posted by Betsy .
a child causes a wagon to accelerate by pulling it with a horizontal force.
newton's third law says that the wagon exerts an equal and opposite force on the child. how can the wagon accelerate
The net force ON the wagon is the force exerted by the child on the handle (minus the drag of the wheels and any air friction but that is not mentioned here).
Forces exerted BY the wagon do not count in the SECOND law which is the Net force ON the object equals the rate of change of momentum which is the mass times the acceleration.
F = m A
where F = net force ON the object