What is missing from this free body diagram of a sled being pulled across level ice by a dog team
the normal force
the force that the dogs exert on the ground because of the weight
the velocity vector arrow for the sled
the reaction force that the ground exerts on the dogs to push them forward

The normal force

Explanation:

There are four forces acgint on the sled:

- the driving force, exerted by the dogs which pull the sled forward

- the frictional force, that act in the opposite direction of the driving force (backward)

- the weight of the sled, which is directed downward

- the normal force, which is the reaction of the surface on which the sled slides, and which pulls opposite to the weight (upward)

From the graph, we see that the only missing force is the one which is directed upward, so it must be the normal force.

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