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!