Again, the array of products on the market was making people having a perfectly normal experiences feel like icky outliers."We were hearing that people wanted a fully reusable product but were getting stuck on the difficulty of removing the cup," Wang says.
Here is a force diagram for a tiny object pulling a massive object with no friction. For the train (object B), a frictional force is pulling to the right.
With that, I can find a force-acceleration equation for both masses. With an accelerating mass, the net force is NOT zero. Notice that if the two masses have the same value, the acceleration would be zero meters per second squared (that's good).