Newton 2nd Law

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Newton 2nd Law

• Today we understand Newtons Second law and how
  it can be used to measure mass is a different way.

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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
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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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.
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.
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.
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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.

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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.

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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
• SF a
• Slope of a F vs a graph is__________.
• What type of mass are you measuring?

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Conclusion

• Are the masses the same?
• What is their percent difference?