# Goal To understand Newtons 3 rd law Objectives

• Slides: 18

Goal: To understand Newton’s 3 rd 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 3 rd 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 3 rd 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 3 rd 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 2 nd 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!