Drag is a force that opposes the motion of an object through a fluid, such as air or water. It is caused by the fluid particles colliding with the surface of the object and pushing it in the opposite direction. For stationary and moving obstacles in the same medium, drag affects both in the same way by slowing down their motion.
The main difference between drag on a stationary and moving obstacle is the magnitude of the force. In a stationary object, the drag force is equal to the weight of the object, while in a moving object, the drag force is equal to the weight plus the force required to overcome the object's inertia.
The shape of an obstacle plays a significant role in determining the drag force it experiences. A streamlined shape, such as an airplane wing, can reduce drag by allowing the fluid to flow smoothly around it. On the other hand, a blunt or irregularly shaped object will experience higher drag due to the disturbance it creates in the fluid flow.
The drag force on an obstacle can be affected by several factors, including the object's size, shape, and speed, the fluid's density and viscosity, and the roughness of the object's surface. Additionally, external factors such as temperature, pressure, and humidity can also impact the drag force.
To reduce the drag force on an obstacle, engineers can use various techniques such as streamlining the object's shape, reducing its surface roughness, or using special coatings to reduce friction. Additionally, reducing the object's speed or changing its orientation with respect to the fluid flow can also help decrease the drag force.