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
Tags: Coast Down, Sighn, Speed Limit
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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May 31st, 2009 at 10:29 pm
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.