마그누스 효과, Magnus effect

 

시계 방향으로 회전하는 공이 오른쪽으로 날아가는 경우를 생각해보자. 이때 운동하는 공과 공기의 충돌에 의해 공의 이동 방향의 반대 방향, 즉 왼쪽으로 주변 공기의 흐름이 생기게 된다. 또한 공이 회전하기 때문에 공의 표면에 아주 가까이 있는 공기는 공과 같은 시계 방향으로 움직이게 된다. 그렇게 되면 공의 위쪽에서는 공에 가까운 공기와 주변의 기류가 서로 반대 방향으로 이동한다. 따라서 위쪽의 공기는 상대적으로 느린 속도를 갖게 된다. 반대로 공의 아래쪽에서는 공에 가까운 공기와 주변의 기류가 서로 같은 방향으로 이동하기 때문에 상대적으로 빠른 속도를 갖게 된다. 베르누이의 법칙에 의하면 유체(액체 또는 기체)에서 물체의 속도가 증가하면 그 물체가 유체로부터 받는 압력은 감소하고, 반대로 물체의 속도가 감소하면 그 압력은 증가한다. 따라서 상대적으로 빠른 속도를 갖게 된 공의 아래쪽의 압력은 감소하고, 상대적으로 느린 속도를 갖게 된 공의 위쪽의 압력은 증가한다. 공의 위, 아래에 발생한 이 압력차이가 양력을 발생시켜 공을 압력이 높은 곳에서 낮은 곳으로 이동시키고 이런 힘을 마그누스 힘이라고 한다. 이 힘에 의해 공이 회전 방향(이 경우에서는 아래쪽)으로 휘게 된다. 마그누스 효과는 물체의 회전 방향에 따라 어느 방향으로도 작용할 수 있다.

 

 

 

Consider a case where a ball rotating clockwise flies to the right. At this time, the collision between the moving ball and the air creates a flow of ambient air to the left, which is the opposite direction of the ball's moving direction. Also, because the ball rotates, the air very close to the surface of the ball moves in the same clockwise direction as the ball. Then, above the ball, the air close to the ball and the surrounding airflow move in opposite directions. Therefore, the air above has a relatively slow speed. On the other hand, below the ball, the air close to the ball and the surrounding airflow move in the same direction, so it has a relatively high speed. Bernoulli's law states that if the speed of an object in a fluid (liquid or gas) increases, the pressure the object receives from the fluid decreases, whereas if the speed of the object decreases, the pressure increases. Therefore, the pressure below the ball with a relatively high speed decreases, and the pressure above the ball with a relatively slow speed increases. This pressure difference that occurs above and below the ball generates positive force, moving the ball from high pressure to low, and this force is called the Magnus force. This force bends the ball in the direction of rotation (downwards in this case). The Magnus effect can act in either direction depending on the direction of rotation of the object.