Download Newton`s 2nd Law

Newton 2nd Law
• Today we understand Newton’s
Second law and how it can be used
to measure mass is a different way.
Second Law of Motion: “The Law of Acceleration”
The acceleration produced by a net force on an object is
directly proportional to the magnitude of the net force, is in
the same direction as the net force, and is inversely
proportional to the mass of the object
v
v
F  ma
“sigma” = sum
F and a
are vectors
What does this law tell us?
• Objects that are not in equilibrium will accelerate.
• Net force (sum of all forces) on an object causes acceleration.
• Note the difference between a force and a net force.
A good analogy is to compare deposits/withdrawals
into a bank account with the account balance.
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applet
Inertial and Gravitational Mass
Inertial mass
Relates to how a mass responds to an external
force (also called a contact force).
If you push a stalled car into motion you are
testing its inertial mass.
Gravitational mass
Relates to how a mass responds to the force
of gravity (also called a field force).
If you lift up a stalled car you are testing its
gravitational mass.
r
F  ma
inertial mass
Fg  mg
gravitational mass
In the equation for weight, g is no longer considered the
acceleration due to gravity, but rather the gravitational
field strength, with units of newtons/kilogram.
Inertial and gravitational masses have been tested and are
believed to always be equal in amount. This is why all
objects freefall at the same rate of acceleration.
Constant acceleration from gravity
• Galileo found that balls of
different masses fell at the
same rate.
• The force of gravity is
stronger for more massive
objects.
• But it requires more force to
accelerate a more massive
object (inertia).
• These two effects cancel each
other out.
How does this relate to gravity?
• We have already talked object that are in free fall
always fall with a acceleration of 9.8 or 10 m/s2.
– Remember the acceleration in the y direction for all of
those fun freefall problems?
• Well this is why they do.
F  ma
i
Fg  mg g
F  F
g
mi a  mg g
ag
Mini Lab: Mass Two Ways
• The first way is that
you will measure the
mass of the car using
a triple beam
balance.
• What type of mass
are you measuring?
• The second way is to
plot the Net Force vs
Acceleration on a graph.
• Y=
x
• ΣF =
a
• Slope of a F vs a graph
is__________.
• What type of mass are
you measuring?
Conclusion
• Are the masses the
same?
• What is their percent
difference?