Lecture Notes
Updated Friday, 06-Jan-2006 19:34:59 EST


where F = external force, m = object's mass, a = object's acceleration (rate at which velocity changes)
where g = 9.8 meters/sec2 if w is in Newtons (N) and m is in kilograms (kg) -- metric system of units If F = 10 N and m = 2 kg, then

  • Newton's Third Law -- when two objects exert forces on each other, the forces are equal in magnitude (strength) and opposite in direction. This law is also stated as "for every action, there is an equal and opposite reaction."
  •  Air molecules inside balloon collide with each other and with balloon walls. For each molecule striking the left side, another strikes the right; for each one that strikes the upper left, one strikes the lower right, etc. -- the forces exerted against the balloon then cancel in pairs. But for molecules striking the upper end (shown as blue molecules here), there are no counterparts striking the lower end since it is open. The net force on the balloon is then in the upward direction in this figure.


    In a rocket engine, combustion (high-temperature chemical reaction) generates energetic particles that collide with engine walls and exert forces. As with the balloon, these forces cancel out pair-wise, except for those particles colliding with the top of the rocket (as shown here). Forces acting upward are not balanced by forces acting downward because opening (the exit nozzle) allows particles to escape as exhaust gases. Engine is attached to main rocket, so engine exerts upward force on rocket and entire vehicle accelerates upward.

    F = T - W = 2700 - 1200 N = 1500 N upward speed at end of 2nd second = 24.4 m/sec

    upward speed at end of 3rd second = 36.6 m/sec

    Copyright Ó 1998, Robert G. Melton

    Updated Friday, 06-Jan-2006 19:34:59 EST