Need help regarding understanding gravity in space vs on Earth!?

My teacher wants to know how a person standing on Earth could easily lift a wrench having a mass of 1 kilogram, but not a scientific instrument with a mass of 100 kilograms. But in the International Space Station, she is quite capable of manipulating both as the will ‘float’, but the scientific instrument still requires more effort to move than the wrench. He wants me to be able to explain this concept and I'm really not sure. I would think that the scientific instrument would be weightless in space, but if it still takes effort to lift it... that can't be correct. Is it something to do with the person having to push against it and then the instrument having to push back according to Newton's Laws??