Saturday, May 30th, 2009

Are you going to get a speeding ticket?

RhondaJo asked:


Yoou are driving at 35km/hr when the road desends suddenly 15m into a valley.You take your foot of the accelorator and coast down hill.Just as you reach the bottom you see the police hiding behind the speed limit sighn that reads 70 km/hr.

speed reading

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2 Responses to “Are you going to get a speeding ticket?”

SWH Says:

speed reading

No. There is no way that you’ll double your velocity. Air drag and inertial will cause your vehicle to decelerate after removing your foot from the gas pedal.

lango77 Says:

speed reading

As you coast down the hill, the potential energy (m*g*h) you had is converted to kinetic energy (1/2*m*v^2). The velocity is determined by sqrt (2*g*h) = sqrt (2*9.81*15) = 17.2 m/s * 1km/1000m * 3600s/hr = 61.9 km/hr.

Now, if you were at rest at time 0, your velocity would be 61.9 km/hr. But, since your initial velocity was 35 km/hr, you’re getting clocked at roughly 97 km/hr. In reality, it would probably be less due to the frictional forces slowing you down (wind drag and tire friction), but you are going to be faster than 70km/hr.

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