When you're on a roller coaster, the true force pressing on you is the seat pushing your body forward. At a certain rate of acceleration, these opposing pressures balance each other out, giving you the impression of weightlessness that a skydiver experiences in free fall.
The reason you don't feel this force when you're walking down the street or inside a car is because those objects are providing some form of stability. A roller coaster, on the other hand, is in constant motion; if you were to try and stop yourself by reaching out with your hands or feet, you'd quickly learn how ineffective that is when accelerating up a hill or diving down one.
Thus, the only thing holding you back from flying off the seat is the fact that you have no choice but to stay there until the ride ends. If you fell asleep, for example, you might find yourself flying through the air at high speed when you wake up.
This illusion is responsible for many a rider losing their lunch on a roller coaster. The feeling of weightlessness can be especially surprising for people who have never ridden a roller coaster before and don't know what to expect. If you suddenly realize that you aren't moving at all, you might just panic and run away from the machine!
In a strange manner, your body feels accelerated. However, due to your body's inertia, you sense a force pushing you into the seat in front of you. This is what causes you to feel like you are falling at high speeds.
Your body is forced backward by its weight, which is why it feels like you are falling even though there is no net force acting on you.
This feeling of falling back causes your muscles to tense up to keep from being injured. This is why riders' legs shake when riding roller coasters for the first time.
The reason you don't feel like you're falling when walking down the street is because your body is able to counterbalance itself. It does this using your muscle groups: muscles along the side of your body help balance you while walking, since they can apply a reverse force to the motion of your body. This is why dancers and gymnasts often look as if they are floating when performing acrobatic moves on the floor or around objects in the room.
However, this isn't always the case.
Acceleration is the other factor at work on you. You only experience the downward force of gravity while riding in a coaster vehicle that travels at a steady pace. However, as the automobile accelerates or decelerates, you feel squeezed against your seat or the restraining bar. Because your inertia differs from that of the coaster vehicle, you experience this force.
Also, there is another reason for feeling the force of acceleration: when you enter a new level in a roller coaster, the ride operator must accelerate quickly to reach the desired speed before entering the next curve or drop. This means that someone (usually an electric motor) is helping to push you and your coaster car forward.
Finally, there is another reason why you might feel acceleration: when you enter a new section of track and the operators signal for you to follow them, they may accelerate quickly ahead of you so they can handle any curves or drops in the track safely. This is done by pressing down on the accelerator pedal which sends more electricity into the motor that works against your weight to get you moving again after each dip in the track.
Acceleration feels like pressure because when you increase the speed of something else without changing direction, it forces itself onto you. Your body wants to keep going in a straight line, but the faster object is pushing you forward.
This article explains why you feel pressure on a roller coaster and how roller coasters work.