A block of mass M is placed on a horizontal floor. A frictionless pulley is attached to the top edge of the block, and a light string passes over the pulley as shown. A cube of mass m is suspended from the string so that the bottom part of the string is vertical, and the cube touches the block. The coefficient of kinetic friction between the block and the cube as well as between the block and the floor is μ.

The string is pulled horizontally by a force of unknown magnitude F. As a result, the cube slides upward, and the block is moving with acceleration a. Find F.

Guidelines for contributors

Many thanks to all contributors and we hope to hear from many more of you in the future!

Note: as always, we would very much appreciate reader-contributed original Challenges.

Boris Korsunsky,...

AAPT members receive access to The Physics Teacher and the American Journal of Physics as a member benefit. To learn more about this member benefit and becoming an AAPT member, visit the Joining AAPT page.