Simulating accurate weight forces in Virtual Reality (VR) is an unsolved challenge. Therefore, providing real weight sensations by transferring liquid mass has emerged as a promising approach. However, key objectives conceptually interfere with each other. In particular, previous designs that support a high range of weight or high flow rate lack mobility. In this work, we present MobileGravity, a system, that decouples the weight-changing object from the liquid supply and the pump. It enables weight changes of up to 1 kg at a rate of 235 g/s and allows the user to walk around freely. Through a study with 30 participants, we show that the system enables users to perceive the weight of different virtual objects and enhances realism, as well as enjoyment.