Transcript Newton`s 1st Law and Applying Newton`s 2nd Law

Newton’s 1st Law of Motion,
Weight & Applying Newton’s 2nd
Law
System
Chapter 6.2
What You Already Learned
• Newton’s 2nd Law of Motion.
a
• Inertia was a term first coined by
.
– He suggested that objects in motion would
remain so unless acted upon by
.
– Inertia is an objects
to
in
.
• For two objects of the same size, the more
an object has, the more
it has.
Newton’s 1st Law of Motion
• Newton later revised Galileo’s theory:
– An object in motion will remain in motion in a
with
unless acted upon by an
force.
and
– An object at
will remain at
unless acted upon by an
Newton’s 1st Law of motion is also known as the
force.
.
Inertia and Motion
• If the net sum of external forces on an object was
equal to zero, would the object change direction or
accelerate?
•
– If the forces are
or change in
, then
will occur.
Fground-on-car
Fforward
Ffriction
Fgravity
System
If the forces are
,
then the vehicle will continue to
travel at a
.
Misconceptions about Forces

Is a force required after a ball is released while
throwing to make it continue in its path?

: once the contact force between the
hand and the ball are broken, there is
a force propelling the ball forward.
Misconceptions about Forces
• Is inertia a force?
–
: Inertia is the tendency for an object
to
a change in
.
– Inertia is a property of
.
– The more
an object has, the more
it has.
– Forces that exist in the environment
on objects.
Misconceptions about Forces
• Does air exert a force?
–
• Objects that are not moving relative to their
surroundings experience a
of
due to the air in the atmosphere
(Barometric pressure = 101.3 Pa = 14.7 psi).
• Objects in motion experience
,a
due to air acting on
the side of the object facing the direction that it is
moving. As the
increases the
increases.
– Ex. Air drag on skydivers, automobiles, etc.
What’s the difference between
mass and weight?
is a property of an object that
(provides a numerical value) for
the amount of
(
and
) that it contains.
•
is a measure of the
exerted
on a body by
, which is directly
related to the amount of
and
due to gravity.
•
is the same everywhere in the
, while
will vary with
the mass and distance from other bodies.
•
NOTE:
AND
ARE THE SAME!
Determining Weight
• If the mass of an object is known, its
weight can be determined using
Newton’s
Law of Motion.
o Fg =
Where:
•m=
•g=
(g =
).
to
Example: Mass vs. Weight
On Earth:
•
–
–
On the Moon:
•
–
–
Mass = __ kg
Weight = _____ N (gm = 1.62 m/s2)
Why is the weight on the moon so much less?
•
–
•
Mass = __ kg
Weight = ___ N
Because the gravitational force of attraction on the moon
is _____ that on Earth.
Note: The
is the same on both the
Earth and the Moon.
Example 1: How will your weight
change?
•
You have a mass of 75 kg and are standing
on a bathroom scale in an elevator. The
elevator accelerates from rest at a rate of
2.0 m/s2 for 2 s and then continues at
constant speed.
1. What is the scale reading during acceleration?
2. How does this reading compare to that of the
scale at rest?
3. How does this reading compare to that of the
scale when the elevator is moving at constant v?
Diagram Problem
+y
System
Fnet =
State the Known and Unknowns
• What is known?
o Mass (m) = ___ kg
o Acceleration (a) = ___ m/s2
o Time (t) = ___ s
• What is not known?
o Fscale = ?
Perform Calculations
• Fnet =
Where:
o Fnet =
o Fgravity =
o Fscale =
Scale Reading at Rest and
Constant Speed
• When the elevator is at rest or not
accelerating, equation (1) [Fnet =
reduces to:
• Since the forces are
and FNet =
.
]
, there is
(Newton’s 1st Law of Motion)
Key Ideas
• Inertia is a measure of an objects
change in motion.
• Newton’s 1st Law of motion is also known as the
.
• Size being equal, the more
a body
contains, the more
it has.
• If the sum of the forces on a body equal
,
then the object will remain at
, or remain in
motion at a
.
to
Key Ideas
is a property of a material that
is a measure of the amount of
it
contains.
•
is a measure of the force on an
object that is proportional to its
and
due to
.
•