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What is the force called when an object such as a Hot Wheels car performs a loop on a looped track and generates force against the surface?

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04/23/2008

This question is actually two questions in one -- what is the force on the Hot Wheels car, and what is the force against the surface (i.e. the track)? First, the force on the car . . . The name of the force described in the question is called centripetal force. This is the force that occurs when a body in motion (the Hot Wheels car), which would continue to travel in a straight line if untouched, is pushed to change direction (in the case of the question, due to the curvature of the track). The curvature of the track forces the car to deviate away from its natural state - travelling in a straight line. This force - the track pushing on the car and changing its direction - is centripetal force. Centripetal force always pushes inward - toward the center of the turn. It is what makes a body (such as you inside a car) turn and change direction, instead of continuing in a straight line. Many people confuse this effect with centrifugal force, which is an imaginary force - that is, centrifugal force does not exist. The imaginary centrifugal force "pushes" outward - away from the centre of the turn. We think centrifugal force is real, but our perceptions are distorted because we are in an accelerating frame of reference. When a car turns suddenly, we feel like we are pushed sideways, and we call the "force" that pushed us the centrifugal force. An independent observer (a bystander on the side of the road), who is in an inertial frame of reference, and who witnesses the same thing, will tell you did not move sideways toward the car's door, but instead the car pushed toward you (centripetal force), forcing you to turn with the car. Without centripetal force, you would have continued traveling in a straight line, and fallen out of the turning car! == == Next, the force on the surface . . . According to Newton's Third Law, "For every force, there is an equal and opposite reaction force." So, if the track pushes inward on the car (to make it travel in its circular path as discussed above), then the car pushes outward on the track with exactly the same amount of force. If you watch a Hot Wheels track as the car goes through the loop, that track will be pushed outward by the car. The tracks on some looping amusement park rides are also visibly distorted as the roller coaster passes through the loop. This "give" is built into the track, which quickly resumes its original shape once the roller coaster has passed. By the way, whenever a surface pushes against an object (like the track pushing on the looping car), the direction of that force is always perpendicular to the surface. For that reason, it is called a normal force, because "normal" is a geometric term meaning perpendicular in all directions. (It doesn't mean the opposite of abnormal.) At any point in a circular track, the normal direction is toward the center of the circle, which is why it is called centripetal, which literally means center - seeking. There is no commonly-used term for the direction of the reaction force, other than to say "outward". Some older texts once called that direction "antinormal," but that isn't used much any more.