Download 12.3 Newton`s Third Law of Motion and Momentum Momentum

12.3 Newton’s Third Law of Motion and Momentum
Newton’s Third Law of Motion – Whenever one object exerts a force on a second object, the
second object exerts and equal and opposite force on the first object. These two forces are called action
and reaction forces. (Sometimes called “Law of action and reaction.”)
Example : When you press against a wall, your hand exerts a force on the wall. This is the
action force. The wall exerts an equal and opposite force on your hand. This is the reaction
force.
Forces always come in pairs – but the action force and the reaction force of a force pair act on
different objects.
Action-Reaction forces do not cancel each other out and produce a net force of zero. Only when
opposite forces act on the same object do they result in a net force of zero.
Momentum is the product of an object’s mass and its velocity. An object with a large momentum is
hard to stop. An object has a large momentum if the product of its mass and velocity is large. The
momentum of any object at rest is zero (since the velocity is zero).
Momentum = Mass • Velocity
P
m=—
v
P = mv
P
v=—
m
Conservation of Momentum
Conservation means that something has constant value. The conservation of momentum means
that the momentum does not increase or decrease.
Closed system means that no other objects or forces enter the system.
Law of Conservation of Momentum
If no other forces act on a system, then the total momentum of the system does not change (is
conserved).
In a closed system, the loss of momentum of one object equals the gain in momentum of another
object – momentum is conserved.