Final answer:
The correct understanding is that the energy of motion, known as kinetic energy, is not the same as force. Forces must act to change motion per Newton's laws. Kinetic and potential energies are interchanged during the motion of a projectile, like a rock thrown into the air.
Step-by-step explanation:
The statement 'The energy of motion is unequal force' is not accurate. Instead, the energy associated with an object's motion is known as kinetic energy, and it does not directly equate to force. Force, according to Newton's laws of motion, is necessary to change the motion of an object, i.e., to accelerate it. The actual statement should reflect that an external force is required to change the state of motion of an object.
Referring to Newton's Third Law of Motion, when a person walks on the ground and exerts force F₁, the ground exerts a reaction force F₂ that is equal in magnitude but opposite in direction. These forces act on different systems; force F₁ acts on the ground, while force F₂ acts on the person. The net force on the person is non-zero due to other forces at play, such as friction, allowing the person to move.
When two unequal forces act on a body, it will accelerate in the direction of the resultant force, which is based on the vector sum of all the forces. An external force is indeed necessary to set a stationary object in motion, even in outer space absent of gravity and atmospheric friction. And as for kinetic energy and potential energy, if a rock is thrown into the air, the kinetic energy will decrease as it reaches greater height, at which point the potential energy is maximum; and as it falls back, the kinetic energy increases again while potential energy decreases.