A 0.5 kg yellow bouncy ball rolls to the right at a speed of 8 m/s while a 0.25 kg blue bouncy ball rolls to the left towards it with a speed of 4 m/s. After the collision, the blue bouncy ball rolls to the right
with a speed of 12 m/s. How fast is the yellow ball rolling and in what direction?

Explanation:

Let yellow ball be m1 = 0.5kg with u1 = 8 m/s and blue ball be m2 = 0.25 kg with u2 = - 4 m /s respectively.

After collision, blue ball travels 12 m/s.

Using conservation of Linear Momentum :

m1u1 + m2u2 = m1v1 + m2v2

0.5* 8 + 0.25 * - 4 = 0.5 * v1 + 0.25 * 12

v1 = 0 m/sec i.e. Yellow ball comes to rest.

if the thermos has a vacuum innner, then heat through conv currents and conduction are reduced = vacuum flask

the independent variable, or the one that is intentionally manipulated is the speed.