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Beginner physics

  1. Oct 5, 2015 #1
    • Member warned about posting vague, ill-defined questions
    How do I know what formula to use?? I'm getting confused.
    Kinematics formulas
    Thank you
     
  2. jcsd
  3. Oct 5, 2015 #2

    SteamKing

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    We share your confusion.

    How do you know what formula to use for what problem? Not every formula is well suited to solving a given problem.
     
  4. Oct 5, 2015 #3

    CWatters

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    The equations of motion typically contain variables such as Initial velocity, final velocity, distance, acceleration, time etc. Perhaps start by reading the problem statement and writing down which are known/given and which are unknown/to be found.
     
  5. Oct 8, 2015 #4
    In your textbook, there will be a section, probably in the Introduction or Foreword to Students, describing how to solve problems. Read it carefully and completely. Looking for a formula which will solve your problem, whatever it is, is always a mistake, and is a common mistake made by beginner students.
     
  6. Oct 8, 2015 #5

    sophiecentaur

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    You write down the variable you want to know and the variables you have been given. You choose the formula which contains just those variables (and no others) and re-arrange it to get what you want.
    I would say that you have to know them all off by heart and not fool about, using the list that may be available on the dreaded 'formula sheet'. (There are not very many such formulae to learn in the whole of 'School" level Physics.)
     
  7. Oct 9, 2015 #6

    CWatters

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    It took me a while to understand how to solve Kinematics/Projectile problems. The trick is to realise that motion in the vertical and horizontal planes can usually be treated separately. eg you can write separate equations of motion in each plane.

    In the vertical plane you normally use one of the equations of motion for constant acceleration (google the SUVAT equations and memorise). Which SUVAT equation you use may depend on what "knowns" are given in the problem statement and what unknown you are asked to find.

    In the horizontal plane the velocity is frequently constant (assuming no air resistance). So in the horizontal plane equations like velocity = distance/time can be used.

    Then the trick is to realise that time is a common to both the vertical and horizontal motion. eg the vertical and horizontal motion must take the same time. After all it's the same projectile! So frequently you can substitute and eliminate T in the equations.

    Beware that sometimes there are two valid answers. For example there are two angles at which a projectile can be launched so that it hits the target.
     
  8. Oct 9, 2015 #7

    ogg

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    Imagine Physics is Magic, not Science. So, how would you "know" what SPELL or incantation to use? (assuming you knew several). The answer is obvious: you use the one which will give you the result you want.

    Just like physics: you use "the formula" which will give you the result you want. Now, this isn't really a very helpful answer, since the result you (probably) want is getting the problem "right" (getting 100% credit for the problem). But we can be a bit more subtle: getting 100% credit means answering the question that is posed (and probably showing your work).

    So, you work backwards, usually. You know (or figure out, somehow) what "kind" of answer you want, and figure out how to get it using one or more formulas. The most important "way" to categorize an answer (a numerical answer, I mean) is by its units of measurement. For instance, speeds can be miles per hour, meters per second or leagues per fortnight, what they all share in common is they are units of distance ÷ units of time.

    So, you need to be comfortable working with different units of measurement AND be familiar with various units. The SI system has units of distance, time, energy, power, voltage, and others as fundamental units with others often derived from them.

    Oh, I forgot mass and weight! I usually give the following example of working backwards: Jane runs at 5 ft per second, how far can she travel in 35 seconds? Now, working backwards what is the desired answer? It asks for a distance (and in this case the units of distance are feet). So, the answer will be X ft. Next is to figure out what we have (what we know) so far. We know the question involves 5 ft/s and 35 s.

    By inspection we see that multiplying 5 ft/s by 35 s will give us 175 ft.s/s and we should easily understand that s/s cancel so that 175 ft.s/s = 175 ft. In this case, we didn't use ANY formula. But the same considerations apply to deciding which formula to use, it's just that when you NEED to use a formula, you better be familiar with the UNITS of what you will get "out" of the formula (and what you "put in", too).

    The formula for voltage, current and resistance of a simple circuit is V = iR (or E=iR ...the symbols don't really matter...) That expression is only part of what you need to learn, in order to fully learn the "formula". The other part is you need to learn the UNITS of V, i and R. (as well as the systems of units, which means which units should "go together".) If you don't know the units, you really don't understand the formula.

    Once you know the units, its pretty easy to figure out whether you can or should use it to solve a given problem. Anyways, you've been warned about posting questions that are too vague to be clearly answered. What you need in order to apply a formula is understanding both the relationship of the variables of the problem AND the context of that problem.

    For instance you probably wouldn't get the right answer in applying a DC formula to a problem with AC current. For most beginning physics problems, the formula is really obvious, IF you know the units being asked for for the answer (and the units that the various formula use).
     
    Last edited by a moderator: Oct 9, 2015
  9. Oct 11, 2015 #8
    Wat is the formula for power
     
  10. Oct 11, 2015 #9

    sophiecentaur

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    Watt?
     
  11. Oct 11, 2015 #10
    First, look at the definition of power: It is the energy given (or work done) per unit time. As for a formula, it depends on what form of energy you are talking about. If the work is done on a particle by a force, then the power is force . velocity. This is a dot product. If it is electrical energy, in the form of a current driven by a battery, then thepower delivered by the battery is the battery voltage multiplied by the current ..........
     
  12. Oct 12, 2015 #11

    sophiecentaur

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    Yes. And look up and learn all the other definitions and equations. If you start with a head full of familiar equations and relationships then the feeling of understanding a new topic has a better chance of emerging. So many people seem to want the 'intuitive' approach and to avoid any maths except when absolutely necessary. Even the simplest bit of Physics can turn out to be very complicated and you need to keep many balls in the air at once. Having a load of formulae at your fingertips gives you a much better chance of appreciating the patterns involved in Science. Using maths when you can, helps to clear the decks of irrelevant clutter and to present the problem to yourself in a clear way.
    Strange how many people can remember the scores and the names involved in past sporting events (they learned them off by heart) and also the singers and words to numerous (sometimes nonsense) songs. Those same people say that they could never learn the SUVAT equations. It's all about motivation.
     
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