In a script for a movie scene, a bridge is remotely detonated by explosives as t

Question

In a script for a movie scene, a bridge is remotely detonated by explosives as the hero races
towards it in her car. After the detonation, she brakes the car as hard as possible, but realizes
that the car won't be able stop in time and jumps out just before it shoots off the edge of the
bombed bridge.
Obviously, the faster the car is going before it begins braking, the more exciting the scene, so
the director wants an initial speed of 90 mph (40 m/s). The stunt coordinator says that the
driver will be safe if the car is going no more than 25 mph (11 m/s) when she jumps out. To
set up the shot as the director wishes, the car can't begin braking until it's 80 m from the edge
of the bridge (where the edge will be after the explosives go off).
The coefficient of kinetic friction between the car's tires and the road surface is known to be
0.78, and wind tunnel tests on the car have established that the it will be subject to an
average air resistance force of no more than 3500 N when slowing down.
Can the stunt be pulled off as the director and stunt coordinator want, or will the plan have to
be changed? Choose a reasonable mass for the car to use in your calculations.

Explanation / Answer

Let the weight of the car be 1000 kg
Now the frictional force = u* N
N (normal weight) = m * g = 1000 * 9.8 = 9800N
Therefore, frictional force = 9800 * 0.78 = 7644 N
Total resistance force = 3500 + 7644 = 11144 N
Now, retarded acceleration of the car will be = 11144/1000 = 11.144 m/s2

Now u = 40 m/s

v= 11 m/s

a = -11.144  m/s2

therefore, s = (v2 - u2)/2*a

s = (402 - 112 )/(2*11.144) = 66.36 m

Since, Car will require 66.36m to slow down to that speed and the stunt man can apply break when he is 80m away from the edge, therefore the stunt can be pulled off as the director and stunt coordinator want

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