What do you mean by normal reaction?

Explanation

### Step-by-Step Solution:

1. **Definition of Normal Reaction**: The normal reaction is a force exerted by a surface that supports the weight of an object resting on it. It acts perpendicular (normal) to the surface.

2. **Understanding the Forces**: When an object of mass \( m \) is placed on a surface, it experiences a downward gravitational force due to its weight, which is given by \( mg \) (where \( g \) is the acceleration due to gravity).

3. **Role of the Surface**: The surface on which the object rests is rigid and cannot change shape. Therefore, it must exert an upward force to counteract the downward gravitational force acting on the object.

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