How Do I/Brake

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BRAKE not BREAK

THE ENERGY IN YOUR SE7EN

"ENERGY CAN NEITHER BE CREATED NOR DESTROYED", it can only be changed from one form to another!

The chemical energy in petrol is released and changed to heat energy by first mixing it with an appropriate amount of air and then setting light to it.
The heat energy is changed to pressure energy by the cylinders.
The pressure energy is changed to kinetic energy by the pistons.
This kinetic energy moves your se7en forwards (hopefully) by friction between the rotating drive wheels and the road.

YOUR WHOLE SE7EN NOW HAS KINETIC ENERGY (Kinetic Energy is the energy possessed by mass because of its motion, and mass is the quantity of matter in a body. Your Se7en has approximately 500kg mass)

THE SPEED OF YOUR SE7EN IS REDUCED BY CHANGING ITS KINETIC ENERGY BACK INTO HEAT ENERGY.

The device fitted to your car to change its kinetic energy back into heat energy is the brakes.

Brakes are a VERY EFFICIENT method of generating heat.
The greater the car's kinetic energy, the more heat the brakes have to generate to bring the car to a complete stop.
Pressing the brake pedal generates FRICTION in the brakes.
Friction in the brakes generates HEAT in the brakes.
Pressing the brake pedal harder increases the friction in the brakes which increases the amount of HEAT generated in the brakes, which increases the speed of kinetic energy conversion.
HOWEVER, if you press the brake pedal TOO HARD the wheels will stop rotating. There will then be no friction in the brakes, no heat generated in the brakes and no kinetic energy conversion in the brakes.

Because the wheels have stopped rotating before all the car's kinetic energy has been changed into heat, the car will still be moving forward.
There will still be some friction, but only between the stationary tyre contact patches and the road surface.

Stationary tyre contact patches and the road surface are an INEFFICIENT method of generating heat, therefore the speed of kinetic energy conversion into heat is much reduced.

KINETIC ENERGY

Kinetic energy is the energy possessed by mass because of its motion.

KE.gif Kinetic energy is proportional to mass(m) and to speed(V) squared

Speed will therefore have a greater influence on the kinetic energy of your Se7en than its mass.

(There is little point here in comparing the braking efficiency of different cars. The intent is to stick to basic principles and concentrate on the influence of speed on the stopping distance OF THE CAR YOU ARE DRIVING AT THE TIME. Increasing the braking efficiency of your Se7en DURING an emergency stop is not an option. The 'correct' units for kinetic energy will not be used. In this case the units don't matter. Comparisons of distances and percentage changes that we can all visualise will be used; so I don't want to hear any air being sucked through the teeth of our experts).

As a starting point, let's assume:

Speed = 30mph
Conditions = dry level road
Maximum braking effort (Emergency Stop; assume a steady 1g deceleration)
Distance = front face of nose cone from point of brake application to its position when the Se7en has come to a complete stop
Example ABSOLUTE MINIMUM STOPPING DISTANCE = 30ft

(The ABSOLUTE MINIMUM STOPPING DISTANCE is that obtained after lots of practice and experimenting with all methods of braking).

Under the same conditions, an additional 5mph will result in a minimum stopping distance of 41ft.

: HOW COME?

: 30mph squared = 900
: 35mph squared = 1225
: 1225 - 900 = 325
: 325/900 x 100 = 36%

: A 5mph increase from 30 to 35mph has resulted in a 36% increase in kinetic energy.

: The percentage increase in kinetic energy will give the same percentage increase in stopping distance.
: So, 30ft x 36% = 11ft
: 30ft + 11ft = 41ft

A seemingly insignificant increase of 5mph means that an obstacle that will be 5 comfortable feet in front of your Se7en if you stop from 30mph, is now laying flat on its back under your gearbox - SCARY THOUGHT!

Perhaps the scariest part: THIS IS A LAW OF THE UNIVERSE AND CANNOT BE CHANGED!

The braking efficiency of a Se7en will NOT be better because it's lighter than the average 'tintop'.

The reduced kinetic energy of a Se7en, that 'might' be thought to give it an advantage while braking also gives reduced friction between the tyre contact patches and the road; the two effects cancel-out - Bummer!

"Just a minute!", you say. "My Se7en has four pot calipers, ventilated 10 inch discs, 'Superdooper' state-of-the-art brake pads, a high performance master cylinder and I can press the brake pedal faster and harder then anybody else!".

Sorry, the laws of the universe don't care!

Remember, we've already set-up an ABSOLUTE MINIMUM STOPPING DISTANCE with our standard brakes. If you try to increase the rate of change of kinetic energy into heat, you will also need more friction between the tyre contact patches and the road - otherwise you will just stop the wheels rotating and revert to a less efficient method of changing kinetic energy into heat.

If you have higher efficiency brakes AND stickier tyres the laws of the universe will still apply. Your braking distance from 30mph will be shorter, say, down to 25ft, but from 35mph it will still be 36% greater than that. It will be 34ft (9ft greater). The object you ran over will be under your radiator instead of the gearbox - GULP!

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