Dynamics: Cause of Motion

The First Law of Motion Better known as Newton’s first law of motion An isolated body at rest will remain at rest, and an isolated body in motion continues its motion with constant velocity along a straight line. The influence of forces… External Frictional Paraconception – Objects need a constant source of force to keep them in motion. True, but only in the presence of friction.

Where Aristotle Went Wrong Motion is a natural state, just like rest. A body in motion will continue in that same state of motion until an unbalance force acts on it. Examples Clouds, birds, and projectiles are not left behind because they have forward motion of their own. A ball dropped on a moving ship hits deck below. The state of physics knowledge in early times was comparatively limited.

Momentum & Inertia Momentum, p, is a vector quantity associated with both mass (arbitrarily defined) & velocity. p = mv (units typically kg*m/s) The momentum of an object at rest is 0. Inertia should not be confused with momentum; inertia is a body’s tendency to resist changes in motion; non-mathematical.

Total Momentum is Conserved Momentum before = momentum after (pi = pf) Holds in explosions, collisions, and some +/- of mass Does not hold for falling bodies UNLESS Earth’s momentum is taken into account Examples of momentum conservation: Recoil of a rifle or handgun. Dropping mass on a moving cart. Ice skaters pushing off one another. Astronaut pushing off a space ship. Rocket in flight.

Collisions, Explosions, & +/- Mass Principle – momentum is conserved, pi = pf Collision example Explosion example Addition of mass example