15. Normal force is defined as resistance of an object to a change in its velocity. rate at which velocity changes force exerted by a surface that opposes motion as an object moves across it. force exerted by a surface to support the weight of an object.

The correct answer is: force exerted by a surface to support the weight of an object.<br /><br />Normal force is the force exerted by a surface to support the weight of an object. It acts perpendicular to the surface and is equal in magnitude and opposite in direction to the weight of the object.