1) In the ground frame, the force felt at one's feet when pushing a cart is γ2σv, where σ is the rate at which mass falls into the cart and v is the cart's velocity. This comes from calculating the rate of change of momentum of the combined cart-mass system. 2) In the cart frame, the apparent force felt would be σv, creating a paradox with the ground frame result. 3) The resolution is that one's own mass is also decreasing in the cart frame at a rate of (γ2-1)σ/γ, as mass transfers from the moving person to the stationary cart. Adding this change in momentum resolves the paradox, giving the