Freight train near Ballyglunin.
Image courtesy The Carlisle Kid
Newton's second law is usually denoted as
or
that is, the rate of acceleration of
an object is equal to the external force acting on it divided by its mass.
However, Newton's original proposition was that the change in the momentum of an object is equal
to the force acting on it.
and since
we can substitute fo momentum
take the "m" out to the front
substitute for acceleration
and we get
The law of conservation of momentum is very important. If a system is isolated
(no external forces or energy), then momentum will be conserved.
Conservation of momentum is very important in space collisions. On Earth, energy can
dissipate in a collision in the forms of heat and sound. Though heat and sound are created in
all collisions, they cannot dissipate and thus increase the damage to the colliding objects,
where they reduce damage in our atmosphere.
Remember that heat is movement at the molecular level (translation, rotation and movement of the molecule shape)
and sound is movement of matter at the compression, rarefaction speeds of gases and liquids)
Collisions where energy is lost from the system are called inelastic collisions. If the total
kinetic energy is conserved then the collision is called elastic.
Newton's Cradles are often given as an example of elastic collisions. Yet, they
rarely "click clack" more than 5 times before they start to take on a general back
forward motion. this is for a number of reasons; The collisions are not elastic
some energy is lost to sound and heat
some energy is lost to air resistance
external forces; the strings convert a "straight line" force to rotational motion
entropy - the natural tendency is to go from a small fast movement to a large slow movement
Newton's Cradle
Image courtesy The Dean of Physics
