Health and safety are two reasons
When it says something like 'on the verge of moving,' it means that the pulling force and static friction force and gravitational force all cancel out! Any more pulling force and it is ready to move!
At some point, you want F as a function of <span>μs</span>, to determine the force needed depending on the coefficient of static friction. This function, <span>F(<span>μs</span>)</span>, will rely on the angle θ as well, but we want to consider just one angle θ in every scenario. One value means it is constant.
But if we know the F, and we know <span>μs</span>, we can find what the constant angle θ must be!
If F is the pulling force, <span>FS</span> is the static friction force, and <span>FG</span> is gravitational force,
<span><span><span>Fnet</span>=0</span><span>=F+<span>FS</span>+<span>FG</span></span><span>=F+<span>FN</span><span>μs</span>+mgsinθ</span><span>=F+mgcosθ<span>μs</span>+mgsinθ</span><span>=0</span></span>
Then you can find <span>F(<span>μs</span>)</span>, but then there is the issue of solving for the θ<span> to make it true.</span>
Visible light or electromagnetic radiation within 400nm to 700nm is responsible for colour of the spectrum.
Explanation:
The electromagnetic spectrum contains radiations of varying wavelength. The radiations with the lowest energy are characterised by the longest wavelength.
Within this spectrum lies the visible light which enables us to see a different colour. The radiations within the range 400nm to 700nm are included in the visible spectrum.
While violet lies at the 400nm spectrum part red colour lies at 700nm part. As the wavelength of the radiation transverses between 400-700 nm, the colour of the object changes accordingly.