Forces and the Laws of
Motion
Newton’s Second and Third Laws
Newton’s Second Law
► Force
is proportional mass and acceleration
 More massive objects require more force to
accelerate
► Newton’s
Second Law – The acceleration of
an object is directly proportional to the net
external force acting on the object and
inversely proportional to the object’s mass
Newton’s Second Law
► Net
external force = mass x acceleration
 ΣF =ma
 ΣF is the vector sum of all of the forces acting on an
object
 m is the mass of the object
 a is the acceleration of the object
► Often
easier to break force and acceleration down
into x and y components and then use
Pythagorean theorem and inverse tangent to find
net force and direction
Newton’s Third Law
► Forces,
including field forces, always exist in pairs
 When you apply force to an object, the object returns
that force to you
► Newton’s
Third Law – If two objects interact, the
magnitude of the force exerted on object 1 by
object 2 is equal to the magnitude of the force
simultaneously exerted on object 2 by object 1,
and those two forces are opposite in direction
Newton’s Third Law
► For
every action force there is an equal and
opposite reaction force
► Action-reaction pair – a pair of simultaneous
equal but opposite forces resulting from the
interaction of two objects
► Action and reaction forces act on different
objects, but involve the same two objects
Newton’s Third Law