122 Balloon Helicopter

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As each blade push air backward and out through the hole in the blade, the ejected air pushes the blade forward (Newton’ 3rd Law). This is how a forward thrust is produced to propel each blade forward, resulting in a torque that causes the blades to spin.

Because the blades are tilted at an angle, the ejected air is also pushed downward and out through the holes in the blades. This implies the ejected air must be pushing the blades upward. (Newton’s 3rd Law). This is how an upward thrust is produced to lift the balloon.

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The two main forces acting on the helicopter are the upward lift and the downward weight of the helicopter. In this video, we can see that initially, the lift force was barely able to support the weight of the helicopter, resulting in the helicopter hovering at about the same height. However, as the helicopter ejects air from the balloon, its weight decreases. So the net upward force becomes larger and larger, and the helicopter accelerates upwards. Right at the end, the balloon ejects the air in a spurt, resulting in a kick in the lift force. This coupled with rapid decrease in mass of the helicopter, results in the obvious jerk up in the last part of the motion.

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