Download Newton`s Laws and Forces

Newton’s Laws and Forces
Inertia

Inertia - the natural tendency of an object
to remain at rest or to remain in motion
with a constant speed in a straight line.


The object wants to keep doing what it was
doing.
Mass – measure of inertia


Units: kg
Will stay the same wherever object is
Newton’s !st Law
( the law of inertia)

NEWTON’S 1ST LAW – every body
continues in its state of rest or of uniform
speed in a straight line unless acted on by a
non-zero net force.

Newton says inertia is the tendency to resist the
change of motion
Inertial reference frames

Inertia Reference Frames – those in which
Newton's 1st law holds true


Example: you in accelerating car where cup
moved but not by any force.
Most reference frames on Earth are assumed to be
inertial (neglect spin)
Force -an action capable of accelerating on object.






a push or pull
changes speed of an object
changes direction of an object
can be touching or long distance
always acts in pairs.
Units of force: Newton (N)


1 N = 1 kg* m/s 2
Vector (so has magnitude and direction)
Newton’s 2nd Law of Motion
(“Law of Acceleration”)

The acceleration of an object is directly
proportional to the unbalanced force acting on
it and inversely proportional to the object’s
mass. The direction of the acceleration is in
the same direction as the unbalanced force.

Mrs. Nixon’s way – “an uneven force causes a
mass to accelerate in the direction of the total
force.”

ΣF = ma
net force = mass x acceleration


Units:
ΣF = vector sum of the Forces acting on a body in
Newtons
m = Mass in kilograms
a = Acceleration in m/s2
You can have a negative sign for force and
acceleration. That sign means that the force and
acceleration are OPPOSITE to the motion of the
object (the object is slowing down)


The sum of all forces are taken in separate
directions.
If the object is moving in a specific direction
we will take the sum of the forces in that
direction.
You have to work with the components of the force and
acceleration if the motion is not in one dimension



ΣFx = max
ΣFy = may
ΣFz = maz
Weight

Weight – the measure of the force of gravity.




Weight is a force
Units are in Newton
Will change from place to place because gravity changes
Always points down



just a special case of Newton’s 2nd law
Fw = m x g
Force of weight = mass x acceleration due to gravity
Third Law

Newton’s Third Law – for every applied force, there
is an equal and opposite force.




If you push on something that something will push on you
the same amount.
Two forces, two objects every time
Ex: Foot pushes down on floor, floor pushes up on foot.
Just because there is no movement doesn’t mean that
there are no forces. No movement just means that all
of the forces acting on one of the objects balance
each other out.
“On” and “by”



2nd law deals with one object and all of the
forces acting “on” it. This tells you about the
motion.
The third law only talks about the interaction
between 2 objects. This tells you nothing
about the motion.
Fgp = F-pg