1、NEWTONS LESSON 14: Newtons Third Law of Motion ContinuedYou are familiar with Newtons third law: For every action there is an equal and opposite reaction. The statement of this law is fairly easy, but the understanding is not. People often say the words without truly understanding their meaning.Acti
2、on-Reaction PairsYou probably know that Earth orbits the Sun because a gravitational force acts between them, so try this question. Which is greater the gravitational force of the Sun on Earth or the gravitational force of Earth on the Sun? ANSWER: If you thought they were exactly the same, congratu
3、lations. If not, lets look at this question using Newtons third law.The action and reaction in Newtons third law are forces. For every force there is an equal and opposite reaction force. The Suns gravitational force acting on Earth is an action. The Newtons third law reaction is Earths gravitationa
4、l force acting on the Sun. Earths gravitational force on the Sun and the Suns gravitational force on Earth are an action-reaction pair.Pair means two, so an action-reaction pair includes only two objects. There is never a third object in a Newtons third law action-reaction pair. If the Sun acts on E
5、arth, the reaction is Earth acting on the Sun with an equal but opposite force.For Newtons third law always remember action-reaction pairs, are always two objects, never three.This part often confuses people. Not all equal and opposite forces form a Newtons third law action-reaction pair.EXAMPLE: If
6、 you weigh 120 pounds that means that Earths gravity is pulling down on you with a force of 120 pounds. If you are standing on a solid floor, you do not accelerate downward because the floor is holding you up. The floor exerts an upward force of 120 pounds on you. These forces are equal and opposite
7、, so you remain at rest on the floor.Are these two equal and opposite forces an action-reaction pair? Lets look at them. The forces are Earth on you and floor on you. There are three objects involved, not two. So these forces are not a Newtons third law action-reaction pair. They are JUST two differ
8、ent forces acting on you that happen to be equal and opposite.The Newtons third law reaction to Earth pulling you down with a 120 pound force is that you pull Earth up with a 120 pound force. The forces, Earth on you and you on Earth, form an action-reaction pair.Diagram of the action-reaction force
9、s:The reaction to earths gravity pulling you down, is you pull the Earth up with exactly the same force. So if the forces are the same, why do we fall down to Earth rather than having Earth fall up towards us? Earth is much more massive than we are. By Newtons second law a 120 pound force will affec
10、t us, but it will not significantly affect the much more massive Earth. So we fall down; Earth does not fall up. Or we can look at it as you and the Earth have the same force acting on each of you. But your mass is much much smaller than the Earths. Therefore, for you both to have the same force, th
11、e lighter mass much have a much much larger acceleration.ma = F = maYOU EARTHNote that the larger the acceleration, the larger the velocity and the larger the distance traveled over a specified period of time. That is why it appears as if we are falling to Earth and not the other way around.! Rememb
12、er: Since action-reaction forces act on different objects they cannot cancel each other out. Balanced and unbalanced forces act on the same object so they can cancel each other out. This concept will become key in further discussions. !ACTION REACTION FORCES DO NOT CANCEL because they influence the
13、motion of different objects.INVESTIGATION:Spring Scales This scale reads force in newtons.Have two people each hold one of the two spring scales.Hook the two spring scales together. Notice how the scales are connected together below, rather than attaching them hook-to-hook, they are aligned hook to
14、top. This makes it easier to compare the readings on both scales.Two spring scales hooked together horizontally always read the same force.Have the people pull back and forth.Notice that the spring scales always read the same value.Trick question, what is the acceleration of each spring scale? What
15、is the net force on the pair of spring scales?The scales are always at rest and remain at rest, their acceleration is zero.Thus, the net force on them is zero from Newtons Law F = m a. Where “a“ is the acceleration, which here is zero, and F is the net force on the object. Here the force of you pull
16、ing on the scale balances the force of the other spring pulling on the scale.Notice that scales do not read the net force of zero. Whats Going On?Scales read the magnitude of one of the two forces acting on them, the force they exert on the thing they are attached to.So one scale reads the magnitude
17、 of the action force the other scale exerts on it.The other scale reads the reaction force. By Newtons third law the action and reaction forces are equal and opposite.Thus, these forces always have the same magnitude and so the scales always read the same value.Tension in the rope = Fab = FbaEXAMPLE
18、: HORSE AND WAGON DOES NEWTONS THIRD LAW PREVENT THE HORSE FROM MOVING THE WAGON?Noah Formula notes that Newtons Third Law says that if I pull on the wagon, the wagon exerts an equal and opposite force on me. He believes that if these two forces are equal and opposite, they will cancel, so that the
19、net force is zero. If the net force is zero, then Newtons Second Law (and Newtons First Law, too) says that the acceleration of the wagon must be zero, so the wagon can never move.Please help Noah. He know that Newtons Laws are correct, and he knows that horses really can pull wagons. There has to b
20、e an error in the argument, but what is it? Can we explain why if the horse pulls the wagon, it will move?After some study and thought, I hope that you will find answers like “The wagon moves because its attached to the horse.“ or “If the horse pushes harder on the ground than the wagon pulls on the
21、 horse, then the wagon accelerates.“ as entertaining as I, your physics teacher does!The KeyEven though a complete answer to the Horse and Wagon Question can get rather involved, a clear explanation only hinges on a couple of simple points: An object accelerates (or not) because of the forces that p
22、ush or pull on it. (Newtons 2nd Law) Only the forces that act on an object can cancel. Forces that act on different objects dont cancel - after all, they affect the motion of different objects! THE HORSE AND WAGON WITHOUT FRICTIONWhat are the Newtons Third Law Force Pairs?1. The two forces colored y
23、ellow in the diagram are a Newtons Third Law force pair - “horse pulls wagon“ and “wagon pulls horse“. They are equal in magnitude and opposite in direction.2. The two forces colored blue in the diagram are a Newtons Third Law force pair - “horse pushes ground“ and “ground pushes horse“. They are al
24、so equal in magnitude and opposite in direction.Why does the wagon accelerate?Newtons 2nd Law says that an object accelerates if there is a net (unbalanced) force on it. Looking at the wagon in the diagram, you can see that there is just one force exerted on the wagon - the force that the horse exer
25、ts on it. The wagon accelerates because the horse pulls on it! The amount of acceleration equals the net force on the wagon divided by its mass (Newtons Second Law). Why does the horse accelerate?There are 2 forces that push or pull on the horse in the diagram above. The wagon pulls the horse backwa
26、rds, and the ground pushes the horse forward. The net force is determined by the relative sizes of these two forces.If the ground pushes harder on the horse than the wagon pulls, there is a net force in the forward direction, and the horse accelerates forward.If the wagon pulls harder on the horse t
27、han the ground pushes, there is a net force in the backward direction, and the horse accelerates backward. (This wouldnt happen on level ground, but it could happen on a hill.)If the force that the wagon exerts on the horse is the same size as the force that the ground exerts, the net force on the h
28、orse is zero, and the horse does not accelerate.In any case, the acceleration of the horse equals the net force on the horse divided by the horses mass (Newtons Second Law).Why does the ground push on the horse, anyway?The force “ground pushes horse“ is the Newtons Third Law reaction force to “horse
29、 pushes ground“. These 2 forces are exactly the same size. If the horse wants the ground to push him forward, he just needs to push backwards on the ground.These two forces do not cancel because they act on different objects. The force “ground pushes horse“ tends to accelerate the horse, and the for
30、ce “horse pushes ground“ tends to accelerate the ground.What about the ground?Looking at the force diagram at the top of the page, you see that there is one horizontal force pushing on the ground - the horse pushes on the ground. Therefore, there is a net force on the ground, so the ground should ac
31、celerate. Does it?Of course it does! However the amount of acceleration equals the size of the net force divided by the mass of the Earth - and the mass of the earth is about 6 x 1024 kg. This means that the acceleration of the ground is much, much too small to notice.Summary:So, it is possible for
32、horses to pull wagons! It is true that the force that the horse exerts on the wagon is the same size as the force that the wagon exerts on the horse, but these forces do not combine to produce a zero net force. The force exerted on the wagon (by the horse) affects the motion of the wagon, and the fo
33、rce exerted on the horse affects the motion of the horse. HOW THE HORSE AND WAGON WORKS WITH FRICTIONThe Forces:Compared to the previous diagram, you can see that two new forces have been added to the diagram at the right. The friction force acting on the wagon (colored red) tries to oppose the moti
34、on of the wagon. It is exerted by the ground. Its Newtons Third Law force partner is the force “wagon pushes ground“. Note that the force pushing the wagon is drawn on the wagon, and the force pushing the ground is drawn on the ground.As always, these two forces dont cancel because they act on diffe
35、rent objects.Why does the wagon accelerate?Consulting the diagram, notice that there are now two forces acting on the wagon. The net force on the wagon equals the force the horse exerts minus the friction force the ground exerts. If the horse pulls harder on the wagon than the friction force, there
36、will be a forward-pointing net force, and the wagon will accelerate forward. If the pull of the horse exactly balances the friction force, then the net force on the wagon will be zero, and the wagon will not accelerate. (This is the situation when the horse is pulling the wagon at constant velocity.
37、)Why does the horse accelerate?The situation for the horse is the same as in the previous (no friction) situation.Does the ground accelerate?There are now 2 forces acting on the ground - the horse pushes it backwards and the wagon pushes it forward. The net force on the ground equals the force that
38、the horse exerts on the ground minus the force that the ground exerts on it. If the horse pushes harder, there will be a backward net force on the ground. If the wagon pushes harder, there will be a forward net force on the ground. If they push equally, the net force on the ground will be zero. In a
39、ny case, the acceleration of the ground will not be noticeable, due to the enormous mass of the earth. EXAMPLE: TUG OF WARAn elephant and a teenager are having a tug of war. Does Newtons third law imply a draw?Answer: No, unless the elephant is very weak, the teenager will definitely lose. It is tru
40、e that the force the elephant exerts on the teenager, Fet is equal and opposite to the force the teenager exerts on the elephant, Fte, but the motion of either “object” depends on the resultant of all the forces acting on it. Both the teenager and the elephant are pushing the ground forward with the
41、ir feet and in each case the ground exerts an opposite reaction force. Diagram shows where Fgt and Fge represent the horizontal forces exerted by the ground on the teenager and on the elephant, respectively. Thus, for example, suppose that Fet = Fte = 250 N. We might have Fgt = 100 N and Fge = 650 N
42、. Then a net force of 150 N acts on the teenager to the left and he moves leftward. Similarly, a net force of 400 N acts on the elephant to the left and the elephant moves leftward. NEWTONS LESSON 14 HOMEWORK DO CONCEPTUAL PHYSICS WORKSHEET p 12.1. In the top picture (below), Kent Budgett is pulling
43、 upon a rope which is attached to a wall. In the bottom picture, the Kent is pulling upon a rope which is attached to an elephant. In each case, the force scale reads 500 Newtons. Kent is pulling .a. with more force when the rope is attached to the wall. b. with more force when the rope is attached
44、to the elephant.c. the same force in each case.2. Two people pull on a rope in a tug-of-war. Each pulls with 400 N of force. What is the tension in the rope?a) Zerob) 400 Nc) 600 Nd) 800 Ne) none of the above3. As a ball falls, the action force is the pull of the earths mass on the ball. What is the
45、 reactionto this force?a) Air resistance acting against the ballb) The acceleration of the ballc) The pull of the balls mass on the earthd) Nonexistent in this casee) none of the above4. A person is attracted towards the center of the earth by a 500-N gravitational force. The forcewith which the ear
46、th is attracted toward the person isa) very very small.b) very very large.c) 500 N.5. An unfortunate bug splatters against the windshield of a moving car. Compared to the forceof the car on the bug, the force of the bug on the car isa) larger.b) smaller.c) the same.d) Need more information to say6.
47、An unfortunate bug splatters against the windshield of a moving car. Compared to thedeceleration of the car, the deceleration of the bug isa) larger.b) smaller.c) the same.7. If a horse pulls on a wagon at rest, the wagon pulls back equally as much on the horse. Willthe wagon be set into motion?a) N
48、o, because the forces cancel each other.b) Yes, because there is a net force acting on the wagon.c) Yes, because there is a time delay between action and reaction.d) Yes - The horses pull on the wagon is larger than the wagons pull on the horse.8. A Mack truck and a Volkswagen traveling at the same
49、speed have a head-on collision. Thevehicle to undergo the greater change in velocity will be thea) Volkswagen.b) Mack truck.c) Both the same9. According to Newtons third law, if you push gently on something, it will pusha) gently on you.b) gently on something else.c) on something only under the right conditions.d) on you only if you arent moving.10. The earth pulls on the moon, and similarly the moon pulls on the earth. This is evidence thatthea) earth and moon are simply pulling on each other.b) earths and moons pulls comprise
