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Fast Car Physics

The average driver doesn’t think about what keeps their car moving or what keeps them on the road, but that’s because they don’t have to. The average driver doesn’t have to worry about having enough downforce to keep them on the road or if they will reach the adhesive limit of their car’s tires around a turn. These are the things are the car designers, professional drivers, racing pit crews, serious sports car owners, and physicist think about. Physics are an important part of every sports and racing car design.

The stylish curves and ground effects on sports cars are usually there not just for form but function as well allowing you to go speeds over 140 mph in most serious sports cars and remain on the road and in reasonable control. The aerodynamic efficiency is the single most important element in designing a competitive car for professional racing or getting the car model on the front of a Car and Driver or Motortrend. Aerodynamics is the study of the motion of gases on objects and the forces created by this motion. The Bernoulli effect is one of the most important behind car design.

The Bernoulli Effect states that the pressure of a fluid, in gaseous or liquid state, varies inversely with speed or velocity and a slower moving fluid will exert more pressure on and object than the same fluid moving slower (Yager). The goal of car designers is to make the air passing under a car move faster than the air passing over the car. This causes the air passing over the car to create more downforce than the air passing under the car creates upforce creating a force additional to the car’s weight pushing the car to the road.

Large amounts of downforce are needed to keep light cars grounded at high speed and keep to cars from sliding around turns at high speeds. The Venturi Effect is also an important in aerodynamic design. The Venturi Effect states that as a fluid, in gaseous or liquid state passes through a narrow space its speed increases (Yager). This is the reasoning behind keeping cars as close to the ground as they can be safely. The narrow space between the car and the ground increases the speed of the air flowing beneath it causing a decrease in pressure to do the Bernoulli Effect and increase in downforce.

The Venturi Effect is the reason for front ground effects, which feature small air ducts or venturi tunnels. Negative lift is the technical term for downforce (Yager). Negative lift is the opposite of the lift used by planes to fly, it forces an object down rather than up (Yager). Negative lift is created by front and rear wings on race cars and by ground effects and spoilers on the average sports car. Most negative lift is used to fight inertia as a car rounds a turn. Inertia is the tendency of an object to remain in the same state of motion (Murphy 77).

When a car rounds a turn at high speeds it often needs more force than it’s weight to resist the car’s tendency to keep traveling straight. The increased downforce puts more weight on the tires helping the tires grip the road. Drag force is the cost of increasing downforce. Drag force is the force acting on an object in motion in the opposite direction the object is moving through a fluid (Yager). To most people drag force is simply known as air resistance. The objective of aerodynamic efficiency is to maximize downforce while minimizing drag force.

Acceleration and Speed are obviously the two defining characteristics of a fast car. Newton’s three laws of motion are an essential part in determining how fast a car will accelerate and how fast it will go. Newton’s second law is the easiest to understand in relation to a car’s acceleration. Newton’s second law mathematically states Force=(mass)(acceleration) (Murphy 78). This law explains why cars that need to accelerate fast should be relatively light in weight compared to other cars.

Removing mass, such as a bumper, radio or fancy upholstery reduces the weight of a car. When the tires create a constant force and the mass is decreased the acceleration will increase. Just for an example if you have a 100kg car and a 250kg car both have a 10,000N force pushing it forward (ignoring outside sources of resistance) the 250kg car will accelerate at 40m/s while the 100kg car will accelerate at 100m/s. So a big force and a little mass will yield a big acceleration.

Acceleration also ties in Newton’s third law of motion, which states the every force is accompanied by an equal but opposite reaction. The tires apply a force on the road and the road applies and equal and opposite force back, but because the road is essentially part of the earth it’s mass is much greater than the mass of a car. The road accelerates very little in it’s opposite reaction to the force of the tire because it’s mass is so huge and the car accelerates allot from the opposite reaction or push applied by the road because it’s mass is so small.

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