When you are in a front end collision in your car, why do you need to wear your seatbelt? What will happen to you if you don't. First think about the following questions.
What happens to the state of motion of the car in a front end collision?
What happens to your body's state of motion if the car is in a front end collision?
Look Here to Find Out (Quicktime Version)
Look Here to Find Out (Animated Gif Version)
As a driver you should never tailgate any vehicle, but you should be especially careful not to tailgate a motorcycle. Why would it be more dangerous to follow a motorcycle too closely? Think about how you measure inertia.
... and for that matter, why do semi trucks need larger engines than cars? The answer to these questions also has to do with what inertia is a mesaure of.