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The Physics of Video Games will be continuing the theme of Out Run coverage here on Retroware for its 30th birthday anniversary. I would rejoice and bask in its glory, but I’ve never played Out Run before, and don’t really have an efficient means to do so. I may be able to import the Japanese PS2 remake someday down the road, but that doesn’t really help me now. It looks like a jolly ol’ good time though and I am excited to play it one day. Ito’s Vinyl Soundtrack review, Nostalgia Nerd’s retrospective, David’s spinoff coverage, and Peter’s personal experiences are all great sources for those who wish to hear about people with experience with the game. So instead of talking about this renowned classic in the fashion that everyone else is, let’s use this time to learn a little bit about friction and how it helps the vehicles in this driving game, and in real life. This is more of a fun fact article as opposed to trying to prove the impossible.

Friction is something everyone reading has probably heard of in some form or another, whether they understand it or not. The basic idea is that it’s a force that’s in play when two objects are in contact with one another and one or both objects are trying to move while maintaining contact. Basically, mobile rubbing causes a force of friction. Friction is a force that opposes motion. The force of friction is dependent on two things: the normal force and coefficient of friction.

The normal force is how much force one object is putting on another one from its contact, this force is here due to that pesky gravity that’s always putting us down. The normal force depends on the force of gravity, which is basically constant on Earth for our purposes, and the mass of the object. The bigger and more dense the object, the greater the force. For example, a crate full of lead on the floor is going to have a much greater normal force than a delicious Oreo on that same floor (unless it happens to be a gargantuan Oreo).

It's pretty big... I guess

It’s pretty big… I guess

The coefficient of friction is the other component in resisting motion. This is what dictates how easy it will be to move an object. For example, it would be much easier for you to run and slide down a greased up, tiled hallway (or ice if you want a more practical scenario) than it would to slide on dry concrete. The coefficient of friction is much lower in the hallway than it is the concrete, making it easier to slide, which means less resistance to motion.

To further this, there are two kinds of coefficients of friction, static and kinetic. Static friction deals with objects that are not in motion, but something is trying to put them into motion. Kinetic friction deals with an object that is already moving around. Static friction is a greater force than kinetic friction. A very shallow proof of this is when you’re trying to move something. Think about when you’re trying to push a snowball to build a big snowman, push a couch across the room, or something like that. Once you start moving the object it becomes much easier to move while in motion as opposed to going from stationary to moving. Static friction tries to keep things in place. Kinetic friction opposes motion too, but not nearly as much. Kinetic friction wants to keep things moving and grooving more so than static friction.

Friction is one of the many things keeping the vehicles in Out Run where they need to be. If friction were non-existent then vehicles would be slipping and sliding all over the place without a means to stop after they got moving. They wouldn’t even have a chance to start moving without help from an external force seeing as the tires would need something to grip in order to move. Now that we have a little bit of context about friction and how it helps our lovely Out Run drivers, it’s onto the fun fact.

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You need friction to start getting those high scores

Fun fact: when we’re talking about friction and vehicles, whether they be in motion or at a complete stop, we’re always talking about static friction between the tires and the respective surface. I know this flies in the face of the context, but I’ll explain. It’s true that kinetic friction deals with the opposition of motion with moving objects. If the vehicle had no tires and was just sliding along the road, it would be kinetic friction. But when we’re considering a tire on the road, the contact almost seems stationary. Tires are constantly in motion while driving, so as the tire rotates, a new piece of tire is touching the ground all the time. Every new little section of tire that is now in contact with the ground doesn’t have the benefit of the previous section moving towards kinetic friction. The old piece of tire is no longer in contact with the ground. It’s a whole new battle of overcoming the much more aggressive static friction with every section of tire. You can replicate what it is like by pushing an object a little bit, picking it up, moving forward, putting it down and pushing again. Picking the object up makes it lose that progress made with overcoming static friction. This is analogous to the tire on the ground.

Well, that about does it for the fun fact. We didn’t prove the impossible, but we learned something fun today. If you want to learn about a more complicated scenario involving friction, then read about the Murderous Death Ball from Trapt and our analysis of that. Otherwise, enjoy the other Out Run content on the site, happy birthday anniversary, Out Run, and I have myself a PS2 game to import.

Dan

My book about physics in video games: http://bit.ly/16MDCxE

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