the other. The goal is to separate them by using water at different temperatures.

Concept of Thermal Expansion and Contraction

When materials are heated, they tend to expand. Conversely, when they are cooled, they tend to contract. This concept can be applied here.

Apply Hot Water to the Outer Glass

The outer glass should be run under hot water. The reason is that heating the outer glass will cause it to expand slightly, increasing the space between the two glasses.

Apply Cold Water to the Inner Glass

The inner glass should have cold water run into it. Cooling the inner glass will cause it to contract slightly, decreasing its size and making it easier to separate from the outer glass.

Combine the Effects

By expanding the outer glass with heat and contracting the inner glass with cold, you create more space between the two glasses, facilitating their separation.

Key concepts

heat transfer

Heat transfer refers to the process of heat moving from one object or area to another. This principle is crucial in understanding thermal expansion and contraction.
When you apply heat to an object, the molecules within it gain energy and begin to move more vigorously. This results in the object feeling warmer and often leads to expansion.
Conversely, when an object is exposed to colder temperatures, the molecules lose energy, moves less, and the object contracts.
In the context of our exercise, running hot water over the outer glass transfers heat, causing it to expand. Similarly, cold water run into the inner glass transfers cold, leading it to contract. Heat transfer underlines the entire method, helping the glasses to separate.

contraction

Contraction is the process where a material decreases in size. When materials are cooled, the molecules slow down and draw closer to each other.
This effect is very useful when trying to separate stuck objects. In our glass-separating exercise, cold water is run inside the inner glass, causing it to contract a bit.
The decrease in its size helps to loosen the grip between the glasses. This process, in conjunction with expansion, makes it possible to use temperature differences to effectively separate nested glasses.

material properties

Material properties play a pivotal role in thermal expansion and contraction. Different materials respond uniquely to temperature changes.
Materials like glass, which are used in this exercise, have specific coefficients of thermal expansion. The coefficient determines how much a material will expand or contract in response to temperature changes.
Understanding these properties helps us predict and utilize the way objects will behave when heated or cooled. It's important to know the properties of the materials involved when attempting to use temperature to alter them for practical solutions like unsticking glasses.

temperature effects

Temperature effects describe how varying temperatures can alter the physical characteristics of materials. As temperature rises, most materials expand. As it decreases, they contract.
These effects allow for practical applications, such as our method of separating glasses. By using hot water on the outer glass and cold water on the inner one, we exploit these natural properties.
This results in the desired separation, showcasing how temperature effects can be practically and successfully applied to everyday problems.