Answer
Ambiguity about what body the forces act upon. Failure to subdivide the system into parts for analysis.
Newton's third law action/reaction forces act on different bodies. Forces that contribute to the net force on a body act on that body, not on different bodies.
Failure to define the system that is accelerating.
To analyze this properly, it is probably best to consider the
Explanation
horse is harnessed to a cart. If the horse tries to pull the cart, the horse must exert a force on the cart. By Newton's third law the cart must then exert an equal and opposite force on the horse. Newton's second law tells us that acceleration is equal to the net force divided by the mass of the system. (F = ma, so a = F/m .) Since the two forces are equal and opposite, they must add to zero, so Newton's second law tells us that the acceleration of the system must be zero. If it doesn't accelerate, and it started it rest, it must remain at rest (by the definition of acceleration), and therefore no matter how hard the horse pulls, it can never move the cart.
List all the physical errors and mistakes in the above paragraph and explain why they are wrong. Show a free-body force diagram of the horse and cart, identifying all relevant forces, and then write a short paragraph describing this situation correctly.
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