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I How to perceive a force in introductory context

  1. May 6, 2016 #1
    *How to perceive the duration of a force

    I am wondering how should I perceive the occurence of a force, at least while reading an introductory university physic textbook. When I try to think about basic vacuum environment examples of an object receiving a push, I always arrive at the dilema of visualizing how long did that push last. Was it instantaneous? Did it last half a second? A few seconds? Does it even matter? Knowing the magnitude of the force, the mass of the object, and the acceleration it underwent doesn't imply how long the force was applied for.
  2. jcsd
  3. May 6, 2016 #2


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    If you start with Momentum - one of the really fundamental quantities in Physics then you can relate that to an applied force.
    The change in momentum is equal to the force times the time it's applied for. A force, applied for zero time will have zero effect (no momentum change).
    We so often get asked the question "What is the force of impact when two cars, ships, bananas collide or someone punches someone on the jaw?" The answer is indeterminate without knowing more about the situation - either the distance (or the time) that the force was applied over.
    So you are right to ask that question about the relevance of Forces to motion.
  4. May 6, 2016 #3
    Ok I think that helps a lot.

    Now that I've seen that though, do you even require the concept of momentum though? If you rearrange the formula F=ma into F=m(V1 - V0)/t, it shows itself that for a given object mass and change in velocity, time is another variable that dictates the magnitude of the force.

    In other words, for a given object mass, and a desired change in velocity, it is not a force per-se, that will accomplish it, but a set of forces with varying strengths that can accomplish it - where their magnitudes vary as the time of their application changes. Is that right?

    Moreover, if it was a known magnitude of force, a known change in velocity, and mass, then we would know that it could have been applied for only one length of time...
    Last edited: May 6, 2016
  5. May 6, 2016 #4
    No, for example your avatar!
    The engine thrust is the only force involved in accelerating the plane, (in level flight).
    OK, as it accelerates the engine thrust will be countered by air resistance, but that's just a facet of this particular situation.
    A rocket in space will accelerate continually as long as the engine is providing thrust, no other force is involved.
  6. May 6, 2016 #5
    Hmm not sure what you mean.

    *Edit - because I understand that the rocket in space would accelerate only for as long as the engine is providing propulsive force. Once the engine stops, the rocket remains at its last speed.
  7. May 6, 2016 #6
    I was addressing your notion that multiple forces need to be applied to an object to get a change in velocity.
    One force applied for 'x' amount of time (where x > 0) is sufficient.

    Per your update:
    Yes that's right, it will accelerate while the engine (a single force) provides thrust, when the engine stops it will 'coast' at it's final velocity.
    (we can ignore gravity and other forces just to understand that basic idea)
    Last edited: May 6, 2016
  8. May 6, 2016 #7
    Maybe I wrote it unclearly, but I don't think I had a problem understanding how many forces are needed to change object's velocity.

    I meant that to change an object's velocity from say 0m/s to 10m/s, varying strengths of forces can be used, each with their own required duration though. Right?

    F = m(V1-Vo)/t ?
  9. May 6, 2016 #8
    Well you could of course have an engine which provides varying amount of thrust over time.
    Depends on what final trajectory you want, but the engine is still is a single force.
    Making that force variable would require complex calculation for the expected result, but no additional forces are needed.
  10. May 6, 2016 #9


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    Correct. A force of 10 N will accelerate a 1 kg object to 10 m/s in 10 seconds. A 100 N force will accelerate the same object to 10 m/s in 1 second. Etc.
  11. May 6, 2016 #10
    Ugh Im kinda confused maybe by what you mean by single force, varying thrust/force. What I am saying, correct me if I'm wrong, is that in the vacuum environment used in school examples, if an object wanted to go from say 0m/s to 10m/s, it could "choose" from a selection of magnitudes of force to do it. Each of which will be applied for different amount of times.

    Ex: Mass of 5kg, wants to go from 0 to 10m/s.

    (1) Can use a 100N force, with .5 second application:
    5kg(10-0)/.5 = 100N

    (2) Can use 50N force, with 1 second application:
    5kg(10-0)/1 = 50N

    (3) So on and so forth
  12. May 6, 2016 #11
    Right, ok.

    I get hung up on this because sometimes reading some text, it will make it sound like there is one single magnitude of force that is able to move object of mass x, by y amount of velocity change..ex: "a force moves an object at rest to a velocity of..." Seeing how that can't be the case, I wanted to investing the length of time that forces are applied etc...

    Thanks everyone for your input :smile:
  13. May 10, 2016 #12


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    Your reactions to all this is interesting because it is a very concrete area of Physics and most people don't actually question it the way you are doing. The 'hang ups' usually start when they get to Electricity and QM. But , fair enough and your questions are perfectly valid.
    It seems to me that you want some fundamental truth to apply here when what Mechanics is all about is to find a way of expressing measured relationships in a simple set of equations. There is a great difference between those two.
    Scientists mostly shy away from 'truth' in favour of 'working models' and that's what all your basic Physics will give you. You can use Mass as a concept without having to go into Higgs Bosons. Likewise for Distance, Work, Momentum and Vectors etc etc. There are no answers to any 'Why" questions in Newtonian Mechanics - just a set of methods to allow you to predict what will happen (to an extremely high degree of accuracy in nearly every case, aamof).
    Getting very familiar with (and frequently using) the equations is a good way forward and will help to bring those 'hang ups' into the realm of acceptance. There is no end to the process of taking it further.
    I watch my young granddaughter as she 'learns' things. She just repeats and repeats puzzles and language until she can 'do it'. Not a hint of worry about a deeper meaning. That's the way we all learn stuff, initially and I don't think it's a cop out to use that method until we reach the real frontiers of Science. (Then most of us have to give up and accept what 'they' tell us, in any case).
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