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!

Newtons law of motion type of question

  1. Apr 30, 2009 #1
    Two forces, of magnitude 10N and 4N, are applied to an object. The relative direction of the forces is unknown. The net force acting on the object __________.

    Check all that apply ?

    a) cannot be equal to 5N
    b)cannot be equal to 10N
    c)cannot be directed the same way as the force of 10N
    d) must be greater than 10N

    Thanks =]
  2. jcsd
  3. Apr 30, 2009 #2


    User Avatar
    Science Advisor
    Homework Helper

    Welcome to PF!

    Hi Kurwa ! Welcome to PF! :wink:

    Show us how far you get, and where you're stuck, and then we'll know how to help! :smile:
  4. Apr 30, 2009 #3
    Re: Welcome to PF!

    ok well first i drew a 2 different systems (dots)

    first dot- both forces to the right i.e 10N and 4N 0-----> 14N
    second dot- forces opposite of each other 10N <--0-> 4N = 6N <--

    now as you can see (a and b) are true

    c) must be true because the net force can act in the same way as the 10N was (second dot)

    d) which i totally dont understand because if the two forces are opposite of each other it holds false but if the two forces are in the same direction it holds true ??

    soo thats why im getting them wrong after 3 attempts ?
  5. Apr 30, 2009 #4
    Re: Welcome to PF!

    For d.) If the statement is false in one case and true in another, what does that tell you about the statement overall?

    for a and b.) Try breaking the 2 forces into x and y components (remember the forces dont have to be applied solely on the axes).
  6. May 1, 2009 #5
    Re: Welcome to PF!

    Hey NBA- I dont understand your post- ???
  7. May 1, 2009 #6


    User Avatar
    Science Advisor
    Homework Helper

    NBAJam100 :smile: means that you've assumed that the forces must be in the same or opposite directions (left or right), but in fact they can be at any angle to each other.

    (and your "c) must be true" contradicts your own reasoning :wink:)
  8. May 1, 2009 #7
    Re: Welcome to PF!

    Try making one of the dots directly up and one to the side, and check all of the statements again - the question didn't say that they're going to be in the same direction.

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook