This is the half life of the substance. The half life is the time it takes for the substance to become reduced by half as a consequence of decay.
The visible radiation produced by new stars within a molecular cloud get absorbed by dust grains and heats up the cloud
A molecular cloud is an interstellar cloud of gas and dust in which molecules can form, the most common of which is hydrogen .
The formation of stars occurs exclusively within molecular clouds because of their low temperatures and high densities, because the gravitational force acting to collapse the cloud must exceed the internal pressures that are acting "outward" to prevent a collapse.
Visible radiation are absorbed by dust grains and heats up the cloud as it cannot penetrate through the cloud instead get absorbed by the cloud . Newly formed stars are visible nearby, their images reddened by blue light which got scattered by the pervasive dust.
To learn more about molecular clouds here
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Its a very syplel thing devipe it by distance and boom u shude have it
Answer: Total work done on the block is 3670.5 Joules.
Step by step:
Work done:

With F the force, d the displacement, and theta the angle of action (which is 0 since the block is pushed along the direction of displacement, and cos 0 = 1)

Given:
F = 75 N
m = 31.8 kg
Final velocity 
In order to calculate the Work we need to determine the displacement, or distance the block travels. We can use the information about F and m to first figure out the acceleration:

Now we can determine the displacement from the following formula:

Here, the initial displacement is 0 and initial velocity is also 0 (at rest):

Now we still have "t" as unknown. But we are given one more bit of information from which this can be determined:

(using vf as final velocity, and tf as final time)
So it takes about 6.44 seconds for the block to move. This allows us to finally calculate the displacement:

and the corresponding work:
