91Ó°ÊÓ

Gravitational force is a fundamental force that attracts two masses toward each other. In the context of holding a brick, the gravitational force is essentially the weight of the brick acting downward towards the Earth.
This force results from the mass of the brick interacting with the Earth's gravitational field. It's always directed towards the center of the Earth, and its magnitude can be calculated using the equation: \( F_g = m \cdot g \), where \( F_g \) is the gravitational force, \( m \) is the mass of the brick, and \( g \) is the acceleration due to gravity (approximately \( 9.8 \text{ m/s}^2 \) on Earth).
Understanding gravitational force helps us comprehend why objects fall and why they have weight.

The normal force is a contact force exerted by a surface, in this case, your hand, to support the weight of the brick and prevent it from falling.
It acts perpendicular to the surface in contact with the object. For the brick sitting in your hand, the normal force acts upward and is equal in magnitude but opposite in direction to the gravitational force pulling downward.
This force ensures the brick is held stable and doesn't accelerate either upwards or downwards, maintaining its position in your hand.

Newton's Third Law of Motion describes action-reaction pairs as equal and opposite forces acting on two different objects. For every action, there is an equal and opposite reaction.
In the case of the brick, although the gravitational and normal forces are equal and opposite, they are not action-reaction pairs. This is because both forces are acting on the brick itself.
An action-reaction pair involving the brick could be identified like the brick pulling up on the Earth with the same gravitational force that the Earth pulls down on the brick. Here, the brick and the Earth are separate objects in this interaction.

Net force is the overall force acting on an object when all the individual forces acting upon it are combined. It determines whether an object accelerates or remains at rest.
When you hold the brick stationary, the net force on it is zero. This is due to the gravitational force downwards being perfectly balanced by the normal force upwards.
Newton's First Law tells us that with no net force acting (net force equaling zero), an object will maintain its current state of motion—either staying at rest or moving at constant velocity.

Force equilibrium occurs when all the forces acting on an object are balanced, resulting in a net force of zero. This concept is fundamental in understanding how objects can remain at rest or move without accelerating.
In our brick example, force equilibrium is achieved because the gravitational force is perfectly counterbalanced by the normal force from your hand.
When an object is in force equilibrium, it neither moves nor changes velocity, illustrating a state of mechanical balance and stability.