Chuck Norris Posted September 17, 2009 Share Posted September 17, 2009 I just can't seem to get my head around Newton's third law of translation. The one about every force having a equal and opposite reaction force. If you push a door with 10N and the reaction force is 10N in the opposite direction of the initial force surely the forces cancel out and the door doesn't move at all. Isn't this the same as if two equal forces are supplied on either side of a moveable object in opposite directions the object doesn't move. Obviously there is a huge missing link that I'm too stupid to see. Link to comment Share on other sites More sharing options...
ajb Posted September 17, 2009 Share Posted September 17, 2009 You need to think carefully about what the forces act on. Link to comment Share on other sites More sharing options...
swansont Posted September 17, 2009 Share Posted September 17, 2009 Obviously there is a huge missing link that I'm too stupid to see. No, it's more like you've hit the conceptual speed-bump that trips up many beginning physics students. Think about ajb's post very carefully. Link to comment Share on other sites More sharing options...
baxtonduglonn Posted September 17, 2009 Share Posted September 17, 2009 If I am a 100kg person and the door is 10kg, then the door experiences 10 times the acceleration that I do (F=ma). I also have friction between my shoes and the floor while the door is on hinges, which are (hopefully) free to move. So the door moves and I do not but we both experience the force (not "the Force", Luke) of 10 N. Link to comment Share on other sites More sharing options...
Externet Posted September 18, 2009 Share Posted September 18, 2009 If I am a 100kg person and the door is 10kg, then the door experiences 10 times the acceleration that I do (F=ma). I also have friction between my shoes and the floor while the door is on hinges, which are (hopefully) free to move. So the door moves and I do not but we both experience the force (not "the Force", Luke) of 10 N. Whaaaat ??? Link to comment Share on other sites More sharing options...
swansont Posted September 18, 2009 Share Posted September 18, 2009 Whaaaat ??? Could you please rephrase the objection? There's nothing wrong with what baxtonduglonn said. Link to comment Share on other sites More sharing options...
Edtharan Posted September 18, 2009 Share Posted September 18, 2009 When you push on the door, the door pushes on you. If you were in space, this would cause the door to move away form you (as you pushed on the door) and as the door pushed back on you, you would move away form the door. If you take the average positions, (and accounting for F=MA) then you will find that the centre of mass has not changed. So in effect, the You/Door system has not actually moved its centre of mass. When you are on Earth, you have friction between your shoes/feet and the floor. When you push on a door, it does push back on you, but that is transferred down your legs to your feet/shoes and the friction between your shoes and the floor convert this force into heat energy. This action/reaction is the basis of rockets and jet engines. They heat up a material into a gas and cause it to expand very quickly. They then funnel this expanding gas into moving in one direction. By forcing it to move in one direction you have to push on it (well it pushes on the engine and that causes the engine to push back on it). The force that the gas exerts on the engine causes the engine to move forwards. The force the engine exerts on the gas causes it to move backwards. Link to comment Share on other sites More sharing options...
Sisyphus Posted September 18, 2009 Share Posted September 18, 2009 When you are on Earth, you have friction between your shoes/feet and the floor. When you push on a door, it does push back on you, but that is transferred down your legs to your feet/shoes and the friction between your shoes and the floor convert this force into heat energy. I don't think this is quite right. If your feet aren't moving, the force isn't being converted to heat energy. It's being transferred to the ground, essentially just making the system bigger. Instead of door-you maintaining the same center of mass, it's door-Earth. Link to comment Share on other sites More sharing options...
Chuck Norris Posted September 19, 2009 Author Share Posted September 19, 2009 I think I get it. The force of the door on you doesn't move you as you are too heavy, and also 'cause of the friction from your feet. I thought about what the reaction force acted upon and it was clear. The door moves because there is no force to counteract or negate the pushing force that you exert. Thanks. Link to comment Share on other sites More sharing options...
Edtharan Posted September 19, 2009 Share Posted September 19, 2009 I don't think this is quite right. If your feet aren't moving, the force isn't being converted to heat energy. It's being transferred to the ground, essentially just making the system bigger. Instead of door-you maintaining the same center of mass, it's door-Earth. The kinetic energy has to be dissipated somehow, and the usual way is to to be converted into heat energy. Maybe it isn't a direct conversion to heat that you get in dynamic friction, but it will cause vibration of molecules (of the floor under your feet) and this can be considered as heat. I think I get it. The force of the door on you doesn't move you as you are too heavy, and also 'cause of the friction from your feet. I thought about what the reaction force acted upon and it was clear. The door moves because there is no force to counteract or negate the pushing force that you exert. Thanks. No matter how big your mass, you will still move. It is only because of friction that you don't move. In space (because there is no friction there), even if I push on a million ton mass, it will still move (not at a great speed, but it will still move). Link to comment Share on other sites More sharing options...
swansont Posted September 19, 2009 Share Posted September 19, 2009 In space (because there is no friction there), even if I push on a million ton mass, it will still move (not at a great speed, but it will still move). A concept for Chuck Norris to understand is that even though the forces are equal in magnitude, they act on different objects. A force acts on you, and you accelerate. The reaction force acts on the other object, and it accelerates. Link to comment Share on other sites More sharing options...
Sisyphus Posted September 19, 2009 Share Posted September 19, 2009 The kinetic energy has to be dissipated somehow, and the usual way is to to be converted into heat energy. Maybe it isn't a direct conversion to heat that you get in dynamic friction, but it will cause vibration of molecules (of the floor under your feet) and this can be considered as heat. Surely the kinetic energy would be accelerating the Earth (or parts of it, since it isn't perfectly rigid). Otherwise you're violating conservation of momentum. Unless by "vibration of molecules" you mean a shock wave? You don't build up heat with static friction, though. Link to comment Share on other sites More sharing options...
Fuzzwood Posted September 19, 2009 Share Posted September 19, 2009 Try to open a door when you are standing on skates on very slippery ice. You won't be able to, because you cannot maintain a foothold. Link to comment Share on other sites More sharing options...
forufes Posted September 30, 2009 Share Posted September 30, 2009 the forces are the same but the displacement isn't.. Link to comment Share on other sites More sharing options...
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