Introduction:
In kinematics, a kinematic pair refers to the connection between two elements that allow relative motion between them. The characteristics of the contact between these elements determine the type of kinematic pair. One of the important distinctions is whether the elements have surface contact while in motion.

Explanation:
When the elements of a kinematic pair have surface contact while in motion, it is classified as a lower pair. Let's understand why.

Lower Pair:
A lower pair is a type of kinematic pair where the contact between the elements occurs on a surface or a line. This type of pair allows relative motion between the elements while maintaining surface contact. The contact surfaces or lines can be flat, cylindrical, or spherical.

Surface Contact:
When the elements of a kinematic pair have surface contact, it means that the contact occurs over a larger area. This provides stability, load-bearing capacity, and the ability to transmit higher forces between the elements. Surface contact also helps in distributing the applied load evenly over the contact area.

Motion:
In a lower pair with surface contact, the motion between the elements can be either sliding or rolling. Sliding motion occurs when one element slides over the surface of the other, while rolling motion occurs when one element rolls over the other.

Advantages of Lower Pair:
Lower pairs with surface contact have several advantages:
- They provide greater stability and resistance to misalignment.
- They can transmit higher forces and moments due to the larger contact area.
- They have lower frictional losses compared to higher pairs, resulting in reduced energy consumption.
- They are more suitable for heavy-duty applications and high-load conditions.

Conclusion:
In a kinematic pair, when the elements have surface contact while in motion, it is classified as a lower pair. Lower pairs provide stability, load-bearing capacity, and the ability to transmit higher forces. They are advantageous in heavy-duty applications and high-load conditions.