This is a very interesting question!! I'm assuming this is from physics. The answer: Kelvin's weight would change between planet x and planet y, because weight is defined as the forcce exerted by gravity. IT's different from mass. Weight changes depending on where you are.
Mental acuity refers to being mentally sharp. If you're mentally sharp, you have an excellent memory, and you find it easy to complete different tasks. Lack of sleep can effect this.
