ch6_owtb

 =Chapter 6 -- Newton's Third Law: Action and Reaction     =

 **   For every action, there is an equal and opposite reaction. **

The basics: <span style="font-family: Georgia,serif;"> <span style="font-family: Georgia,serif;"><span style="font-family: Georgia,serif;"><span style="color: rgb(91,0,255); font-family: Georgia,serif;"><span style="color: rgb(0,0,0); font-family: Georgia,serif;">**Real life examples of action-reaction force pairs:** A fish uses its fins to push water backwards. But a push on the water will only serve to accelerate the water. Since forces result from mutual interactions, the water must also be pushing the fish forwards, propelling the fish through the water. The size of the force on the water equals the size of the force on the fish; the direction of the force on the water (backwards) is opposite the direction of the force on the fish (forwards). For every action, there is an equal (in size) and opposite (in direction) reaction force. Action-reaction force pairs make it possible for fish to swim.
 * <span style="font-family: Georgia,serif;">In every interaction, there is a pair of forces acting on the two interacting objects.
 * <span style="font-family: Georgia,serif;">The size of the forces on the first object equals the size of the force on the second object.
 * <span style="font-family: Georgia,serif;">The direction of the force on the first object is opposite to the direction of the force on the second object.
 * <span style="font-family: Georgia,serif;">Forces always come in pairs - equal and opposite action-reaction force pairs.

Consider the flying motion of birds. A bird flies by use of its wings. The wings of a bird push air downwards. Since forces result from mutual interactions, the air must also be pushing the bird upwards. The size of the force on the air equals the size of the force on the bird; the direction of the force on the air (downwards) is opposite the direction of the force on the bird (upwards). For every action, there is an equal (in size) and opposite (in direction) reaction. Action-reaction force pairs make it possible for birds to fly.

Consider the motion of a car on the way to school. A car is equipped with wheels which spin backwards. As the wheels spin backwards, they grip the road and push the road backwards. Since forces result from mutual interactions, the road must also be pushing the wheels forward. The size of the force on the road equals the size of the force on the wheels (or car); the direction of the force on the road (backwards) is opposite the direction of the force on the wheels (forwards). For every action, there is an equal (in size) and opposite (in direction) reaction. Action-reaction force pairs make it possible for cars to move along a roadway surface.

<span style="font-family: Georgia,serif;">http://www.glenbrook.k12.il.us/GBSSCI/PHYS/CLASS/newtlaws/u2l4a.html
 * Check Your Understanding: **

=<span style="color: #f278d4; font-family: Georgia, serif;">//Outline of Chapter 6 from Physics Text Book// =

<span style="font-family: Georgia, serif;">**6.1 --** **Forces and Interactions** A force is a push or pull A force is not a thing in itself, but an interaction Interaction -- Mutual action between one thing and another Examples: Interaction between hammer and nail

**<span style="color: #f575e0; font-family: Georgia, serif;">6.2 -- Newton's Third Law ** Two forces: Action and reaction Coparts of a single interaction Neither force exists without the other Equal in strength, opposite in direction In every interaction, the forces always occur in pairs Examples: You interact with the floor when you walk on it When swimming, you interact with the water Car tires interact with the road

<span style="font-family: Georgia, serif;">**6.3 --** **Indentifying Action and Reaction** One object (A) interacts with another object (B) Action: A exerts force on B Reaction: B exerts force on A Other examples: Action: Tire pushes road Reaction: Road pushes tire Action: Rocket pushes gas Reaction: Gas pushes rocket Action: Earth pulls ball Reaction: Ball pulls earth

When a rifle is fired, there is an interaction between the rifle and the bullet. The force the rifle exerts on the bullet is exactly equal and opposite to the force the bullet exerts on the rifle, so the bullet shoots through the air (large acceleration & small mass) and the rifle "kicks" (small acceleration & large mass).
 * <span style="color: #cc1935; font-family: Georgia, serif;">6.4 -- Action and Reaction on Different Masses **

F= action and reaction forces **m**= Mass of rifle m= mass of bullet a=acceleration of rifle **a**=acceleration of bullet Bullet: F/m = **a** Rifle: F/**m** = a

<span style="font-family: Georgia, serif;"><span style="font-family: Georgia, serif;"> **6.5** --  **Do Action & Reaction Forces Cancel?**    <span style="font-family: Georgia, serif;"><span style="font-family: Georgia, serif;"> <span style="font-family: Georgia, serif;">Why it might be initially <span style="font-family: Georgia, serif;">confusing : > <span style="font-family: Georgia, serif;">And if -16 + 16 = 0 > <span style="font-family: Georgia, serif;">Shouldn't there be a net value of zero when considering > <span style="font-family: Georgia, serif;">The equal and opposite forces? > <span style="font-family: Georgia, serif;"><span style="font-family: Georgia, serif;"> <span style="font-family: Georgia, serif;"><span style="font-family: Georgia, serif;"><span style="font-family: Georgia, serif;">Easy & Simple: <span style="font-family: Georgia, serif;"> <span style="font-family: Georgia, serif;">* Basically, you have to consider all of the systems <span style="font-family: Georgia, serif;">-*Internal <span style="font-family: Georgia, serif;"> -*External <span style="font-family: Georgia, serif;">Reminder: <span style="font-family: Georgia, serif;">*Newton's Third Law: <span style="font-family: Georgia, serif;"> <span style="font-family: Georgia, serif;"> Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.
 * <span style="font-family: Georgia, serif;">Actions & Reactions are equal and opposite.
 * <span style="font-family: Georgia, serif;"> <span style="font-family: Georgia, serif;">That's not quite all there is to consider when answering...

<span style="font-family: Georgia, serif;">*Important statements from 6.5: <span style="font-family: Georgia, serif;">- "If action and reaction forces are **internal** to a system, they **cancel** each other and produce **no acceleration** of the system."

<span style="font-family: Georgia, serif;"><span style="font-family: Georgia, serif;"><span style="font-family: Georgia, serif;"> <span style="font-family: Georgia, serif;">** 6.6 The Horse-Cart Problem ** <span style="font-family: Georgia, serif;">AKA <span style="font-family: Georgia, serif;">"Horse Sense"  <span style="font-family: Georgia, serif;">(Text 81) <span style="font-family: Georgia, serif;"> <span style="font-family: Georgia, serif;">  <span style="font-family: Georgia, serif;"> <span style="font-family: Georgia, serif;"> <span style="font-family: Georgia, serif;"> <span style="font-family: Georgia, serif;">1. Cart and <span style="font-family: Georgia, serif;">Horse Pulling in Opposite Directions. <span style="font-size: 90%; font-family: Georgia, serif;"> =<span style="font-family: Georgia, serif;"><span style="font-family: Georgia, serif;"> = <span style="font-size: 12pt; font-family: 'Times New Roman'; mso-fareast-font-family: 'Times New Roman'; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA;"><span style="font-family: Georgia, serif;">(F2)<-> (P) (P) <-> (F1) <span style="font-family: Georgia, serif;">Force 1 (horse pulling forward) is cancelled out because Force 2 (cart pulling back) is equal and opposite to the force exerted on it. <span style="font-family: Georgia, serif;">= Zero Net Value & No Acceleration
 * <span style="font-family: Georgia, serif;"><span style="font-family: Georgia, serif;">Context of the Problem:    <span style="font-family: Georgia, serif;"><span style="font-family: Georgia, serif;">There is a farmer with a cart and a horse who needs to get to the market by moving forward. Horse argues that if he pulls on the cart (Force 1) the cart will pull backwards on him (Force 2) with an equal and opposite force, resulting in a net force of 0 and no acceleration of the cart or the horse.
 * <span style="font-family: Georgia, serif;"><span style="font-family: Georgia, serif;"> <span style="font-family: Georgia, serif;">* Consider the different systems in the problem…

<span style="font-family: Georgia, serif;"><span style="font-family: Georgia, serif;"><span style="font-size: 12pt; font-family: 'Times New Roman', Times, serif; msofareastfontfamily: 'Georgia'mso-ansi-language;">2. Cart Alone <span style="font-family: Georgia, serif;">So it needs more force to move it forward. [needs more force--->] The farmer knows that if the horse moves forward so will the cart. He needs the horse to provide the force needed to pull the cart, regardless of the force exerted on the horse. <span style="font-family: 'Times New Roman', Times, serif;">3. Horse Alone
 * <span style="font-family: Georgia, serif;"><span style="font-family: Georgia, serif;">Has to overcome...
 * <span style="font-family: Georgia, serif;"><span style="font-family: Georgia, serif;">mass of cart
 * <span style="font-family: Georgia, serif;"><span style="font-family: Georgia, serif;">wheel friction
 * <span style="font-family: Georgia, serif;"><span style="font-family: Georgia, serif;">air resistance
 * <span style="font-family: Georgia, serif;"><span style="font-family: Georgia, serif;">farmer knows that...
 * <span style="font-family: Georgia, serif;">A(cart) = F(cart) / M(cart)
 * without the cart the horse moves forward easily
 * - How could it increase the force it uses to move forward in order to overcome the force with which the ground (FG) pulls the mass of the cart backward?
 * <span style="font-size: 12pt; font-family: 'Times New Roman'; mso-fareast-font-family: 'Times New Roman'; mso-ansi-language: EN-US; mso-fareast-language: EN-US; mso-bidi-language: AR-SA;">The horse is able to move forward by interacting with the ground the same way we do every day. By pushing down against the ground, the horse is using the equal and opposite force that the ground exerts back against it to push its body forward.

4. The Entire Horse-Cart System Cart Horse (F2)<-> (P) (P) <---> (F1) (FG)< > (Fa) *friction of wheels *friction of ground
 * -->( F1+ Fa) **

<span style="font-family: Georgia, serif;">So by pushing its hooves (Force a) against the ground (Force G) the horse is able to use the equal and opposite force from the ground is pushing back against it to move forward with a greater force. <span style="font-family: Georgia, serif;">The force that the horse receives from the ground is used to increase the force it uses to pull the cart. <span style="font-family: Georgia, serif;">There's now an external force (ground) in addition to the internal forces (cart & horse). <span style="font-family: Georgia, serif;">= Positive Net Force, & Forward Acceleration

<span style="font-family: Georgia, serif;">Net Forces: <span style="font-family: Georgia, serif;">On Farmer – P (pull from horse) - f (friction) <span style="font-family: Georgia, serif;">On Horse – F(force 1) - P (pull from cart) <span style="font-family: Georgia, serif;">On Ground – F (force G) - f (friction) <span style="font-family: Georgia, serif;">*In order to accelerate, an external force is needed.

** <(F2)Cart (P)>  <(P)---Horse ---(F1)> - 12 N +12 N  <(FG)Ground (Fa)> + 10 N Net Force = - 12 N + 12 N + 10 N = 10 N
 * <span style="font-family: Georgia, serif;">Here's the Horse-Cart problem again with numbers to represent the forces…

<span style="font-family: Georgia, serif;">**Another Example....** <span style="font-family: Georgia, serif;"><span style="font-family: Georgia, serif;"><span style="font-family: Georgia, serif;">IF YOU WANT TO MAKE SURE YOU UNDERSTAND, <span style="font-family: Georgia, serif;">READ OVER THIS SIMILAR EXAMPLE: <span style="font-family: Georgia, serif;">Imagine your car has stalled. Would you sit in your front seat, pushing against the dashboard in order to move it forward? Obviously, this action would not help you move your car forward (the reaction would just be the dashboard pushing back against your hands). <span style="font-family: Georgia, serif;"> If you needed to move your car forward, you would get out of it first of all, then push against it with your hands as you pushed against the ground with your feet. <span style="font-family: Georgia, serif;"> Do you see how this is like the horse-cart example now? <span style="font-family: Georgia, serif;">Pushing on the dashboard as it pushes against you is an internal force and there will be no net force in this system alone. (Like the first part of the horse-cart example.) <span style="font-family: Georgia, serif;">When you push against the ground, it pushes against you with an equal and opposite amount of force, which you use to push against the car. With this force your car will move along the road because a net force will be created. (Like when the horse used the ground to move the cart.)

<span style="font-family: Georgia, serif;">** 6.7 --  Action Equals Reaction ** <span style="font-family: Georgia, serif;"><span style="font-family: Georgia, serif;"><span style="font-family: Georgia, serif;">**The Wall Example** – With every action force, an equal and opposite reaction force occurs. <span style="font-family: Georgia, serif;"> -You can not hit the wall any harder than the wall can hit you back. <span style="font-family: Georgia, serif;">-If you decide to hit a wall, be warned that the wall will exert a force on your hand as hard as you exert on the wall. The wall will hit you as hard as you hit it. <span style="font-family: Georgia, serif;">-> <- Equal & Opposite Directions <span style="font-family: Georgia, serif;"><span style="font-family: Georgia, serif;">**The Paper and the Heavyweight Champ Example** – You can only act on an object with as much force as it can react (equal & opposite) with on you <span style="font-family: Georgia, serif;"> -You cant hit paper a piece of paper in the air with a 250 N force. <span style="font-family: Georgia, serif;">-Why? - Because the paper cannot hit you back with that amount of force. <span style="font-family: Georgia, serif;"> - There is no action force without an equal and opposite reaction force! <span style="font-family: Georgia, serif;">- But if your heart is set on punching a piece of paper with 250 N of force, you could do so if you taped the paper to the wall and then hit it. The wall will hit you back with an equal and opposite amount of force.