Because of gravity and friction.
Atoms found in nature are either stable or unstable. An atom is stable if the forces among the particles that makeup the nucleus are balanced. An atom is unstable (radioactive) if these forces are unbalanced; if the nucleus has an excess of internal energy. Instability of an atom's nucleus may result from an excess of either neutrons or protons. Ccredits: internet source
Im not sure about this one but the one i will go for is a.
Force is a vector quantity
so pulling from opposite side will be negative
so
750+(-500)= 250N
C is the right answer
becauseause the man on the right applies greater force.
Hooke's Law
F = k. Δx
Δx = 30 cm = 0.3 m
200 = k . 0.3
the spring stretch for 100 N: