$A$ heap of rope with mass density $\lambda$ (per unit length) lies on a table. You grab one end and pull horizontally with constant speed $v$,as shown in the figure. (Assume that the rope has no friction with itself in the heap.) The force that you must apply to maintain the constant speed $v$ is

• A
  $2\lambda v^2$
• B
  $\lambda v^2$
• C
  $\lambda lg$,where $l$ is the length that you have pulled straight
• D
  $\frac{\lambda v^2}{2}$