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Summary: The Physics Of Stopping

Good Essays
Jason Fry
Constance Nelson
Composition I 30127
29 June 2015
The Physics of Stopping: A Brief Tutorial

Anyone who has been driving for any significant amount of time has had that moment. Driving down the road minding your own business, when suddenly in front of you a chicken is crossing the road. You slam your foot onto the brake pedal, bringing the car to a stop with inches to spare. The chicken goes about its business completely oblivious, safely making it to the other side of the road. Heart pounding, you carry on with your drive, shaking your head at the close call and the chicken’s lack of judgement in choosing this particular moment to cross the road. Your brakes worked flawlessly, arresting the motion of a multi-ton object traveling
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While much greater than the force you have applied, it is still not nearly enough to stop a car. There is another factor in the system on modern vehicles, called a booster. This is basically another hydraulic circuit, except it uses vacuum from the engine instead of fluid, and it “boosts” the amount of force applied by the pedal to the hydraulic part of the brake system, rather than multiplying it. Usually around 300 to 400lbs of additional force is generated by the booster. Adding this to the equation, the driver applies 10lbs of force to the pedal, which is multiplied to 20lbs through the mechanical advantage of the pedal ratio of 2:1, this is “boosted” to 420lbs, and multiplied by the hydraulic circuit at a ratio of 9:1 to generate a pressure of 3780lbs at the brakes. As you can see, a booster makes a huge difference in the equation. If you have ever driven a classic car without power brakes, you know that to stop you have to apply much more pressure to the pedal, than you would with a modern car and a boosted system. In this case, to stop in the same distance from the same speed, the driver would have to apply 210lbs of force to the…show more content…
The final part of the braking system is one that is easy to overlook, a lot of people don’t associate with braking at all. The tires. Tires are, as a major brand’s advertisment says, “the only safety feature on the road that actually touches the road.” Stopping the brake rotors, and thus the wheels and tires does little good, and is usually of some detriment if trying to make a controlled stop. This is because when the tires stop moving, or “lock up” their grip on the road changes because the static friction between the tire and the road changed to dynamic friction. The easiest way to imagine this is if you were trying to push a heavy box across the floor. While the box is stationary there is static friction between itself and the floor, once it starts to move it becomes dynamic friction. You might have noticed before if you have ever tried to slide something heavy that it takes more effort to get the object to move initially, than it does to keep it moving. This is because static friction takes more force to overcome than dynamic friction. Similarly, once the tires are sliding on the road, there is less friction, and thus a reduction in braking force relative to the vehicle and the road, not to mention the possibility of spinning out of control. The tires gripping the road is what actually slows the vehicle down and brings it to a controlled
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