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Q & A: Acceleration and de-acceleration rates of cars

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Most recent answer: 07/11/2012
Q:
For most cars, the stopping distance from 30-0 is nearly exactly one-fourth of the distance from 60-0. As I understand, that makes sense, since energy increases as the square of speed. However, the reverse is not true: 0-60 acceleration times generally are quite a bit less than four times 0-30 times. Any idea why? Thanks very much.
- David (age 49)
Mill Valley, CA, USA
A:
That's a nice little question David.
The basic reason is that for a constant de-acceleration rate, which maxes out when the tires start to skid on the pavement,  you get a quadratic velocity-stopping distance relation.  On the other hand the power  going into acceleration is more or less a constant given by the engine.  (The spinning of tires in drag races makes things worse.)  For a given  constant power input the acceleration rate is not a constant. The velocity only grows as the square root of time and the distance as the 3/2 power.    That's the main difference.  There are other subtle things like wind resistance, skidding, etc.

Lee


(published on 07/11/2012)

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