Final answer:
The net force acting on the cart when pulled by a donkey with a force of 100N against a frictional force of 50N is 50N.
Step-by-step explanation:
When a donkey pulls a cart with a constant force of 100N and a constant force of friction of 50N acts in the opposite direction, the net force can be determined by subtracting the force of friction from the pulling force. The calculation would be 100N (force exerted by the donkey) minus 50N (frictional force) resulting in a net force of 50N acting on the cart. Since the cart's acceleration is determined by this net force, the cart will accelerate according to Newton's second law of motion, which states that the acceleration of an object is directly proportional to the net force exerted on it and inversely proportional to its mass (F = ma).