In an experiment, students release a block from rest at the top of an inclined plane. The block slides down the plane through a vertical displacement h, and the students measure the speed v of the block using a photogate. The students expect the speed of the block at the end of this motion would be v=2gh−−−√. However, the measured v is significantly different. Which of the following is the most probable source of error the students should cite in their lab report?.