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The Conservation of Momentum - Physics 30

  1. Apr 16, 2008 #1
    I'm taking Physics for the first time in three years by correspondence... I'm struggling with a few questions, so any help would be greatly appreciated! I promise there isn't actually THAT many questions here. For the most part I have the answers to the parts a and b. Thanks in advance for any help!

    1. You head across the Battle River in your boat with a velocity of 8.0 m/s, south. The river is flowing at 2.8 m/s, east.
    a. Make a sketch indicating both velocity vectors. Show the resultant vector.
    I've done this and I'm faily certain what I have is correct.
    b. Determine the magnitude and direction of the resultant velocity vector.
    I got an answer of 8.5 m/s (which, again, I'm fairly certain is correct) what I don't get, is how to find the direction of the resultant velocity vector. I tried it and got a rather bizarre answer.
    c. Assume that you and your boat have a total mass of 192 kg. Calculate the magnitude and direction of the momentum of you and your boat as you cross the river.
    I calculated the momentum to be 1632 kg*m/s. Again, that seemed pretty straight forward to me. I'm lost on the direction again though. I believe it would be the same as the direction of the resultant vector?

    2. A railway car with a mass of 8.30 x 10^4 kg is moving west at 6.25 m/s when it collides with another car with a mass of 9.64 x 10^4 kg moving east at 7.00 m/s. The two freight cars join together at impact.
    a. What is the common velocity of the two cars after they join together?
    I got an answer of 0.870 m/s, East.
    b. Calculate the magnitude of the impulse provided to each freight car in this collision.
    I used mvf-mvi... I got the same answer of 5.91 x 10^5 for both cars... does that seem right?

    c. Now calculate the percent loss of kinetic energy.
    I used % Ek lost = ((deltaEk)/(Ekinitial))... I'm not sure if that's correct though.

    3. Sara's favourite game is marbles. Her favourite marble is a large steel marble with a mass of 32 g. She shoots it with a speed of 24 cm/s directly at a staionary, smaller glass marble with a mass of 15 g. After the collision, the glass marble moves ahead with a speed of 30 cm/s.
    a. Determine the resulting speed of the steel marble.
    I got an answer of 0.23 m/s. Again, I'm not positive that's correct.

    b. If this collision had been perfectly elastic, determine the resulting velocities of both the glass and the steel marbles.
    I don't really know how to do this. I understand that elastic means there isn't a change in the kinetic energy, but that's about all I understand.
  2. jcsd
  3. Apr 17, 2008 #2


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    For part 1 b) you will notice that all three vectors form a right angled triangle. All you need to do to do is find the angle of the resultant vector and state what that angle is from a fixed reference point (i.e. x degrees north of west for example). This uses simple trigonometry.

    Part 1 c). Yes, the direction will be the same as the velocity.

    Question 2 seems fine. Part c) is the fraction of kinetic energy lost, be sure to convert that to a percentage.

    I'd check part a) again. If you want post your working and be careful of the units. For part b) and elastic collision is one in which the kinetic energy is preserved and velocity has direction.
  4. Dec 23, 2009 #3
    If you're going to post another question, post it in a seperate thread don't just hijack somebody elses.
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