Download Newton`s First Law

PHYS 220
General Physics
Aristotle
“The natural state of objects is at
rest.
Objects have velocity because a
force acts on it.”
Aristotle 384-322
BC
Galileo



There were problems
with Aristotle's ideas
(e.g. any item that
coasts to a stop)
Galileo experimented
with balls on an
inclined plane
Conclusion: If the
slope is 0, the
acceleration should be
0.
Inertia



An object will maintain its state of motion (its
velocity) unless it is acted upon by a force.
Does not yet explain how force is related to
motion – Newton's laws will do this.
Breaks Aristotle's link between force and
velocity.
Newton 1642-1727 AD
Newton's First Law
If the total force acting on an object is zero, the
object will maintain its velocity (magnitude and
direction) forever.


Inertia is a measure of an object's resistance to
change in motion – depends on mass.
Mass is a measure of how much matter an
object contains. An intrinsic property.
Abracadabra!
Table Cloth Demonstration
Newton's Second Law


Many different forces can be acting on an object
simultaneously.
The effect of all these forces is the same as the
effect of one force with the same magnitude and
direction as the sum of all the forces.

F tot =

F
The acceleration of an
object is related to the
net force and the mass


F
ma
of the object.
Units of Force:
1newton
N = 1 kg
m/s
2
Direction


Acceleration must be
parallel to the total force
Velocity does not need to
be parallel to total force.
Example
Consider the moon as it orbits Earth. Is the
moon's acceleration zero or non-zero? If the
acceleration is non-zero, what force(s) is(are)
responsible?
Newton's Third Law


When on object exerts a
force on a second object,
the second object exerts a
force of the same
magnitude and opposite
direction on the first object.
The action – reaction
principle.
Action – Reaction Pairs
Concept Check: Which of these is not an action –
reaction pair of forces?
– The force exerted by a pitcher on a baseball
and the force exerted when it hits the bat.
– When you lean against a wall, the force
exerted by your hands on the wall and the
force exerted by the wall on your hands.
– When you walk, you foot exerts a force on the
the ground and that ground exerts a force on
your foot.
Cart Pull Demonstration
After Newton
Newton's Laws do not cover everything
– Newton's Laws work in the classical regime
– Fail with quantum regime (electrons, protons,
neutrons, etc.)
– They don't tell us anything about energy.