I think the correct answer would be that because electromagnets are powerful and can be turned off and on anytime. Electromagnet is a magnet in which the magnetic field is made by the electric current that is induced to the system.
Answer:

Explanation:
= normal force acting on the coin
Normal force in the upward direction balances the weight of the coin, hence

= frequency of rotation
Angular velocity of turntable is hence given as

= distance from the axis of rotation
= minimum coefficient of static friction
static frictional force is given as

The static frictional force provides the necessary centripetal force , hence
Centripetal force = Static frictional force

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B. Fusion requires very high pressure and temperature.
Explanation:
Nuclear fusion is the combination of atomic nuclei into larger ones with the release of a large amount of energy.
Nuclear fission is the decay process in which a heavy nuclide is bombarded with with neutrons.
- Both processes release a huge amount of energy.
- But fusion produces far more energy.
- The reason why nuclear fusion is not in use is because of the high pressure and temperature needed start it.
- The activation energy is high and the energy barrier is very great.
- Only the core of the sun naturally favors fusion
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Answer:
Betelgeuse is 640 light years away from earth.
Explanation:
A light-year is an astronomical unit to measure the distance the light travels in a calendar year.
If the light from a star takes 640 years to reach us, then that star its 640ly away from us.
Betelgeuse has been labeled as a Variant Star, which means that its brightness can fluctuate over the course of years, this has made difficult for astronomers to measure the exact distance of the star. Right now the star is estimated to be around 613 and 881ly away from earth, although, for the sake of your second question, we will take 640 years as our estimated value.
In 1380 (640 years ago) the Battle of Kulikovo took place. A battle of remarkable importance to Russian history, in which the Russian army, led by Prince Dmitry of Moscow, defeated the Mongol army, defining a turning point in the Mongol dominance, and setting the bases for what Russia is today.