When the bag lands on the cart, it follows that vf < vo given the fact that some mechanical energy is now lost by the system.
The term friction refers to the force that opposes motion. The direction of the frictional force is opposite to the force that causes motion.
When the bag lands on the cart, it follows that vf < vo given the fact that some mechanical energy is now lost by the system. Note that the potential energy of the bag is converted to kinetic energy.
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