Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Newton's Laws Questions + Friction. Very tricky and help appreciated!

  1. Oct 11, 2010 #1
    I AM NOT INTERESTED IN DIRECT ANSWERS! I'm just stuck on how to process these questions and just need a little guidance on what I'm doing wrong. I will be forever indebted and I am more interested in learning how to solve for these. Thank you!

    1. A person weighing 0.9 kN rides in an elevator that has a downward acceleration of 2.6 m/s2.The acceleration of gravity is 9.8 m/s2. What is the magnitude of the force of the elevator floor on the person? Answer in units of k

    1a. Relevant equations

    f=m x a

    1b. The attempt at a solution
    Find mass of person (should be 900 N) but should I find the mass in kg?
    Combine both accelerations and multiply by mass in kg?

    2.When you drop a 0.35 kg apple, Earth exerts a force on it that accelerates it at 9.8 m/s2 toward the earth’s surface. According to Newton’s third law, the apple must exert an equal but opposite force on Earth.
    If the mass of the earth 5.98 × 1024 kg, what is the magnitude of the earth’s acceleration toward the apple?
    Answer in units of m/s2.

    2a. Relevant equations.
    f= m x a?

    2b. Attempt
    The force the apple creates is 3.43. Do I set that equal to Eaarth's force and divide by Earth's mass? I get something like 5.74e-25. That can't be right.

    3.The force of friction acting on a sliding crate is 232 N. How much force must be applied to maintain a constant velocity? Answer in units of N

    3a. Equation
    Not sure which one to use.

    Do I find the mass of the crate? Acc. has to be 9.8 but how would i find for constant velocity?
  2. jcsd
  3. Oct 11, 2010 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    It's f_net = m x a, where f_net is the algebraic sum of the force of gravity (the weight) on the person and the force of the floor (the normal force) on the person
    the weight of the person is 900 N. The person's mass in kg is calculated by m=weight/g
    No, find the net force acting on the person , and set it equal to m x a. The net force is always in the direction of the acceleration.
    Why not? you don't expect the apple to lift up the earth significantly, do you? :wink:
    This problem assumes the crate is on a rough horizontal surface. If it's moving at constant velocity, there is NO acceleration (per Newton 1). So what force must be applied if it is moving at constant velocity?

    Welcome to Physics Forums!
  4. Oct 11, 2010 #3
    Ah thank you! I think I'm getting somewhere now! Problem 2 was correct after all :smile:

    For problem 3, I'm still not understanding what I'm looking for exactly. The follow up question to that problem asks for a net force so I must be looking for some kind of acceleration, no?
  5. Oct 11, 2010 #4


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Not if the net force is zero, that's Newton 1:wink: 'An object in motion at constant speed moving in a straight line will remain at constant speed in a straight line unless acted on by a net unbalanced force. No net force, then no acceleration, F_net =0.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook