Final answer:
An example of unbalanced forces is when a person pushes a car that is parked, causing it to start moving. According to Newton's third law, there is an equal and opposite reaction force between the person and the car.
Step-by-step explanation:
a. An example of a scenario in which the forces acting on an object are unbalanced is when a person pushes a car that is parked on a flat road. In this scenario, the force exerted by the person on the car is greater than the force of friction between the car's tires and the road. This causes the car to start moving.
To know that the forces are unbalanced, we can observe that the car's acceleration is not zero. If the forces were balanced, the car would not accelerate.
b. According to Newton's third law of motion, for every action, there is an equal and opposite reaction. In the scenario described above, when the person pushes the car forward, the car exerts an equal and opposite force backward on the person. This is why the person feels a backward force while pushing the car.