Passage Summary: Running on hard tracks is faster than on soft ones because your feet spend less time touching the ground, which outweighs the fact that your steps get a bit shorter.

Conclusion: Hard running surfaces allow for higher speeds than soft surfaces because they reduce the time the foot stays in contact with the ground.

Reasoning: Even though a runner's stride is shorter on hard ground, the decrease in contact time more than makes up for the loss in step length.

Analysis: The prompt asks for evidence that the provided explanation is only 'partial,' which means we need to find another factor that contributes to the speed difference. The current argument focuses entirely on the trade-off between step length and contact time. To show this is incomplete, look for an answer that introduces a third variable—perhaps something about energy return, muscle fatigue, or traction—that also helps explain why hard surfaces are faster. We aren't trying to prove the original explanation wrong, just that it doesn't tell the whole story of the physics involved.