What is Drag Force?

 When we were children, we always enjoyed pulling our hand out of window of car and experiencing air exerting force on our hand. We have also liked the scenes from action movies where heroes have their hair flying backward in air while running bikes at high speed. Some of us were amazed when they saw men jumping from air planes and coming down with the help of parachute. If you have ever swum in a pond of water, you would have known that moving through water takes a lot more energy compared to moving through air.

All examples cited above involve a special force called drag force. When an object moves through a medium, a retarding force acts on it. This retarding force is called drag force. Drag force depends upon different factors like speed of object, shape of object and nature of medium. Nature of medium is specified by viscosity. Viscosity is a resistive effect offered by medium due to resistance between various layers of medium. More viscous a medium is, greater will be drag force. A higher viscosity and hence greater drag force means greater force will be needed by the object to move through medium. As the velocity or speed of object is increased, drag force is also increased. In order to decrease the drag force and still be able to move at higher speed, objects are designed in special shapes. If we observe a ship and fish we can see similarity between two shapes. Both are narrow at the start and gradually increase in with. This is known as streamline body shape. Transporting vehicles on land, in air or water are designed while keeping this property in mind. This type of designs help in enhancing speed with greater stability and lower fuel consumption. For an object of spherical shape and speed v drag force is F=6πηrv. This relation is known as stoke's law.

In winter season, fog is a great example where we can observe drag force in action. Fog consists of tiny water droplets falling from high altitudes. As the fog droplets start falling down towards earth in air (medium of movement), air offers drag force to droplets. As the time passes, speed of fog droplet increases due to gravitational acceleration. As mentioned above, increase in speed increases drag force hence retarding force also act on fog droplet in addition to gravitational force. Hence speed of fog droplets depends upon net force and not only downward gravitational pull. For most droplets, weight i.e gravitational pull becomes equal to drag force. At this stage, net force becomes zero and hence droplets fall with constant velocity. Usually this velocity is very small and hence these droplets appear to be at rest. Due to this a large number of water droplets gather in air and we can see these droplets as fog. The constant velocity with which these droplets come down is known as terminal velocity. In coming posts we will discuss terminal velocity in detail.

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