Flipped Pizza
When you bite into a hot pizza, the roof of your mouth gets burnt, but your tongue doesn't. What happens if you flip the pizza upside down?
Figure 1
This article recalls a familiar phenomenon and explains it using thermodynamics. The situation is shown in Figure 1. Usually, when we take a bite into a hot pizza, the roof of your mouth in contact with the cheese would get burnt. However, your tongue, which is in contact with the crust does not get burnt. To try and explain this many assume that the temperature of the cheese is higher than that of the crust, but this is not the case. A video demonstration on the right/below gives proof of it. If the temperature of the cheese and crust are the same, how come the cheese burns our mouths, but the crust doesn't? Furthermore, would the "hot" side change if the pizza were flipped when we ate it?
First, we need to talk about inertia. Newton's First Law says that an object in motion will tend to stay in that motion unless acted upon by a net external force. If an object is at rest, it will stay at rest. Most of us are familiar with this concept from our everyday lives. An object's tendency to resist its change of motion is called inertia. It can be measured by what we call inertial mass, or for our purposes, just mass. This implies that if an object has more mass, it will take a greater force to change its motion by a specific amount. A simple example is that it is easier to speed up something heavier than something lighter. Other types of inertia also exist to define how an object resists change. For instance, rotational inertia is used to define how much torque is needed to change the rotational motion of an object by a specific amount. Among all the different types of inertia, the one that matters to us today is thermal inertia. This is also called specific heat or the specific heat capacity. Just like mass, it is an intrinsic property of a substance that defines how easy or hard it is for the object to transfer thermal energy.
Now that we understand how "hot" the pizza feels depends on the specific heat of the part we are touching, we can solve our problem. It is important to note that water has one of the highest specific heats of all substances. The content of cheese has much more water than that of the bread under it, so it is safe to say that the specific heat of the cheese is greater than that of the bread. Furthermore, it is estimated that the specific heat capacity of cheese is 3270 J/kg*C and the specific heat of bread is 2800 J/kg*C. Therefore, the cheese will transfer more heat energy to your mouth than the bread will. When we eat pizza, the cheese is conventionally facing up, and this is why only the roof of your mouth gets burnt. This brings us to the answer to our first question. If you ate a hot pizza upside down, instead of burning the roof of your mouth, you would burn your tongue!