PHYSICS physics help me pls!

When a book falls from a shelf to the ground without anyone pushing on it, no work is done on the book. true or false

Assume you measured the mass of the cart to be (500 ± 1) g and the mass of the additional mass you put on the cart to be (500 ± 1) g as well. since the scale you are using in the lab cannot measure objects heavier than 600g you will have to sum up individual pieces and propagate the error. so what would be the mass and the standard error of the cart and the mass

Which system of units is used by only a small number of countries in the world, including the u.s.