Motion transmitted between the teeth of two spur gears is generally partly sliding and partly rolling.

Explanation:
Spur gears are one of the most common types of gears used in mechanical systems. They consist of cylindrical teeth that are parallel to the axis of rotation. When two spur gears mesh together, the motion between their teeth is a combination of sliding and rolling.

Sliding Motion:
Sliding motion occurs when the teeth of the gears slide past each other. This happens because the teeth of the driving gear push against the teeth of the driven gear. As a result, there is a relative motion between the contacting surfaces of the teeth, leading to sliding. This sliding motion creates friction, which can cause wear and tear on the gear teeth.

Rolling Motion:
Rolling motion occurs when the teeth of the gears roll against each other. This happens because the teeth of the driving gear engage with the teeth of the driven gear, causing the contact point to move along the tooth profiles. The rolling motion reduces friction and wear between the teeth, making it a more efficient mode of transmission.

Combination of Sliding and Rolling:
In reality, the motion between the teeth of spur gears is a combination of sliding and rolling. The sliding motion occurs at the entry and exit of the gear mesh, where the teeth first engage and disengage. During this phase, the teeth slide past each other, leading to sliding friction.

However, once the teeth are fully engaged, the rolling motion takes over. The teeth roll against each other as the gears rotate, minimizing sliding friction and improving the efficiency of power transmission. The rolling motion also helps distribute the load evenly across the teeth, reducing the risk of tooth failure.

Conclusion:
In summary, the motion transmitted between the teeth of two spur gears is generally partly sliding and partly rolling. While sliding occurs at the entry and exit of the gear mesh, the majority of the motion is rolling, which reduces friction and wear, and improves the efficiency of power transmission.