Consider the following scenario. You are driving your friend home from a coffee shop. Your friend momentarily places her coffee on the dashboard so she can riffle through her backpack to look for her cell phone. At that instant, a squirrel darts in front of your car. You slam on the breaks and your friend’s coffee flies into the windshield splattering the entire dash with mocha latte. Why did the coffee fly into the windshield when you slammed on the breaks?

Was it pushed forward by the breaking automobile, or did it simply fly forward without any push or pull compelling it to do so?

Most first-time physics students believe that of course there was something pushing the coffee forward. Why else would it move so violently. However, this is incorrect.

The correct description of the events surrounding the exploding coffee cup is as follows.

As you drive down the street, you, your car, your friend, your friend’s coffee, and everything else inside the automobile are in motion. So, the instant before you slam on the breaks, the coffee cup is in a state of motion. When the breaks are applied, the car’s motion changes; it slows down. Thus everything that is a part of the car also slows down. The coffee is in the car but it is not a part of the car because it is just resting on the dash and not firmly attached to anything. So, while the car’s motion changes as a result of the breaks being applied, the coffee’s motion does not change. It continues to move as it was doing before the squirrel so daringly darted in front of you. The coffee cup moves directly into the windshield, which is slowing down right in front of the cup. A mocha shower ensues.

The essential physics required to understand these events is a principle that was first proposed by Galileo Galilei and latter refined by Sir Isaac Newton. It has come to be called Newton’s First Law of Motion. This principle states that:

An object in motion will continue to move with a constant speed and 
in a straight line unless some push or pull compels its motion to 
change. At the same time, an object at rest will remain at rest 
unless a push or pull compels it to start moving.

In other words, objects have a property that causes them to resist changes to their motion. This property is called inertia. When we talk about Newton’s Second Law of Motion, we will discover that physics tells us that objects which are more massive resist changes in there motion better than less massive objects. But we don’t say that because of this, massive objects have more inertia. Inertia is the property which all objects have regardless of their mass. Mass is the property that tells us the degree to which objects resist changes in their motion.

Why do first-time physics students get the interpretation of the coffee explosion wrong? Why do they insist that something pushes the coffee forward. Because their experience tells them that when in a car that breaks suddenly, it “feels” like they are pushed forward.

What you actually feel in such a scenario is your seatbelt pushing you backward. There is in fact nothing pushing you forward. You have inertia and thus will tend to move in a straight line with a constant speed until something, in this case the seatbelt, pushes on you and compels you to slow down. The coffee isn’t wearing a seat belt. If you weren’t wearing a seat belt then you too would collide with the dashboard or windshield like the coffee cup. Moral of the story: always were a seatbelt.