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Confused about Newton's third law again?(Doc Al and others help)

  1. May 28, 2010 #1
    Hello everyone,

    Ok I'm no longer worried about why Newton's third law is occuring. Doc Al advised me in the last thread to look at some action reaction pairs instead, and I think that is what I needed to do. Ok I'm having difficulty understanding newton's third law when two objects are always in contact with each other.

    Eg: There is an elastic spring attached to a wall and I try to compress it using my hand. My hand is always in contact with the spring.

    Now my hand applies a force of let's say 10 N on the spring. Then spring applies the same force on me. Now these are the questions I have.

    1.Why am I not moving back due to springs reaction force? Is it because due to friction of the ground, the net force acting on me is 0.
    2.Now after I compress the spring for a bit, I can no longer compress? This means that my 10 N force must be balanced? If this is due to elasticity of the spring, does it build up as I push further.
    3. Now this is my most important question. If the spring didn't have an elastic property does that mean, I would be able to compress it right up to the wall, with no problem. So lets say I was only able to compress a spring halfway with a 10 N force, then I suddenly supplied a bigger force and it caused the spring to lose its elasticity, does this mean that from now on the resistance force(elastic force is zero).

    Thank you very much :smile:
    Last edited: May 28, 2010
  2. jcsd
  3. May 28, 2010 #2
    1. Imagine performing the same experiment on an ice rink. The spring will compress until the force it exerts on you (and you on it!) overcomes the friction, and you will slide backwards.

    2. Springs with a given constant k require force equal to k * the distance you compress them. When you can no longer compress the spring, this just means you cannot exert enough of a force - aren't strong enough!

    3. If you snap the spring when it is compressed, which is what i think you are referring to, There is no longer a force on you, since you are no longer applying a force to the spring.
  4. May 28, 2010 #3
    I'm a bit confused on Newton's 3rd law as well.

    Let's say I have two wooden blocks one touching the other and I apply 10N of force on one of the blocks such that it also pushes the 2nd block (me and the two blocks are all on a frictionless surface), would I accelerate backwards while the two blocks accelerate forwards?

    Also, if I have 2 blocks next to each other and we push one block with a force of 20N and the other with 30N with the forces directing in opposite directions, what is the force that the blocks apply on each other?

    Also, if there's always an action/reaction pair, how can there be net force at all? Does it only exist when we limit what objects we consider to be part of the system?
  5. May 28, 2010 #4
    Yes, you would accelerate backwards. At some point, you won't be able to reach. (imagine doing the experiment while you're on a skateboard)

    The next question I'm not sure of - but if the blocks are incompressible, you can work some stuff out with pressure. Pressure should be transmitted perfectly so the force should "come out the other side" (and also imagine doing the experiment with a bigger block instead - the end result should be the same with regards to the motion of the blocks).
    Intuition might be thwarted by friction, so always imagine doing things on ice or skateboards.

    In a N3 pair, the opposite forces always act on opposing bodies. So they can't cancel.
    If you add up all forces on all bodies though, yes, they go to zero.
  6. May 28, 2010 #5

    Doc Al

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    Staff: Mentor

    Whether you move or not depends on the net force on you. If the net force is zero, your center of mass will not accelerate. Yes, probably friction is exerting an equal force on you, balancing out the force from the spring.
    I don't understand the question. I thought you are pushing with 10 N of force? That will compress the spring a certain amount. You can certainly press harder if you want, but as long as you keep pushing with 10 N of force nothing 'builds up'.
    I'm not sure what you mean by not having an elastic property, so you might have to ask your question again in other words. If you push on something harder than it can support, then it will not be able to sustain a force (and you will not be able to continue exerting the force). Compressing a spring isn't a good example, since even if you compress it hard enough to destroy its ability to uncompress once you release the force, you're just crushing it. Most things will allow you to crush them with whatever force you like (up to some extreme limit, I suppose).

    A slightly better example: Push against the wall, it pushes back; push too hard, it collapses and you go flying through it.
  7. May 28, 2010 #6

    You are not moving backwards because there is no net backwards force there.
    Guess what happens if the spring and wall disappeared? You would fall flat on you face because you had to unbalance yourself to apply a force to the spring.

    Yes the force in the spring is proportional to it's extension, it's called Hooks Law.

    If the spring fails as you describe then yes the force effectively disappears and you will fall against the wall and apply a force to the wall.
  8. May 28, 2010 #7

    Doc Al

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    Note that since you are exerting a net force on the two blocks, they will accelerate. The blocks will exert a force of 25N on each other.

    Action/reaction force pairs are always exerted on different bodies. Looking at each body separately, those force never cancel each other.
    If two parts of a system exert equal and opposite forces on each other, then those forces do not contribute to the net force on the system as a whole. (They are examples of internal forces.)
  9. May 29, 2010 #8
    Ok thanks for all the replies. Yes I think I wasn't clear in this one. What I mean is let's say there is a tripod. I attach a string to the tripod and then attach a mass to the string. The string with the mass ,stretches a bit and then the mass hangs in the air. So this means the weight force of the mass must be balanced by the tension of the string. Now let's say this tension is coming from the elasticity of the string. Now after the mass is hung, I take it off and look at the string, now it is stretched and loss its elasticty. So if I attach this string to the tripod again, and hang the mass, and it stretches a bit and stays stationary, where is the tension of the string coming from. It can't come from the string because it has lost its elasticity.

    I don't understand how you worked out 25N. Isn't this a momentum collision, so if you don't know about how long the collision lasted how would you know the force.
  10. May 29, 2010 #9

    Doc Al

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    Elasticity, in the sense you mean, is just the property of a body to return to its original shape once the stress (the added weight) is removed. It's not the source of tension.
    Again, you are misusing the concept of 'elasticity'. If the string could support a given weight one moment, why do you think it couldn't support it the next? (At some point you'll reach the breaking strength of the material. Then it will break.)

    Why do you keep bringing up collisions? To do the calculation, I presumed that a steady state has been achieved. (As opposed to applying the given forces for say a billionth of a second then releasing them.) You have the applied forces, so you can find the acceleration and thus the net force on each block. Use Newton's 2nd law. That's all you need.

    None of this has much to do with Newton's 3rd law.
  11. May 29, 2010 #10
    Ok you are correct. I didn't read the question properly.

    Now my question is simply what is the reason behind tension force. I know air resistance is caused by air but what causes tension? Also if I hang a mass to a string, and then it stretches a bit and then forces balance remains stationary. I hang a bigger mass and it stretches further and forces balance remain stationary. Why did it stretch further in the 2 nd situation for the forces to balance. Thanks :smile:
  12. May 30, 2010 #11

    Doc Al

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    Think of the molecules in a solid. If they are pushed too close together, they repel each other; if pulled apart, they attract. When a pull is applied to each end of an object, the molecules separate a bit until the attractive force balances the applied force.

    For the same reason it stretched the first time! Applying additional force pulls the molecules apart until their attractive force balances the applied force.
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