A loaded penguin sled weighing $\displaystyle 80 \ \mbox{N}$ rests on a plane inclined at angle $\displaystyle \theta=20^{\circ}$ to the horizontal. Between the sled and the plane, the coefficient of static friction is $\displaystyle 0.25$, and the coefficient of kinetic friction is $\displaystyle 0.15$.

(a)What is the least magnitude of the force $\displaystyle \overrightarrow{F}$, parallel to the plane, that will prevent the sled from slippig down the plane?

(b)What is the minimum magnitude $\displaystyle F$ that will start the sled moving up the plane?

(c)What value of $\displaystyle F$ is required to move the sled up the plane at constant velocity?

Thankyou... I'm having a hard time grasping the concept of static & kinetic friction.