1. The problem statement, all variables and given/known data This is a problem from a post back in 2006. I have a fairly good understanding of how to do it; however, there is one part about the answer that I cannot figure out at all. (See the relevant equations) Problem: A 5kg penguin sits on a 10kg sled. A horizontal force of 45 N is applied to the sled, but the penguin attempts to impede the motion by holding onto a cord attached to a wall. The coefficient of kinetic friction between the sled and snow as well as that between the sled and the penguin is .2 b) Determine the tension in the cord and c) the acceleration of the sled if you try to solve for the tension using F=ma, do you could both the weights as the normal force? or do you only count the penguin's weight? Thanks Reference https://www.physicsforums.com/threads/newtons-law-problem-got-the-first-half.141064/ 2. Relevant equations On the post it said that the net force in the x direction of the sled was as follows: Sum of all forces sub x = ma sub x Fg + F sub f + (-Fx) = ma sub x I do not understand why Fg was included as a force acting in the x direction. Is this wrong or am I missing something. This is the equation I used: Sum of all forces sub x = ma F sub f + (-Fx) = ma Any insight would be greatly appreciated!!