Ok, if you need to determine a tension in a string that is being pulled by a human that is on a sled (the sled is accelerating outwards). Is the normal force the two weights combined?? I have Friction force + (- Tension)= ma But for the friction force I don't know whether or not I need to find the human's normal force or the sled or both. The human isn't moving, the sled is pulling away ---> but he is holding onto a string attatched to a wall l------O The O is the human and he is standing on the sled. Thanks!