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 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. People can understand a lot about inertia by considering it as it works in cars. First, as a person is driving, as long as he is not using cruise control, he has to carefully decide when to use the brakes, when to depress the accelerator, and when to ease off the accelerator to maintain the same speed. The car cannot stay moving at a constant rate, however, because it is acted upon by gravity and friction, among other forces.
The second proviso to the principle of inertia is that no force acts on the moving object, but many forces act on a car when it is driven. 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 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.
Drivers can notice how friction affects a car’s speed if they drive off a paved road onto an unpaved one. The less smooth surface will slow the car down, and might ultimately bring it to a halt if the driver does not apply his foot to the accelerator. Further, the car itself comes with its own friction devices, chiefly brakes, which slow down the motion of the car when they are applied to tires. Even if the brakes suddenly failed, however, the car would eventually come to a stop because friction from the road would oppose the constant motion forward. If the car ran out of gas, it would continue to run for a some time if the driver doesn't apply the brakes, since it would tend to stay in motion without acceleration.
In terms of constant speed and motion, the car — even without using the gas — will speed up when descending a hill, which may be counteracted by using the brakes or by downshifting. Speed would increase, appearing to violate the rules of inertia, but again, it’s helpful to understand that there is external force acting on the car: gravity. In addition, the very weight of the car will increase its speed when descending a hill.
Understanding these rules is also helpful in designing cars that are safe. If the car comes to an abrupt stop, for instance, the driver and any passengers will keep moving. Head on collisions can result in people flying forward out of front windows if they are not restrained. This is where the seatbelt and airbag help provide opposite friction to stop this movement, and why it is so important to wear a seatbelt. By providing a counterforce, the bodies in the car are restrained from moving, which helps to keep people from getting terribly injured if there is 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 a person would be accelerating, that they could cause injury, especially to a child. Unfortunately, fatal accidents have taught that no children under 12 should sit in the front seat. 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 5 feet (152.4 cm) tall, have to meet the impact of an opening airbag when a car stops abruptly.