1. The problem statement, all variables and given/known data A horse of mass M pulls a wagon of mass m so that the horse and wagon have an acceleration a. Assume the wagon wheels have frictionless bearings. Write expressions for the following in terms of m, M, and a. (A) The horizontal frictional force that the ground exerts on the horse. 2. Relevant equations F=ma Ffr=μFn 3. The attempt at a solution I found the answer to be Ma-ma, but that's not right.