Force can change the object's state of motion, can also change the shape of the object. This figure changes the magnitude of the velocity in the state of motion of the electric fan and the cart, so that it moves to the right. Helicopter and this figure is similar to the principle, the propeller to the air to exert a downward force, the air on the propeller to exert an upward reaction force, so that the helicopter can fly upward. But there is no air on the moon, so there is no force object, can not produce force, and can not work on the moon.
The answer is:
Changing the state of motion of an object, right, the role of objects is mutual, helicopters, there is no air on the moon, so there is no force object, can not produce force.