Any help would be greatly appreciated

A cart of mass 10kg is initially at rest on a horizontal surface. A rope is attached to the cart and Sue applies a constant horizontal force on the rope, so that the tensional force of 20N acts.

a) Find the work done on the cart, if it moves 4.0m in the direction of the force.
        W=Fs
        W=20 x 4.0
        W= 80J
     I think this answer is correct, the working seems to all make sense.


b) Explain why the work that Sue has done, is greater than your answer to part a).
    I am not sure on this one. Is it because the cart moved in the direction of the force? Or that because Sue has to turn chemical potential energy in her body into kinetic energy to pull the cart? Or something else?


c) If there is negligible friction acting on the cart, what is its kinetic energy and its speed after moving the 4.0m?

      K= Fs                   <---------- is this the right formula to use?
      K= 20 x 4.0
      K= 80J

    and how can i work out the cart's speed if i do not know how long it took to move the 4.0m?



d) If a constant frictional force of 5.0N did act on the cart, recalculate the kinetic energy of the cart and account for any different between your new answer and your previous answer

    K= Fs
    K= 15 x 4.0       <----------- was taking the frictional force off the tensional force the right thing to do?
    K= 60J

how do i account for the difference?



e) Neglecting friction again, recalculate the work done on the cart, if rope with its tensional force of 20N now makes an angle of 300 above the horizontal cart moves 4.0m

   I do not know where to begin with this question. I do not have any notes on how to do this (I missed a lot of lessons due to illness)



As I said at the start, any help would be GREATLY appreciated