Final answer:
It is true that objects only accelerate with an unbalanced force. The position vs time graph of an accelerating object is curved, not straight. Also, it's true that in walking, the person and the ground exert equal and opposite forces on each other, as described by Newton's Third Law.
Step-by-step explanation:
Objects only accelerate (change velocity) if an unbalanced force is applied. This statement is true. If there's no net external force acting on an object, it will either remain at rest or continue moving at a constant velocity, according to Newton's First Law of Motion. Acceleration occurs only when there's a net force resulting in a change in velocity.
As for the statement that "The position vs time graph of an object that is speeding up is a straight line," the correct answer is false. A straight line on a position vs time graph indicates constant velocity, not acceleration. When an object is speeding up, the graph would be curved, indicating a change in velocity over time.
Concerning Newton's Third Law of Motion, it is true that a person accelerates while walking on the ground due to the action-reaction force pairs. The person pushes against the ground (F1), and the ground pushes back with an equal and opposite force (F2). These forces are equal in magnitude and opposite in direction. However, they do not cancel each other out in this situation because they act on different systems - the force exerted by the person is on the ground, while the force exerted by the ground is on the person. This is why the person is able to move forward.