Goal: To understand Newton’s 3 rd law. Objectives: 1)To understand how actions create Opposite...
-
Upload
chad-obrien -
Category
Documents
-
view
212 -
download
0
Transcript of Goal: To understand Newton’s 3 rd law. Objectives: 1)To understand how actions create Opposite...
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!