Transcript Newton`s First Law

Newton’s First Law
Free Body Diagrams
Newton’s First Law States:
• An object at rest tends to stay at rest and an object
in motion tends to stay in motion with the same
speed and in the same direction unless acted upon
by an unbalanced force.
F=ma
In fact, it is the natural tendency of objects to resist changes
in their state of motion. This tendency to resist changes in
their state of motion is described as inertia.
Mass as a Measure of the
Amount of Inertia
All objects resist changes in their state of motion. All objects have this tendency - they
have inertia. But do some objects have more of a tendency to resist changes than others?
Absolutely yes! The tendency of an object to resist changes in its state of motion varies
with mass. Mass is that quantity which is solely dependent upon the inertia of an object.
The more inertia which an object has, the more mass it has. A more massive object has a
greater tendency to resist changes in its state of motion.
Suppose that there are two seemingly identical bricks at rest on the physics lecture table.
Yet one brick consists of mortar and the other brick consists of Styrofoam. Without
lifting the bricks, how could you tell which brick was the Styrofoam brick? You could
give the bricks an identical push in an effort to change their state of motion. The brick
which offers the least resistance is the brick with the least inertia - and therefore the brick
with the least mass (i.e., the Styrofoam brick).
A common physics demonstration relies on this principle that the more
massive the object, the more that object tends to resist changes in its state of motion. The
demonstration goes as follows: several massive books are placed upon a teacher's head.
A wooden board is placed on top of the books and a hammer is used to drive a nail into
the board. Due to the large mass of the books, the force of the hammer is sufficiently
resisted (inertia). This is demonstrated by the fact that the hammer blow is not felt by the
teacher. (Of course, this story may explain many of the observations which you
previously have made concerning your "weird physics teacher.") A common variation of
this demonstration involves braking a brick over the teacher's hand using the swift blow
of a hammer. The massive bricks resist the force and the hand is not hurt. (CAUTION:
do not try these demonstrations at home.)
Forces Don't Keep Objects
Moving
• Newton's first law of motion declares that a force is not needed to
keep an object in motion. Slide a book across a table and watch it slide
to a rest position. The book in motion on the table top does not come to
a rest position because of the absence of a force; rather it is the
presence of a force - that force being the force of friction - which
brings the book to a rest position. In the absence of a force of friction,
the book would continue in motion with the same speed and direction forever! (Or at least to the end of the table top.) A force is not required
to keep a moving book in motion. In actuality, it is a force which
brings the book to rest.
Free Body Diagrams
Free-body diagrams
are used to show
the relative
magnitude and
direction of all
forces acting on an
object.
This diagram
shows four
forces acting
upon an object.
There aren’t
always four
forces, For
example, there
could be one,
two, or three
forces.
Show Animation
Problem 1
A book is at rest on a table top. Diagram the
forces acting on the book.
Problem 2
An egg is free-falling from a nest in a tree.
Neglect air resistance. Draw a free-body
diagram showing the forces involved.
Gravity is the
only force
acting on the
egg as it falls.
Problem 3
A flying squirrel is gliding (no wing flaps) from
a tree to the ground at constant velocity.
Consider air resistance. A free body diagram
for this situation looks like…
Gravity pulls down
on the squirrel
while air
resistance keeps
the squirrel in the
air for a while.
Problem 4
A rightward force is applied to a book in order to
move it across a desk. Consider frictional
forces. Neglect air resistance. Construct a freebody diagram. Let’s see what this one looks
like.
Note the applied force arrow pointing to
the right. Notice how friction force
points in the opposite direction.
Finally, there is still gravity and normal
forces involved.
Problem 5
A skydiver is descending with a constant
velocity. Consider air resistance. Draw a freebody diagram.
Gravity pulls down on
the skydiver, while air
resistance pushes up
as she falls.
Problem 6
A man drags a sled across loosely packed snow
with a rightward acceleration. Draw a freebody diagram.
The rightward force arrow points to the right.
Friction slows his progress and pulls in the
opposite direction. Since there is not
information that we are in a blizzard, normal
forces still apply as does gravitational force
since we are on planet Earth.
Problem 7
A football is moving upwards toward its peak
after having been booted by the punter. Draw a
free-body diagram.
The force of
gravity is the
only force
described. It is
not a windy day
(no air
resistance).
Problem 8
A car runs out of gas and is coasting down a hill.
Even though the car
is coasting down
the hill, there is
still the dragging
friction of the road
(left pointing
arrow) as well as
gravity and normal
forces.
Net Forces
Now let’s take a look at what happens when
unbalanced forces do not become completely
balanced (or cancelled) by other individual
forces.
An unbalanced forces exists when the vertical
and horizontal forces do not cancel each other
out.
Example 1
Notice the upward
force of 1200
Newtons (N) is
more than gravity
(800 N). The net
force is 400 N up.
Example 2
Notice that while the normal force and gravitation
forces are balanced (each are 50 N) the force of
friction results in unbalanced force on the horizontal
axis. The net force is 20 N left.
Another way to look at balances and
unbalanced forces