Forces not in equilibrium

In the case of the glider mentioned above, what would happen if the pilot of the towing aircraft suddenly opened the engine throttle? The pulling force on the tow-rope would increase, but at first the aerodynamic resistance on the glider would not change. The forces would therefore no longer be in equilib­rium. The air resistance force is still there of course, so some of the pull on the tow-rope must go into overcoming it, but the remainder of the force will cause the glider to accelerate as shown in Fig. 1.2 (overleaf), which is called a free – body diagram.

This brings us to Newton’s second law, which says in effect that if the forces are not in balance, then the acceleration will be proportional to force and inversely proportional to the mass of the object:

a = F/m

,________ ^

Pull applied Aerodynamic

by towing resistance

aircraft force

1000 N

Fig 1.2 Forces not in equilibrium

where a is the acceleration, m is the mass of the body, and F is the force. This relationship is more familiarly written as:

F = m X a