The basic supposition regarding inertia is that a body (object) in motion tends to stay in motion at a constant rate. This supposes that no force acts on the body/object to slow it down or speed it up. When other forces act on the object motion will not remain constant, and might in fact end. If you fall off your bed in the middle of the night, the bedroom floor ultimately stops this motion of falling.

You can understand a lot about inertia by considering it as it works in cars. First, as you’re driving, as long as you’re not using cruise control, you have to carefully decide when to use your brakes, when to depress the accelerator and when to ease off the accelerator to maintain the same speed. But shouldn’t the car stay in motion at a constant rate once you’ve got it up to the right speed? Actually no it shouldn’t, and no it can’t.

This is because of the second proviso to the principle of inertia that no force acts on the moving object. You have many forces acting on your car when your drive. Chief among these is friction from the road, which car designers may mitigate slightly by the size of the car, the type of tires, and the shape of the car. Other forces include gravity, if you are climbing or descending a hill, air pressure as speed increases, and even weather. Strong winds blowing at you may mean having to use more gas in order to try to maintain a constant speed. The more aerodynamic the car, the less wind speed and air pressure act as a force, so shape can be important.

You can notice how friction affects a car’s speed/inertia if you drive off a paved road onto an unpaved one. The less smooth surface of the unpaved road will slow the car down, and might ultimately bring it to a halt if you do not apply your foot to the accelerator. Further, the car itself comes with its own friction devices, chiefly brakes, which when they are applied to tires slow down the motion of the car. However, even if your brakes suddenly failed, your car would eventually come to a stop. Friction from the road would oppose constant motion forward. If your car ran out of gas, it would continue to run for a few seconds to minutes if you didn’t apply the brakes, since it would tend to stay in motion without acceleration.

In terms of constant speed and motion, you’ll note what occurs when you start descending a hill. The car even without your using the gas will speed up, which may be counteracted by using your brakes or if you drive a manual transmission by downshifting. Speed would seem to increase, thereby violating inertia rules. It’s helpful to understand that there is external force acting on your car, gravity, and further the very weight of your car will increase speed when descending a hill.

Understanding inertia as it applies to a car is also helpful in designing cars that are safe. If the car comes to an abrupt stop for instance, it’s quite possible that you and any passengers would keep moving. Head on collisions can result in people flying forward out of front windows. This is where your seatbelt, and possibly airbag help provide opposite friction to stop this movement, and why it is so important to wear a seatbelt. By providing a counterforce, your body is restrained from moving, which helps to keep you from getting terribly injured if you have an accident.

Of course, the principles of inertia, even though they’ve been well understood by scientists for hundreds of years, haven’t always led to the best safety inventions in cars. It was not understood until recently that front airbags could actually provide so large a counterforce, given the rate at which you would be accelerating, that they could create injury especially to children. Unfortunately, fatal accidents have taught us the “no children under twelve” in the front seat rule. Children should be provided with friction, via car seats and seatbelts so their bodies stop moving safely, but they should never, especially if they are under five feet tall, have to meet the impact of an opening airbag when a car stops abruptly.