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il0seonpurpose
Korea (South)5638 Posts
Also, there's this little project where we get pictures and identify the laws being portrayed. I have a picture of a basketball player shooting, it's the second law but I don't know how to explain it well. I also have a picture of a baseball player and I have the same problem. Thanks  | ||
micronesia
United States24670 Posts
What does this mean? For every force, there is an equal and opposite reaction force. It's best to see this with examples, I think. Example 1: You push on a wall with your hands. You apply a 100 Newton force. What happens? You fall backwards. Why? Because the reaction force (the wall pushing back on you) was 100 N which accelerated you backwards. Example 2: The earth pulls you down with a force of gravity equal to 9.8 times your mass in kilograms. According to Newton's Third law, this means you pull the Earth back up with the same force. Is this true? Well, yes it is. Even though your gravity field is much weaker, the earth has such a huge mass that it experiences a force towards you equal to your weight. However, the Earth is so massive that it shrugs off the ~500 Newton force and doesn't move. edit: for the basketball player, I guess you should consider that while your hand is pushing the ball, you are applying a force, so the ball accelerates (up until the moment the ball isn't in contact with the hand anymore) | ||
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Nitrogen
United States5345 Posts
On December 10 2008 12:22 il0seonpurpose wrote: I guess to most people on teamliquid, this is really basic physics, I'm taking it in high school because its required and its basically just advanced. But I don't get the third law, where for every action there is an equal and opposite reaction. The problem I have understanding is that we learned the second law of F=ma and other forces, learning about free body diagrams; how Fa (applied) and Ff(friction) are opposites and they don't always equal each other out. This paper I'm reading off of says "Both forces are exactly the same size. They are equal in magnitude." Also, there's this little project where we get pictures and identify the laws being portrayed. I have a picture of a basketball player shooting, it's the second law but I don't know how to explain it well. I also have a picture of a baseball player and I have the same problem. Thanks i think the paper means before the object is put into motion, if you push on an object the static friction will be equal to the force applied until you go over the coefficient of static friction multiplied by the normal force. and the basketball one is probably just how when he throws the basketball he receives the same force as the basketball, but his mass is greater so he gets less of an acceleration. hope that helps | ||
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Abydos1
United States832 Posts
Edit: as nitrogen pointed out if an object is stationary force applied equals the force from friction since the object isn't moving; otherwise the difference causes an acceleration on the object. Hope that helps... | ||
OneOther
United States10774 Posts
micronesia pretty much covered the third law. As for the picture of a basketball player shooting, is it referring to the ball or the player? Either way there are a lot of things you could talk about. You apply a force to the ball, so it starts moving. You can also talk about the player jumping vs F=mg. There are tons of other stuff as well | ||
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micronesia
United States24670 Posts
On December 10 2008 12:42 Abydos1 wrote: Friction and force applied are not an action/reaction pair. If you use that as an example, say a car wheel's friction on the ground, the action/reaction pair is the force of friction on the car wheel and the force of friction on the road(earth). Think of a spaceship, the spaceship burns fuel to propel itself forward, the fuel is expelled backwards with the same force in the opposite direction. Edit: as nitrogen pointed out if an object is stationary force applied equals the force from friction since the object isn't moving; otherwise the difference causes an acceleration on the object. Hope that helps... 100% accurate but you might want to say tire instead of wheel :p | ||
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Abydos1
United States832 Posts
On December 10 2008 12:45 micronesia wrote: 100% accurate but you might want to say tire instead of wheel :p Fine fine, changed it to tire to avoid ambiguity  | ||
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Kentor
United States5784 Posts
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HeavOnEarth
United States7087 Posts
;p | ||
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eth0x
Canada208 Posts
Forces always act in pairs by the way. | ||
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HeavOnEarth
United States7087 Posts
For freebody diagrams just label everything, draw a picture , then calculate your forces Some general equations are Fn*u(coeffcient of friction) = Ff(Force of friction, acts against Applied force or Fa) And Applied force - force of Friction = MA. Sometimes you aren't given Fn, and you have to know for Newton's third law, (MG) force due to gravity = Fn, for example if you are 100 Kg standing still, Fn = 100Kg*9.81m/s^2 = Newton (kg*m/s^2); because you push on the floor with that, it pushes back Fn= perpendicular force acting against an object w/ wall, etc. For example if you push horizontally with a force of 150N against 10Kg box and it slides across a surface with a coefficient of friction of .5, what's the acceleration? Fn= Mg ( g = 9.81 gravity ) Fn*.5 = Ff 150N - Ff = MA MA/ 10Kg = A + Show Spoiler + you should get about 10m/s for acclerationi think xd | ||
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thoraxe
United States1449 Posts
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Abydos1
United States832 Posts
On December 10 2008 13:34 thoraxe wrote: Its pretty weird how a wall can push back at you. Why do you think it hurts when you punch one? | ||
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Faronel
United States658 Posts
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il0seonpurpose
Korea (South)5638 Posts
On December 10 2008 14:06 Faronel wrote: I imagine micronesia as some sort of cape wearing hero in a physics classroom. Once the Teamliquid Forum Physics Thread Detector goes off, he immediately comes and resolves the topic. LOL Ok thanks guys, I think I got it | ||
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Tynuji
127 Posts
Also, from what I've seen, a mastery of equation manipulation will really help you in short answer questions. Meaning, manipulating the equations before substituting numbers in. You also cannot push a rope. | ||
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OmgIRok
Taiwan2699 Posts
EX: A-Rod is up to bat, the pitcher throws a fastball and A-Rod swings. His lower body will turn the opposite direction of the upper body swing, because of Newton's 3rd Law (Action and Reaction) Let's say that he wasn't standing on the dirt (where there is friction, so the body turning the opposite direction is very minor), and he was standing on a rotating plate (like the ones when you go out to eat at a restaurant, you can spin around to pass food from one person to the next). There is less friction on the plate, so as A-Rod swings, his upper body turns counter-clock wise (he's a right handed person). ![[image loading]](http://i.tsn.com/i/photos/20080618/91496.jpg) Back to the body turning counter-clock wise. Now in order for his body to swing with that force and turn that direction, he is pushing his feet into the ground at an opposite direction as the swing. Now let's imagine that instead of standing in an area with high friction, he his now standing, ready to bat on the plate. As he try to apply force in his feet for the out-of-the-park Grand Slam, it would cause the plate to spin, taking his feet along with it, causing his feet to go in the opposite direction of A-Rod's swinging direction. ![[image loading]](http://imagecache2.allposters.com/images/pic/PHO/AAFV021~Ted-Williams-Batting-sepia-Posters.jpg) Here we see Ted Williams swinging. He is a left-handed batter so we'll have to look at it from a left handed players POV. He swings clockwise, but take a look at the right foot, it is pushing in the counter-clockwise direction. Hope this helps with your Baseball picture understanding. =] I learned all this in 5th grade I can't believe I haven't forgotten. | ||
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Tynuji
127 Posts
Another good example of Newton's third law is being hit by a car. This really expresses the point that the force acts on DIFFERENT objects. That is, you have two equal but opposite forces, a lot of people (beginners) may say, doesn't that mean the net force is zero? That's incorrect because the action/reaction pair apply to different objects. So when a man is hit by a car, the reason he goes flying while the car barely moves is because of combining Newton's second law with that law. You learned F = ma. Since you know that the force they experience will have equal magnitudes but opposite directions. So you have an average joe that I'll weigh as about 75kg. A car we'll make 1000kg. So if the car hits the man with a let's say 1000 N force, the man will also apply a force back with 1000 N. However, the acceleration of both objects will be different when you solve for acceleration for both objects because of the differences in their masses. So for the guy, he would be accelerating at F/m = a. 1000N/75kg = 13.333 m/s^2 (remember Newtons is kg*m/s^2 I think). The car however would be accelerating 1000N/1000kg = 1 m/s^2. You may also need to learn some acceleration constraints. Such as when an object on a frictionless surface is has a rope on a massless pulley with another block hanging from the edge. I never took high school physics but the university course (which first year is apparently review, damn you philosophy) makes use of a buttload of word problem analysis. So just knowing the concepts aren't enough, but applying them to solve problems is very important. | ||
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