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: Applying newtons laws (intro to physics problem!)

  1. Sep 25, 2009 #1
    A 20000 rocket has a rocket motor that generates 3.0×105 of thrust.

    What is the rocket's initial upward acceleration?
    Express your answer using two significant figures.

    At an altitude of 5.0 km the rocket's acceleration has increased to 6.0 m/s^2 . What mass of fuel has it burned?
    Express your answer using two significant figures.
  2. jcsd
  3. Sep 26, 2009 #2


    User Avatar
    Science Advisor
    Homework Helper

    There are reasons for using units in physics, most notably that we know what kind of quantities we are talking about.

    Let's assume 20000 is the mass in kilograms, and by 3.0×105 you mean 3.0×105 (rather than 315), and the corresponding unit is Newtons.

    Then you are given the mass and upward force. What other forces act? What is the relation between force, mass and acceleration?

    For the second question, let's assume that the thrust remains equal. So now the acceleration and force are given and you are asked for the mass. Again: what are the forces acting and what physical law do you know?

    (By the way, when posting this question, you should have gotten a template in the posting form. May I be so bold as to inquire why you didn't use it?)
  4. Sep 26, 2009 #3
    Thrust is the force. Use the basic equation [tex]F=ma[/tex] to find acceleration (don't forget the influence of gravity).

    Since the thrust is assumed to be constant, then you can use that equation again to find the new mass of the rocket+fuel at the higher acceleration.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook