Lift and Drag

Up Smooth Sailing Projectile Motion Lab Lift and Drag Angular momentum The football in flight

When air in laminar flow streams past an object, it exerts a viscous drag on the object.  The drag point into the direction of the air flow and therefore opposite to the direction of the velocity of the object relative to the air.

 

 

 When an object's shape changes the direction of flow and the flow becomes asymmetric, then the object is acted on by lift as well as drag.   If the flow is asymmetric, then the air flows faster past one side of the object than past the other side.  The lift force points away from the surface over which the air flow has the highest speed.

In the above picture, the airplane wing redirects the air flow.  The air flowing over the airplane wing moves with greater speed than the air flowing underneath.  This results in a net upward force on the wing.

 

 For an airplane wing, the lift force depends on the shape of the wing and on the angle of attack.  Shapes that produce more lift also produce more drag.

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When the airspeed falls below a critical speed,  a turbulent wake develops.  The wing looses all lift, and pressure drag increases dramatically.  The airplane stalls, and without intervention by the pilot falls nearly straight to the ground.

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Demonstration:

A ping-pong ball is suspended in the air flow from a hair dryer.
When the air streams symmetrically around the ball in the upward direction, air drag and gravity point in opposite directions, and drag balances gravity When the air flow makes an angle with the vertical, drag and gravity do no longer point in exactly opposite directions, and drag alone cannot balance gravity.  The ball starts to drop out of the stream.  This redirects the stream and air has to flow faster over the upper surface of the ball, creating lift.  Lift and drag together now can balance gravity.

Link:

Bernoulli Ball   (This is an experiment you can try at home.)

A football in flight is also acted on by lift and drag.  Drag is always present.  The lift vanishes if the symmetry axis of the football points in the direction of its velocity.  But if the football is spinning, drag not only exerts a force, but also a torque on the football.  Football physics then gets rather complicated.

To understand what is happening to the spinning ball, we have to investigate in more detail the physics of spinning objects.