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Force, I just don't GET it ?

  1. Jun 19, 2009 #1
    Force, I just don't GET it ???

    If we neglect the force needed to accelerate the weights. It takes the same force to maintain a constant speed on the bar, no matter what the speed is. If the weight is 200 pounds, then it takes exactly 200 pounds to maintain a constant speed of 3 ft/sec or 1 inch/sec, and everything in between.

    The above seems to be true, but I just don’t GET it ???

    Ok Two rockets going up or down or horizontal, air resistance at a constant speed, both are going at 5omph, the behind shunts the front rocket, and accelerates it up to 60mph, then keeps it there at a constant 60mph for a day, if the behind rocket backs of any of its constant force in that day, the front rocket will/must slow down,
    Then accelerates it up to 70mph, and then keeps it there at a constant 70mph for another day. Then accelerates it up to 80mph, and then keeps it there at a constant 80mph for another day, and so on.

    Thus the behind rocket, uses more forces, and more constant forces, the faster it accelerates the front rocket, and the more forces the more it shunts at a constant speed ???

    Or what if the behind rocket just stopped pushing at any of the speeds, the behind rocket would immediately start to slow down.

    How can the behind rocket use the same force to keep the front rocket

    Or let us say we have a machine that can produce any accelerations and constant speeds, the machine needs to lift 200 pounds to 5omph and keep it at a constant 50mph for one day. Then accelerates it up to 60mph, and then keeps it there at a constant 60mph for another day. Then accelerates it up to 70mph, and then keeps it there at a constant 70mph for another day. Then accelerates it up to 80mph, and then keeps it there at a constant 80mph for another day, and so on.

    Thus the behind rocket/machine, uses more forces, and more constant forces, the faster it accelerates the front rocket, and the more constant forces the more it shunts at a different constant speeds ???

    Wayne
     
  2. jcsd
  3. Jun 19, 2009 #2
    Re: Force, I just don't GET it ???

    Speaking just to the first part of your question, what you're describing is the principle of inertia. Don't feel bad that you don't get it yet, it's very unintuitive because it's very alien to our common experiences. When we throw a baseball, eventually it comes to a stop. It's hard to imagine that it's natural for objects to keep moving when left to themselves.

    But that's what inertia is all about. An object at rest tends to remain at rest, and an object in motion tends to remain in motion, unless acted on by an unbalanced force. This counterintuitive idea was advanced by Alhazen, and then Galileo, and then enshrined by Newton as the first law of motion.

    The key here is that word "unbalanced." The force that's holding up the 200 pound object is exactly balanced against the gravitational force that's pulling it down. So, whatever the speed of the object, the key is that the weight is balanced by the lifting force of 200 pounds.
     
  4. Jun 20, 2009 #3
    Re: Force, I just don't GET it ???

    Hi and thx,

    However in my examples you are using different amounts of constant force to move objects at different speeds. So if the weight in my examples are 200 pounds, then it takes more/less force from the engines/machine to 200 pounds to maintain a constant speed of 3 ft/sec or 1 inch/sec, and everything in between.

    Did these great physicists take into account modern powered vehicles/machines ??? Saying that with tongue in cheek, ROL, but then again in their day there was no modern powered vehicles/powered.

    Wayne
     
  5. Jun 20, 2009 #4
    Re: Force, I just don't GET it ???

    I'm sorry, your post is very confusing to me. I can't understand what you're asking.

    An object in motion will remain at motion when there are no forces acting on it. Similarly, an object at rest will remain at rest as long as there are no forces acting on it.

    This seems counter intuitive, but only because in the real world there is always friction, a force!
    If you roll a ball on the ground, it's the friction force that makes it slow down and eventually stop. If there was no friction, there would be no (horizontal) forces, and it would never stop rolling!
     
  6. Jun 20, 2009 #5
    Re: Force, I just don't GET it ???


    If we neglect the force needed to accelerate the weights or front rocket. It takes the same force to maintain a constant speed on the bar/weight, no matter what the speed is. If the weight is 200 pounds, then it takes exactly 200 pounds to maintain a constant speed of 3 ft/sec or 1 inch/sec, and everything in between.

    However I am talking about moving objects on this Planet with air resistance and gravity, and it seems to me that the law of inertia is wrong, by what I wrote on the rocket behind and machine which is using more forces, and more constant forces, the faster the behind rocket accelerates the front rocket, {same with the machine} the more forces and the more constant forces it uses to shunt and maintain the front rocket at different speed ???

    Let me rephrase the mahicne bit slightly at the end.

    A machine can produce any accelerations and constant speeds, the machine needs to lift 200 pounds to 5omph and keep it at a constant 50mph for one day. Then accelerates it up to 60mph, and then keeps it there at a constant 60mph for another day. Then accelerates it up to 70mph, and then keeps it there at a constant 70mph for another day. Then accelerates it up to 80mph, and then keeps it there at a constant 80mph for another day, and so on.

    How can the machine use the same amount of force after its accelerated the 200 pounds up to 50, 60, 70 and 80mph and keep the 200 pounds at those speeds for one day ??? As the 200 pounds needs force to keep it going up, as soon as its say up to 70mph for half a day, if the machine took off some force, the 200 pounds would just about immediately start to slow down, and then be moving at another constant speed but with less force used, not the same as what it says in the law of inertia.

    Wayne
     
  7. Jun 20, 2009 #6
    Re: Force, I just don't GET it ???

    Just did a little experiment, held a half pound weight in my palm and outstretched arm, and started turning around, the half pound weight stated in my palm, as I accelerated and then went around three times at a roughly constant speed.

    Then accelerated around and got to another constant speed, then went around three more times, at a faster constant speed.

    The second time I must surely have been using more force, as if you had got in the way the second time, you would have felt it far more than the first time.

    Wayne
     
  8. Jun 20, 2009 #7
    Re: Force, I just don't GET it ???

    I really can't understand your question, sorry :S

    What kind of machine are you talking about?

    To keep something moving at a constant speed, there must be NO forces acting on it (including frictional forces). So on earth, if you need to keep something moving at a constant speed, you must overcome the friction force. The sum of the friction force ("backwards") and the force you need to exert ("forwards") should be zero.

    Air friction is usually dependent on the velocity of the object, so if you move it faster, you need to exert more force on it to balance the friction and keep it moving at a constant speed.


    So if I understand you right, your machine should exert more force on the object when it is moving faster, because it has more air friction to overcome.

    If your machine is in space however, with no (or at least negligible) friction, it does not need to exert any force on the object to keep it moving. It has to exert a force at first, to accelerate it from rest to some constant speed, but as soon as the object is moving at that constant speed, you can release it and it will keep going forever, without the help of your machine at all.
     
  9. Jun 20, 2009 #8
    Re: Force, I just don't GET it ???

    OP can you please state what you are having trouble with in one simple sentence please?

    I attempted to answer earlier, but like Nick I didnt fully understand just what you were asking so I decided against it incase it confused the situation.
     
  10. Jun 20, 2009 #9

    HallsofIvy

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    Re: Force, I just don't GET it ???

    This is the first example that is causing everyone to ask you to clarify. What bar are you talking about? What "weights" are you talking about?

    A weight of 200 pounds would have a force (of gravity) on it of 200 pounds straight down. You would have to apply a force of 200 pounds straight up to offset that so there is no net force. Then there would be no acceleration and there would be a constant speed. On the other hand, if there were no gravity, such as far from any massive object such as a planet, you do not need to apply any force to maintain constant velocity. Or if your object is on a friction-free surface, so that the surface itself applies the upward force, you need apply no horizontal force to maintain a constant horizontal force.

    If it applies more force than the friction force, then the front rocket will accelerate according to "F= ma" where the "F" is the net force, the force from the rear rocket minus the friction force. That's the key: F= ma where F is the net force.

    Because you postulated a constant friction force.

     
  11. Jun 20, 2009 #10
    Re: Force, I just don't GET it ???

    Hi, Wayne. I think this is where we are going astray. It requires zero force -- no force at all -- for an object to move at constant speed. It's perfectly natural for us humans to think that forces are required to keep objects moving. This is exactly what Aristotle thought in his Physics, so you're in pretty good company. But, it's wrong.

    Things only change speed because a force acts on them. When a falling object accelerates towards the Earth, it's because of the gravitational force. When an arrow leaves its bow, the bowstring is no longer pushing on the arrow, but the arrow continues to fly away. The rolling bowling ball continues to roll, at least until friction grinds it to a halt. Because friction is everywhere in our surroundings, it's hard for our intuition to accept the principle of inertia, but it's really the way the world works.

    Imagine yourself jumping up off the ground. Your legs are no longer pushing you up into the air once your feet lose contact with the Earth, but you continue to move up anyway. (At least for those few seconds before gravity has pulled you back down.) The harder you jump, the better your hang time.

    Now imagine doing the same thing, but you're wearing a James Bond jetpack. The thrust is not dialed up all the way, but its set exactly to counter your weight and the weight of the jetpack itself. This time when you jump, you'll continue to move up at a constant speed (until you run out of fuel).

    Now here's the point: The speed at which you move up has nothing to do with the jetpack's thrust. It has everything to do with how hard your legs pushed off from the ground. Don't confuse the force required to counter gravity with the force needed to accelerate you in the first place.

    I hope this illustrates how a 200 pound weight can be moving at different speeds, even though the same 200 pounds of force is used to counteract gravity. It's a completely different force that accelerated the object to its speed in the first place.
     
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