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A dump truck has a load that is secured by a tarp stretched loosely across the top of the truck.
When the truck is driving down the highway at $60\:mph$, what will the tarp do?
Blow inwards down towards the floor of the truck.
Billow upwards away from the floor.
Flap in the wind, but remain essentially flat.
Alternate between billowing upwards and collapsing inwards.