What is the mass of the Milky Way?

We can consider the Sun to be a satellite of the Milky Way. The Sun revolves around the center of the Milky Way with a radius of 8 kpc. The period of revolution is 230 million years. What is the mass of the Milky Way?
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A motion of a Conical Pendulum


A ball is attached to a string and swung so that it travels in a horizontal circle.

Suddenly the angular velocity is increased. Will the angle with the vertical increase, decrease or stay the same? Justify your answer.

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Non-Ideal Voltmeter & Non-Ideal Ammeter

A student built an electric circuit that consists of an ideal battery, a voltmeter with resistance RV, an ammeter with resistance RA and a resistor R. She connected all elements in the circuit as shown in Fig. (1). The voltmeter showed a reading of 1V and the ammeter showed a current of 1A. Then, another student switched between the voltmeter and ammeter Fig. (2). As a result, the voltmeter showed the reading of 2V and the ammeter showed the current of 0.5A. What is the resistance of the resistor?

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STAR connection

Three batteries have their negative terminals connected together.
Find the voltage read by an ideal voltmeter.

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A rod carries a charge uniformly distributed along its length

A rod of length L carries a charge Q uniformly distributed along its length.ROD
Find the electrical field at point P on the axis of the rod, a distance a away from the end of the rod.

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The rod is “flying” above the cable

The rod is flying above the cableA conducting rod of length l, mass m, and resistance R is suspended by two vertical conducting wires of lengths d each. The circuit consists of a battery ε, the horizontal fixed wires, two parallel vertical wires , and the conducting rod. All conducting wires have negligible resistance and mass. There is a conducting long cable. The cable is below the rod and at a distance x.
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The equation of radioactive decay

The half-life of Molybdenum-93 is 4000 years. A sample of Molybdenum-93 has a mass of 10 mg. When will the mass be reduced to 1 mg?

Image Source: Virtual museum of chemical elements.

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