Newton’s Laws
As simple as
One,
Two,
Three!
Inertia
Newton’s First Law is often called the
Law of Inertia.
But what is inertia?
Inertia is the natural tendency for an
object to maintain a state of rest or to
maintain a state of uniform motion in a
straight line.
In other words, any object likes to
maintain its constant velocity.
Mass and Inertia
Mass and Inertia are very much linked
together. In fact, Newton said that mass
was a measurement of inertia.
The more mass something has, the
more inertia it has.
If a 350 lb person and a 85 lb person are
both walking towards you at a constant
velocity. Which one would be easier to
stop?
The one with the smaller mass, thus
smaller inertia.
Newton’s First Law
An object at rest will remain at rest, and
an object in motion will continue in
motion with constant velocity (that is
constant speed in a straight line) unless
acted upon by a net external force.
What is true for both of these situations is
that all forces equal zero. At rest, the net
force is zero.
Fnormal=Fweight
Fnormal
Fweight
In constant velocity, the net force is also zero.
Fnormal=Fweight
Fresistance=Fforward
Fnormal
Fresistance
Fweight
Fforward
Newton’s Second Law
Newton’s Second Law is looked at as
one of the most important laws in
physics. This is because it is used
throughout. You will use this law in
equation-form more than any other
Newton’s Second Law
a  Fnet
1
a
m
Fnet
a
or Fnet  ma
m
Problem Solving Strategies
When solving problems that involve Fnet=ma, it
is important to separate the problem into the
different coordinates (i.e. x,y, and z).
Fx=max
Fy=may
Fz=maz
Fnormal
Fresistance
Fweight
Fx = Fforward – Fresistance = max
Fy = Fnormal – Fweight = may
Fforward
Newton’s Second Law Problems
A 88 kg crate is being pushed with horizontal
force of 300 Newtons over a flat ice rink (there
are no resistive forces). If started from rest,
find the net force in both the x and y direction,
the acceleration in the x direction, and the
normal force on the crate.
a) Fx = Fapplied (push) - 0 = max
Fy = Fnormal - Fweight = may
b) Fx = Fapplied (push) - 0 = max
300 N-0 N = (88 kg) (ax)
ax = 300 N / 88 kg = 3.41 m/s2
c) Fy = Fnormal - Fweight = may
Fnormal = Fweight – may
Fnormal = (88 kg) (9.8 m/s2) - (88 kg) (0 m/s2)
Fnormal = 862.4 N
(Notice the normal is the same as the weight. That will always
be true as long as the object is on a horizontal surface and it is
at a constant velocity – in this case zero.)
Newton’s Third Law
Newton’s Third Law is probably the most
misunderstood of the three laws.
It states:
For every action, there is an equal but
opposite reaction.
This is not as simple as it sounds!
An example of this is a big moving truck and a little
Volkswagen Carmengia hitting each other in a headon collision.
It is our nature to say that the moving truck hits the
Carmengia with a greater force than the Carmengia
hits the moving truck with. Why is that?
Well, this assumption is not true. They hit each other
with the same force, just in opposite directions.
WATCH OUT!!!
Why is this true?
The forces may be the same, but the
masses are very different.
Since Fnet=ma, the acceleration must go up
for the Carmengia because the mass is so
small.
Likewise, the truck has a very small
acceleration because it has such a large
mass.
Ftruck on car=Fcar on truck
Fcar on truck=ma (of truck) Ftruck on car =ma (of car)
Newton’s Third Law Problems
A 2000 kg truck rear-ends a 400 kg car (both
traveling to the right). If the impact force was
5000 Newtons, what are the accelerations of
the truck and car respectfully?
The force on the truck is going to be negative
(to the left), while the force on the car will be
positive (to the right).
atruck=Fcar on truck/mtruck
atruck= -5000 N/2000 kg
atruck= -2.5 m/s2 (it slows down)
acar=Ftruck on car/mcar
acar= 5000 N/400 kg
acar=12.5 m/s2 (it speeds up)