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Homework Help: A level questions help

  1. Jun 7, 2010 #1
    I'd like to know why a young modulus for a material is constant whereas the spring constant isn't.Is it because the spring constant depends on the extension which depends on the length and area of the wire?Just wanted to make sure :)

    Also,I have these two questions I'm not getting the right answers for so please help !

    Q . A student standing on a platform at a railway station notices that the first two carraiges of an arriving train pass her in 2 s and the next two in 2.4s.The train is decelerating uniformly.Each carraige is 20m long.When the train stops,the student is opposite the last carraige.How many carraiges are there in the train?
    The correct answer is eight but I keep getting a wrong answer.I tried finding out the acceleration by finding out the two velocities at 2 and 2.4s and then using v^2=u^2+2as but don't get the right answer.

    Q .Two equations for power dissipated in a resistor are P=I^2R and P=V^2/R.The first suggests that the greater the resistance R of the resistor,the more power is disspated.The second siggests the opposite,the greater the resistance,the less the power.Explain this inconsistency

    Thank you!
  2. jcsd
  3. Jun 16, 2010 #2
    1. I've not studied equilibrium extensively but Young's modulus isn't always constant.

    2. The formula you used expresses an instantaneous velocity, whereas the student's observations express an average velocity. You need to somehow describe the total distance traveled and time, and the acceleration.

    3. If you change the resistance only, then you must assume that all other factors are being held constant. For example, the temperature of the circuit, the gravitational forces on it, the current going through it, etc. must all be constant. Consider how a change in resistance would affect the voltage if all other factors must be held constant.
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