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: Calculate the force F necessary to keep the velocity constant.

  1. Nov 12, 2007 #1
    I need help!!!

    Ok, so here's the problem.
    A 10-kilogram block is pushed along a rough horizontal surface by a constant horizontal force F as shown above. At time t=0, the velocity v of the block is 6.0 meters per second in the same direction as the force. The coefficient of sliding friction is 0.2. Assume g= 10 meters per second squared.
    A) Calculate the force F necessary to keep the velocity constant.

    The force is now changed to a larger constant value F'. The block accelerates so that its kinetic energy increases by 60 joules while it slides a distance of 4.0 meters.

    B) Calculate the force F'.

    C) Calculate the acceleration of the block.

    Ok, so far I have that the work done by kinetic energy is 1/2mv^2 by which would be simplified to 180 Joules. Since there is friction involved, I would account for the Force of friction which is the coefficient of friction multiplied by the normal force in which the total would be 2.
    Now would the force F be 180 Joules to keep it constant?

    and for B since the kinetic energy is increased by 60 Joules, then the total kinetic energy put on the object would be 240 Joules. So would that be force F'?

    and for the acceleration of the block, would I use V^2= Vo^2 + 2a(deltax)?
  2. jcsd
  3. Nov 12, 2007 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    the normal force is mg which is 100 newtons, so the friction force is 20 newtons.
    you are confusing force with energy. What does newton 1 tell you about objects moving at constant velocity? What then must be the applied force F?
    you can't equate force and energy, they are 2 different animals. Are you familiar with the work-energy theorem, which states that the work done by all forces acting on the block must be equal to the change in Kinetic energy of the block?
  4. Nov 12, 2007 #3
    So then how would I go about solving these problems?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook