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??

Weight and mass are not the same thing.
Gravity acts on mass to give weight.
On earth, (in it's gravity field), to action of gravity must be overcome
as well as the inertia of the mass involved.
In orbit, (free fall, no gravity), only the inertia of the mass need be considered.
You astronaut would still need to exert 100 times more force to move the larger mass
than the smaller one at the same speed, (and to stop them).
On the Moon, the objects would 'weigh' 1/6 as much as they do on earth,
but their inertias would still be the same as in orbit, or on Earth.

There is some gravity in space. Just not enough to actually notice it. When you are in the ISS the gravity acts on the station and you equally along with the weights er go it still requires little force.

The satellite is in constant free fall, and hence feels no gravitational force. Objects in orbit are "falling towards" the Earth, but are moving so fast and in such a way that the earth curves away before they can hit the surface.

Irv S is correct.