Newton s 3 rd Law of Motion. Action and Reaction

Transcription

1 Newton s 3 rd Law of Motion Action and Reaction

2 A Force is Part of an Interaction Up till now, a force has been a single and simple push or pull From now on, a force is part of a mutual action between two objects This is called an interaction

3 A Force is Part of an Interaction Every interaction is between a pair of forces You push on something, it pushes back on you Something pushes on you, you push back on that

4 A Force is Part of an Interaction Tires push on the road and the road pushes back On ice, the push of the tires (or your feet) is not enough to cause the opposite force

5 A Force is Part of an Interaction

6 A Force is Part of an Interaction Does a flying ball have force? Force is not something an object possesses (like mass or volume) The ball exerts a force when it hits something

7 A Force is Part of an Interaction It would appear that the ball has definitely interacted with the head But Cub hunting is so boring

8 A Force is Part of an Interaction Preferably, we would like to be huntin Yankees

9 A Force is Part of an Interaction Xeno s Paradox In order to make contact you first have to cross half the distance and half again and again ad infinitum 10m to 5m to 2.5m to 1.25m to.625m to.3125m to.156m to.078m. How can an interaction take place if it needs contact?

10 Newton s Third Law Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force of the first To every action there is always an equal and opposite reaction

11 Newton s Third Law Which one is the action and which one the reaction? Who cares? It is not important. It is important to realize that both are part of a single interaction Neither force/action exists without the other

12 Newton s Third Law While these forces are equal and opposite, they do not necessarily produce equilibrium Equilibrium usually refers to forces acting on a single object N 3 refers to an interaction between two objects

13 Newton s Third Law The forces are equal and opposite, but the effects of the equal forces can be very unequal For example

14 Stupid people with chain saws

15 Stupid people on mountain bikes

16 Stupid people on surf boards

17 People in the wrong place at the wrong time

18 Ummm Not sure what s going on here.

19 Newton s Third Law We can see action reaction at work here Equal and opposite forces between two different objects producing very unequal effects

20 Rule for Action Reaction Action: Object A exerts a force on object B Reaction Object B exerts a force on object A

21 Rule for Action Reaction Action: Space shuttle pushes on gas Reaction: Gas pushes on space shuttle

22 Action Reaction on Different Masses When a cannon is fired, there is an interaction between cannon and cannonball Equal and opposite This is why the cannon kicks or recoils But the effects of the equal and opposite forces are unequal

23 Action Reaction on Different Masses Based on Newton s 2 nd Law A = F/M Cannonball: A = f/m Cannon: a = f/m Acceleration is inversely proportional to mass!!

24 Action Reaction on Different Masses Recoil can be exampled in other ways Balloons recoil when their air is expelled The balloon is recoiling in the opposite direction from the expelled gas Squid, cuttlefish and octopi (all very good eats) move in the same way

25 Action Reaction on Different Masses A rocket continually recoils from ejected exhausted gas Each molecule is like a tiny cannonball shot from rocket

26 Action Reaction on Different Objects Equal and opposite many times for N 3 does not indicate equilibrium This is because the forces are acting on two different objects Forces can only cancel when they act on the same object

27 Action Equals Reaction Remember, these are two sides of the same coin You can t have action without reaction or reaction without action

28 Action Equals Reaction Pull on a rope attached to the wall The tension you produce by pulling is cancelled by the pull on the rope by the wall Otherwise there is a net force and the rope would accelerate

29 Action Equals Reaction Pull on a rope attached to a person (tug-of-war) The tension you produce you pulling is cancelled by the other person pulling on the rope The tension can t be different along points on the same rope So how does you win tug-of-war?

30 Action Equals Reaction Helicopter stuff Action: the blades force air particles down Reaction: the air forces the blades up Lift When lift = weight the helicopter hovers When lift > weight, the helicopter rises

31 Action Equals Reaction Birds fly using the same lift principle Wings push on the air, air pushes on the wings

32 Action Equals Reaction Can t punch your way out of a paper bag? You re not alone

33 Action Equals Reaction According to Newton: You can t hit a piece of paper any harder than it can hit you back (action/reaction) Try dropping a piece of paper and punching it

34 Summary of Newton s Three Laws Law of Inertia Net Force gets you out of Equilibrium Law of Acceleration Acceleration is inversely related to mass Acceleration is directly related to force Acceleration is in the direction of the net force Law of Action Reaction Force on different objects are equal and opposite Acceleration is different