<h2>Answer: Light waves have a redshift due to the Doppler effect
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The astronomer Edwin Powell Hubble observed several celestial bodies, and when obtaining the spectra of distant galaxies he observed the spectral lines were displaced towards the red (red shift), whereas the nearby galaxies showed a spectrum displaced to the blue.
From there, Hubble deduced that the farther the galaxy is, the more redshifted it is in its spectrum. <u>The same happens with the stars and this phenomenom is known as the Doppler effect.
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This phenomenon refers to the change in a wave perceived frequency (or wavelength=color) when the emitter of the waves, and the receiver (or observer in the case of light) move relative to each other. For example, as a star moves away from the Earth, its espectrum turns towards the red.
Answer:
The friction force is 250 N
Explanation:
The desk is moving at constant velocity. This means that its acceleration is zero: a = 0. Newton's second law states that the resultant of the forces acting on the desk is equal to the product between mass (m) and acceleration (a):

In this case, we know that the acceleration is zero: a = 0, so also the resultant of the forces must be zero:
(1)
We are only interested in the forces acting along the horizontal direction, since it is the direction of motion. There are two forces acting in this direction:
- the pull, forward, F = 250 N
- the friction force, backward, 
Given (1), we have

So the force of friction must be equal to the pull:

Answer:
No - It is connected in parallel instead of series
Explanation:
Answer:

Explanation:
Since the pulley has a mass concentrated on its rim, the pulley can be considered as a ring.
The moment of inertia of a ring is

The mass on the left is heavier, that is the pulley is rotating counterclockwise.
By Newton's Second Law, the net torque is equal to moment of inertia times angular acceleration.

Here, the net torque is the sum of the weight on the left and the weight on the right.

Applying Newton's Second Law gives the angular acceleration

The relation between angular acceleration and linear acceleration is

Then, the linear acceleration of the masses is
