Answer: C. Binary star systemsLet’s see your options:
a) The <em>color of the star</em>: the color is not used in calculating the mass of a star, because it has no relation to it. Think about a red supergiant and a red dwarf: they have the same color, but they are completely different stars, with respectively a big and a small mass.
b) <em>Kepler’s laws</em>: these laws can be applied in what is called the “approximation of 1 body”, which means that is assumed that one body has a much bigger mass than the other and can be considered at rest. This is the case of a star-planet system and the mass that can be calculated is that of the planet.
c)<em> Binary star systems</em>: these are the only cases in which is possible the direct measure of the mass of the stars. Binary systems are classified as follows:
- Visual binaries: each star can be resolved and the motion around the center of mass can be measured.
- Astrometric binaries: only one star is visible, but the presence of the companion can be inferred by the movement of the first star around the system’s center of mass.
- Eclipse binaries: the two stars are not resolved (separated), but the luminosity varies periodically when one star eclipses the other.
- Spectroscopic binaries: the two stars are not resolved, but their spectrum reveals that they are a binary system.
In all these cases we have a “two-body problem” that can be solved by changing system of reference: the motion of bodies 1 and 2 is equivalent to the motion of a body of mass equal to the system’s reduced mass

moving in the potential generated by the total mass (M1 + M2) considered at rest. Hence, we can determine the masses of the two stars.
Answer:

Explanation:
Given:
Length of a rope,
Position of Canary on the rope from one end, 
Position of Grackle on the rope from another end, 
Tension in the rope, 
linear mass distribution on the rope, 
We have for the speed of wave on the string:



<em>For canary to be undisturbed we need a node at this location.</em>
<em>Also, at the end close to Canary there must be a node to avoid any change in pattern of vibration.</em>
So,
the distance between Canary and the closer end must be equal to half the wavelength.


∴Wavelength of wave to be produced = 20 m. This will give us nodes at the multiples of 10 and anti-nodes at the multiples of 5.
Now, frequency:



Answer:
AN electric current is required for an electric charge
Explanation:
Answer:

Explanation:
The peak wavelength of the spectral distribution can be found by using Wien's displacement law:

where
is Wien's displacement constant
T is the absolute temperature
For the cosmic background radiation, the temperature is
T = 2.7 K
So, the corresponding peak wavelength is

Answer:
D
A machine can help decrease the input force and increase the output force.