Mechanics

# Reaction time

Reaction time is the time interval between the generation of a stimulus and a motor action.

Reaction time is the time interval between the generation of a visual or audible stimulus, etc., and the motor action. This time revolves around values ​​that correspond to milliseconds and depends on factors such as emotional state, gender (men are faster), age, physical conditioning and cognitive level. High performance athletes, after a lot of training, achieve a 15% decrease, on average, in their reaction time. This decrease is important for sprinters, for example, who, having a shorter reaction time than normal, are able to get ahead after hearing the starting gun.

The time taken to react to a stimulus influences several everyday situations, such as traffic. When seeing an obstacle ahead or perceiving any other stimulus, before the brake is applied, the driver’s brain needs to process the information and generate the muscle contraction that will apply the brakes. The time taken for this to occur corresponds to the reaction time. While this process takes place, the car continues its displacement.

The table below provides an estimate of the distance traveled by a vehicle during a driver’s reaction time, as well as showing the distance traveled with the brakes on and the total distance. Note that the greater the vehicle speed, the greater the distance traveled during the reaction time.

Through the equation of kinetic energy , energy of motion, it is possible to determine how much energy the vehicle must lose in order to be able to stop. Notice in the table above that when the speed value doubles, the number corresponding to the total distance traveled quadruples. This is because kinetic energy depends on the square of velocity. The greater the energy involved, the greater the braking action, which significantly increases the total distance traveled required for braking.

Below are calculated kinetic energies for speeds of 20 m/s (72 km/h) and 30 m/s (108 km/h), assuming a mass of 900 kg for a vehicle. Note the difference between the values:

And c = mv 2
2

And c = 900.(20) 2
2

And c = 450,400 = 180,000 JOULES

And c = mv 2
2

And c = 900.(30) 2
2

And c = 450,900 = 405,000 JOULES

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