1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Hi guys I hope with your help I will be successful

  1. Oct 1, 2011 #1
    Hi guys. I'd like to make an introduction somewhere but there doesn't seem to be a forum for that here. Basically Im an older student who, despite having low grades all throughout school, always loved science. I decided to go back to adult school to upgrade my marks and then go to university for geology. I'd like to discuss geology and career paths but I will do that on the other topics on this forum.
    So far Im doing alright in grade 12 physics. I've jumped into to grade 12 physics without grade 11 and its definitely harder than I thought, but of course it all depends on how much work you put into it. Everything is in the book and I've had no problem learning from that. Right now were on chapter 5 which is about the law of conservation of energy, keplers laws, planetary motion, stuff like that. I have no problems so far but when I do run into problems Il ask here.

    I do however need help with communication. I have 4 questions from my last test that just need an explanation but I only got half marks for them. My teacher said that if I can give her a better explanation for these question, she will give me these marks back, plus an extra 10 marks for having corrected everything on that test. (shes really nice like that haha)
    Il start with the first question
    Define "internal force" and "external force" and explain how these terms relate to the law of conservation of momentum.

    My answer: Internal forces are all the forces contained in a system where external forces cannot affect them. External forces are all the forces outside of a system. These forces relate to the law of conservation of momentum because if a system is not isolated and external forces can affect the internal forces of that system, they will affect the momentum of that system as well.

    I got only half marks for that. I guess shes looking for some algebra to prove it maybe, or in better words? Im not sure.

    Any advice/help on this this would be greatly appreciated.
  2. jcsd
  3. Oct 1, 2011 #2
    I believe you have neglected to mention that the momentum must be conserved in the internal system as well, this and your final statement may be a bit confusing. Try re writing this in bullet points.
  4. Oct 2, 2011 #3
    Ya Il have to rewrite it better I guess. Shes tough when it comes to explanations. You have to explain it as a physicist would you know. Im looking at my answers and they seem to make perfect sence. For example one of the questions is blowing air into a sail.. would it work or not and explain.

    I wrote: This would not work because of recoil. The fan attached to the boat is pushing air in one direction and thus giving the air momentum, therefore the boat will move in the opposite direction according to the law of conservation of momentum (if the water friction is negligible). For this to work, they would have to turn the fan around, facing the back of the boat and that will give it forward momentum.

    Is that not a perfect answer?
  5. Oct 2, 2011 #4
    Hi, well there is no such thing as a perfect answer really as it will depend on who you are addressing it two. And you should always consider that whatever work you do, it can always be improved that is just the nature of the world.

    While this answer is coherent, and logical I will try to suggest an alternative. Please remember this is only my opinion just to give you some ideas and in no way meant to be a perfect answer

    My version would read: For a fan attached to a boat, blowing into the sail, the system is closed. Therefore the overall momentum, of the system must be conserved. The boat will have zero acceleration, since there is no external forces, and hence there will be no change in motion due to the fan.
  6. Oct 2, 2011 #5
    True.. Thanks for the input man. Il keep your explanation in mind as I try to persuade her to give me the marks :D
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook