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Newton's Laws Discussion
Created for Operation Physics
By
Dick Heckathorn
20 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?
Any
other?
No
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”!
Is
it force that is
It is
What is the reaction
the hand
the
apple
equal and opposite to
the
on the gravitational
or
force?on the
apple?
earth?
Fhand on apple
Fearth on apple
Fapple on earth
When I let go
of the apple,
it will be in
free fall,
Fhand on apple
No
What force(s)
are acting on
the apple as
it falls?
Any
other?
No
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 downward,
could the forces acting on
the apple
add up to be zero?
No.
A hint…..
When you are asked to identify
the force(s) acting on an object,
You can eliminate forces
from discussion
if their reaction force
cannot be found.
When we asked if the hand
was acting on the apple
after it left the hand,
Fhand on apple
Could one say that the apple
was acting on the hand?
No.
What is the reaction force
that is equal and opposite
to the gravitational force exerted
by the earth on the apple?
Fearth on apple
Fapple on earth
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 apple.
We know forces:
always come in pairs,
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 object
to which the force(s) is applied
and the agent
causing each force.
Object:
Apple
Agent:
Earth
Then one writes:
Force earth on apple
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
object and
the agent.
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
Force rig on man = Force man on rig
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
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? No
or…
is the Mack truck pushing with a
greater force on the Volkswagen
because of its inertia and
additional downhill force? No
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? No
or…
is the SUV exerting a larger
force on the trailer since it needs
to keep the trailer behind itself?
No
A Volkswagen is pushing a large
truck down the road causing the
truck to undergo a constant
acceleration.
Is there a net force on the truck?
Yes
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?
Neither, both forces
are the same.
Is it not true that the encounter
will leave the Volkswagen
“ready for recycling,”
while the Mack truck escapes
with “minor” dents?
Probably
FVW on T = FT on VW
mT
.
aT
=
mVW
.
aVW
It has been said,
“Newton never saw a Mack truck
and therefore Newton’s third law
is most likely not applicable
in this situation.”
Sorry, no case has been observed
where Newton’s 3rd has not been
observed to be true.
A bug collides with a car. Which
experience the greater force, the
bug or the car?
Neither, both forces are the same.
Which is affected the most, the
bug or the car?
FB on C = FC on B
mC
.
ac
=
mB
.
aB
That’s all tonight.
That’s all folks!