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The example that represents a movement sustained by the force of gravity is the orbit of the moon around the planet Earth. This is because the gravitational force between the Earth and the moon keeps the moon in its orbit. The force of gravity acts as a centripetal force, pulling the moon towards the Earth and preventing it from moving off into space.
In contrast, the other options do not involve movements solely sustained by gravity:
1. The takeoff of an airplane involves the thrust generated by the engines to overcome gravity and lift the plane into the air.
2. Accelerating a car involves the engine's power to increase the car's speed, not just gravity.
3. Exploding a balloon involves internal forces causing the balloon to burst, not gravity.
Therefore, the orbit of the moon around the Earth is the example that best represents a movement sustained by the force of gravity.
