Molecular machines---assemblies of macromolecules, often fueled by nucleotide hydrolysis---are fascinating devices and crucial for driving self-organization in cells. While protein components of many biological machines have been identified, and in many cases their structures have been solved, the mechanical principles that govern the operation of biological machines are poorly understood. For example, howmuch force can they generate; and what limits their speed and efficiency? These questions have been difficult to answer because the tools needed to study nanometer-sized machines that generate minute forces on the order of piconewtons have not been available until recently. Friction arises between proteins when they interact by making and breaking weak intermolecular bonds. When a bond breaks, the energystored in its deformation is dissipated. Protein friction is a useful concept because it provides mechanical insight and allows for quantitative theoretical understanding of the dynamics and energy balance of mechanical cellular processes. In cells, many motor proteins often cooperate to drive motility. I will ask how friction and force-generation arise and scale with the number of motors to elucidate how collective behavior and self-organization emerge. The goals of this interdisciplinary project address the role that protein friction plays in limiting the dynamics and efficiency of microtubule-based motor proteins using a novel, combined optical tweezers and single-molecule fluorescence apparatus. In the long term, I hope that our avant-garde nanotechnological tools will be applicable to other molecular machines and that the studies on microtubule-based motors will shed light on the way that cells use energy to create pattern and order.