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Newton's Laws Discussion
Created for Operation Physics
By
Dick Heckathorn
12 October 2K + 4
I am holding
an apple in my hand.
What forces are acting on the
apple as it lies motionless in
my hand?
Fhand on apple
Fearth on apple
Is it true that all forces:
come in pairs,
are equal in magnitude,
but opposite in direction?
I hope you said “yes”!
What then is the reaction force
that is equal and opposite to the
gravitational force exerted by the
earth on the apple?
Fearth on apple
The reaction force to earth
on apple is…
Fhand on apple
Fapple on earth
or
the apple
on the
earth?
the hand
on the
apple?
Fearth on apple
When I let go
of the apple,
it will be in
free fall?
What forces
are acting on
the apple as
it falls?
Fearth on apple
What is the reaction force
that is equal and opposite
to the gravitational force exerted
by the earth on the apple?
Fapple on earth
Fearth on apple
Since there is a net force down on
the apple, the apple must be
accelerating downward.
Fearth on apple
Since the apple is
accelerating,
could the forces acting on
the apple
add up to be zero?
No.
Once again, what then is the force
that is equal and opposite
to the gravitational force
exerted by the earth on the apple?
It is the force of the apple on the
earth.
Note…It is not acting on the ball.
We know forces:
always come in pairs,
are directed away from
each other,
are equal in magnitude, and
opposite in direction.
When we examined the apple
accelerating as it fell, we
wanted to know only the
forces acting on the apple.
We were not interested in
Newton’s 3rd law
which deals with
action-reaction forces.
When dealing with
the laws of Newton,
if we want to know what an
object is doing,
we will concern ourselves
with only the forces
acting on the object.
We will not concern our self with
Newton’s 3rd law.
Summary
When dealing with forces,
one must, at all times identify
the agent
(cause of the force)
and the object
(to which the force is applied).
Agent:
earth
Object:
apple
Then one writes:
Force earth on apple
Agent:
apple
Object:
earth
Then one writes:
Force apple on earth
Newton’s Third Law
There are paired forces
that act on
different objects,
never on the same object.
Newton’s Third Law
Forces are identified by
interchanging the
agent and
the object.
Force earth on apple = Force apple on earth
and are in opposite directions
Some things
to
think about.
A man is trying to push
a big rig up a steep hill,
but the big rig is
beginning to overtake him
and
he is slipping back down.
Is the big rig
pushing on the man
harder than
the man
is pushing on the big rig?
No
A student flexes his muscles
and pushes against the wall.
Does the wall push back?
Yes
How does the wall know enough
to push back?
Does Newton’s law cause it?
No, it just describes
what is happening.
A Volkswagen
pushes
a Mack truck
causing them to move
with a
constant velocity.
Does the Volkswagen exert a
larger force on Mack truck
since it is moving the truck
in a forward direction?
or…
does the Mack truck exert a
larger force on the Volkswagen
since it is much more massive?
Neither, they are the same
magnitude
There’s more.
Next
the Volkswagen
is pushing
the Mack truck uphill.
Is the Volkswagen
pushing on the truck
with a greater force
since it has to overcome the
additional downward force
of the Mack truck?
or…
is the Mack truck pushing
with a greater force on the
Volkswagen
because of its inertia
and additional downhill force?
A SUV
with a heavy trailer
is slowly
driving down
a steep incline.
Is the trailer
exerting a larger force
on the SUV
since it would like to roll down
the incline faster
than the slow-moving SUV?
or…
is the SUV exerting
a larger force on the trailer
since it needs to
keep the trailer behind itself?
A Volkswagen is pushing a large
truck down the road causing the
truck to undergo a constant
acceleration ‘a’.
Is there a net force on the truck?
What can you conclude
about the force of the
Volkswagen on the truck
compared to the force of the
truck on the Volkswagen?
A Mack truck collides head-on
with a Volkswagen, both
moving at, let’s say, 50 km/hr.
During the collision, which of the
two vehicles exerts the larger
force on the other?
Is it not true that the encounter
will leave the Volkswagen
“ready for recycling,”
while the Mack truck escapes
with “minor” dents?
Why or why not?
“Newton never saw a Mack truck
and therefore Newton’s third
law is most likely not
applicable in this situation.”
That’s all folks!