Transcript Goal: To understand Newton’s 3rd law.

Goal: To understand
Newton’s 3rd law.
Objectives:
1) To understand how actions
create Opposite reactions
2) To examine Applications to this
law
3) To learn how to Save Batman!
Law of Opposite Reaction
• For every action there is an equal and
opposite reaction!
• So, when you exert a force on something,
it exerts and equal force back on you, but
in the opposite direction!
What does this mean?
• I push on a wall.
• The wall pushes back!
• If I apply a 1000 N force to the wall in the
West direction, the wall applies a 1000 N
in the opposite (East) direction, or you can
say -1000 N West.
Skateboard example
• As we saw last time when 2 people
pushed off, no matter how strong the 2
people were, both had the same force
applied to them.
• Each of us apply a force to the other, but
the other then ends up applying an equal
but opposite force back.
Open door?
• Wait, wait, hold the phones!
• I push on a door, it pushes back….
• Wouldn’t that negate everything?
• Why does the door open? Talk to your
neighbor about this.
Forces on each
• To see what will happen to something you
just have to consider the forces acting ON
it.
• Whatever forces it is generating do not
affect it directly.
• Since there is no force to oppose the force
on the door by me, it will open.
• Now, the door pushes on me. Why don’t I
recoil the other way?
Walk like an Egyptian
• So, how does walking work?
• Walking works because you are generating a
force in the sideways direction on the ground.
• The ground therefore pushes back on you – and
you go forward.
• This is done with friction.
• Thus, no friction, no walk.
Rocket Science!
• Here is the basics of rocket science:
• You have a liquid or gas inside the rocket at
tremendous pressure.
• That pressure pushes out the liquid/gas with a
force.
• That means that the rocket exerts a force on the
gas.
• The gas therefore exerts an equal and opposite
force on the rocket.
• The rocket lifts off!
So, to recap
• Rocket exerts force on gas.
• Gas exerts force on rocket.
• Gas and rocket go in opposite directions.
• And all because of Newton’s 3rd law.
Recoil
• This recoil action works for anything.
• If you shoot a gun, the bullet gets pushed
forward.
• Your gun gets pushed back into you.
• A cannon: you shoot the cannonball, and the
cannon goes the other way.
• On a ship a loose cannon was bad because the
recoil of an improperly supported cannon could
damage the ship.
But what about gravity?
• I drop a rock from the top of a building.
• Have I applied any force to this rock?
What happens?
• The rock falls.
• Clearly a force is being applied to the rock.
• Does this mean the rock will apply a equal
and opposite force to me?
• Has Newton’s 3rd law been violated?
The big picture!
• Sometimes you have to look at the bigger
picture to complete the forces.
• In this case you have to include the earth
itself.
• The earth is applying a gravitational force
to the rock.
• So, what does the reaction have to be?
Earth does not move!
• The earth doesn’t budge in this problem.
• Why not?
Saving Batman!
• Previously we saw that to save Batman
from the frozen lake that we needed to
give Batman a force somehow.
• And once we give him a force he will
move, and continue to move until
something stops him.
• Where oh where can he get a force from?
And the saving force is:
• All Batman has to do is exert a force on
something he is carrying.
• He can shoot off his grappling hook and
he will go the other way.
• Granted, he will go at a fraction of the
speed as we saw last time, but he will still
go!
• He could even throw a shoe!
Batman recap
• To generate a force he uses the 3rd law.
• He generates a force on something he has
handy to send it in 1 direction while he gets an
equal and opposite force from whatever he
sends off (you don’t just throw a shoe, the shoe
throws you!).
• This force due to the 2nd law generates an
acceleration changing Batman’s velocity from 0
to some non-zero velocity.
• The first law allows him to keep that velocity just
as long as there is no other force on him, and he
narrowly escapes doom on the icy lake!
Conclusion
• When a force is applied and equal and
opposite force is applied back.
• This allows you to walk.
• This allows for rockets.
• This saves Batman!