ACTION AND REACTION

The third law of motion establishes that action and reaction are equal and opposite. When a model is resting on the ground, its weight, a force acting downwards, is opposed and exactly balanced by the equal and opposite reaction from the ground. A car running at constant speed is under the influence of a similar vertical pair, weight against ground reaction, but there is also a traction force moving the vehicle along. This is opposed by reaction forces in the other direction: frictional resistance from the ground, and air resistance or drag.

Any imbalance of forces produces acceleration (Fig. 1.2). A model beginning a take off run along the ground accelerates from standstill because the operator suddenly releases it The thrust before release is opposed by the holding force; action and reaction are equal so equilibrium prevails. When the reaction force (holding) stops, the model accelerates. As soon as it begins to move, however, air and ground resistance begin and the faster the model goes the larger these resisting forces become. The model will continue to accelerate only so long as the total resistance remains less than the thrust When the two are equal, with the model flying at some speed, equilibrium is restored (Fig. 1.1c).

In level flight, the weight force acting vertically downwards is opposed by a vertically upwards reaction. This reaction comes, in normal models, from die lift of wings and possibly other surfaces, but it may be supplied by other types of force. A helicopter is supported by its rotors, and a jet-lift aircraft is held up by foe thrust of its motor. If foe upward reaction against weight fails, or is reduced, foe model accelerates downwards. To stop this acceleration it is necessary to restore an upward reaction to equal weight This brings equilibrium but will not stop foe descent To do this an additional force must bring about deceleration. All such acceleration and deceleration will be resisted by foe mass of foe model, i. e. by inertia.