You are correct that force is typically defined as mass times acceleration (F = m * a). However, in the context of lifting heavy objects, we need to consider a few additional factors to understand why it is considered a force.
When you lift a heavy object, you apply an upward force on it to counteract the force of gravity pulling it downward. Even though you may not accelerate the object upward (assuming a constant velocity), you still exert a force to oppose the gravitational force acting on the object.
In this case, the force you apply is equal in magnitude but opposite in direction to the gravitational force acting on the object. According to Newton's third law of motion, for every action, there is an equal and opposite reaction. So, when you apply an upward force on the object, the object exerts an equal and opposite force downward on you (commonly referred to as the weight of the object).
Therefore, lifting heavy objects does count as a force because you are exerting a force to counteract the gravitational force acting on the object. Although the object may not accelerate, the force you apply is necessary to maintain the object's equilibrium and prevent it from falling due to gravity.
It's important to note that when we talk about force in the context of Newton's laws of motion, it refers to the net force acting on an object that causes its acceleration. In the case of lifting a heavy object, the net force is zero (since there is no acceleration), but forces are still being exerted to maintain the object's position.