Textbook

Thrust

Any force pushing an airplane (or bird) forward is called thrust. Thrust is generated by the engines of the airplane (or by the muscles that flap a bird's wings). The engines push fast moving air out behind the plane, by either propeller or jet, causing the plane to move forward.

Sports balls, like tennis, baseball, cricket, golf, as well as sports projectiles like javelins do not use thrust. Neither does a glider or sailplane. There's an entire slue of animals that are gliders: flying fish, flying squirrels, flying lemurs, flying snakes and flying lizards. Don't let the names deceive you. None of these animals fly, they glide. Even the space shuttle is a glider on re-entry from space. No "go-arounds" for this vehicle; there's no engine on it!

Unlike "true flyers" such as birds, bats, insects, airplanes and helicopters, gliding flight is only affected by three aerodynamic forces: weight, lift and drag. So while an airplane will have 4 aerodynamic forces acting on it (lift, drag, thrust and weight), our tennis ball will only have 3 (lift, drag and weight).

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