Since sound is a mechanical wave it needs a to travel through

Which statement is true for light passing into a medium that is more optically dense than the first medium through which it past

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

Why does gravity have a lesser than grater effect on higher altitudes?