3.2 How does speed affect the drag force and how can we increase or reduce it?

In these next examples you are reducing the loss from a kinetic energy store to the surrounding air/water thermal energy store - the destination of most wasted or dissipated energy.

The faster an object moves through a fluid the greater the rate of particle collisions between the object's surface and the fluid (e.g. air or water.

Therefore the faster the speed of an object the greater the drag effect it experiences.

The greater the speed of a boat in water the greater the drag effect on the surface of the hull.

The greater the speed of an aircraft or skydiver the greater the air resistance on the object's surface.

To reduce the drag effect its not always easy to reduce the surface area, hence reduce friction, but you can design the shape of an object to allow the fluid to flow more easily across the surface.

The hull of a boat is designed to 'cut' through the water to reduce friction. The prow is the forward-most part of a ship's bow that cuts through the water. The 'pointed' sharp shape means the hull-fluid particle collisions occur at a sharper angle than a flat surface at 90^o to the ship's movement. The prow can be quite blunt in a slow moving barge efficiently carrying a bulk cargo but not so for a fast moving destroyer class warship!

In the case of cars, trains and aircraft, the streamlined shape of the bodywork is designed to reduce the friction-drag effect of air resistance. You can use a wind tunnel to test different bodywork shapes to find the design of minimum air resistance - the shape that allows the smoothest flow of air across the bodywork.

There are times when we wish to increase the drag effect - see parachuting further down the page.

A parachute is used to slow down military aircraft landing on an aircraft carrier. As soon as the jet touches down on the runway a parachute is ejected from the back of the aircraft to produce a large surface area of friction with the atmosphere. There is a rapid deceleration of the aircraft and reduces the risk of it overshooting into the sea!

Know and be able to explain how speed/velocity and shape affects the drag force from friction.

Be able to describe how can we increase or reduce the drag force e.g. of air resistance (parachute), water resistance (shape of a boat's bow-hull), streamlining road vehicles.

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Physics notes index on acceleration of falling objects, experiments, friction, drag effects, gravity, terminal velocity