1) A boy throws a stone up with a velocity of 60 m/s. How long will it take to reach the maximum height ( g = -10 m/s2 ) 2) A certain...

Hello!

1. The speed will decrease uniformly under the gravity force. The speed of decrease is g, i.e. the speed `V` will be `V(t)=V_0-g*t,` where `V_0` is the initial speed and `t` is a time in seconds (`g` is positive here).

The maximum height will be reached when the speed becomes zero. After that a stone will begin to fall down. The speed will be zero at `t=V_0/(g)=60/10=6(s).`

2. If a particle is in rest, then its weight is equal to gravity force, `mg.` So the mass of a particle is `30/9.8 approx 3.1 (kg).`

A mass remains unchanged regardless of the gravity force or other forces. As long as a particle remains the same (the same matter in the same quantity), its mass is preserved.

However, the weight due to gravity will be zero (no gravity = no force).