2Mg + 2HCl →2MgCl + 2H₂
Balanced is:
4Mg + 4HCl → 4MgCl + 2H₂
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Solution :
Given :
M = 0.35 kg

Total mechanical energy = constant
or 
But
and 
Therefore, potential energy at the top = kinetic energy at the bottom


(h = 35 cm = 0.35 m)
= 2.62 m/s
It is the velocity of M just before collision of 'm' at the bottom.
We know that in elastic collision velocity after collision is given by :

here, 
∴ 

= 0.33 m/s
Therefore, velocity after the collision of mass M = 0.33 m/s
Density = mass / volume
mass = 1.1 g
volume = length of side ^ 3 = [1.2 * 10^-5 km * 100000 cm/km]^3 = [1.2 cm]^3 = 1.728 cm^3
density = 1.1 g / 1.728 cm^3 = 0.64 g / cm^3