1. Newton's Laws Discussion
Created for Operation Physics By
Dick Heckathorn
20 October 2K + 4

2. I am holding
an apple in my hand.

3. Fhand on apple Fearth on apple
What forces are acting on the apple as it lies motionless in my hand?
Fhand on apple
Any other? No
Fearth on apple

4. I hope you said “yes”! Is it true that all forces: come in pairs,
are equal in
magnitude,
but opposite
in direction?
I hope you said “yes”!

5. Fhand on apple Fearth on apple Fapple on earth
What is the reaction force that is equal and opposite to the gravitational force?
Is it
It is
the hand on the apple?
the apple
on the earth? or
Fhand on apple
Fearth on apple
Fapple on earth

6. When I let go of the apple, What force(s) are acting on
it will be in free fall,
What force(s) are acting on
the apple as it falls?
Fhand on apple
Any other? No No
Fearth on apple

7. Since there is a net force down on the apple, the apple must be
accelerating downward.
Fearth on apple

8. Since the apple is accelerating downward,
could the forces acting on the apple
add up to be zero?
No.

9. 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.

10. Fhand on apple 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.

11. Fearth on apple Fapple on earth 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

12. It is the force of the apple on the 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.

13. are equal in magnitude, and
We know forces:
always come in pairs,
are equal in magnitude, and
opposite in direction.

14. We were not interested in Newton’s 3rd law action-reaction forces.
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.

15. if we want to know what an object is doing, we will concern ourselves
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.

16. When dealing with forces, one must, at all times identify the object
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.

17. Object:
Apple
Agent:
Earth
Then one writes:
Force earth on apple

18. There are paired forces never on the same object.
Newton’s Third Law
There are paired forces
that act on
different objects,
never on the same object.

19. Forces are identified by interchanging the object and the agent.
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

20. Some things to
think about.

21. beginning to overtake him and he is slipping back down.
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.

22. is pushing on the big rig? Force rig on man = Force man on rig
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

23. 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.

24. A Volkswagen
pushes
a Mack truck
causing them to move
with a
constant velocity.

25. 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

26. Next
the Volkswagen
is pushing
the Mack truck uphill.

27. 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

28. A SUV
with a heavy trailer
is slowly
driving down
a steep incline.

29. 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

30. Is there a net force on the truck?
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

31. A Mack truck collides head-on moving at, let’s say, 50 km/hr.
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.

32. mT . aT = mVW . aVW 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

33. “Newton never saw a Mack truck and therefore Newton’s third law
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.

34. A bug collides with a car
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

35. That’s all tonight.

36. That’s all folks!