Net charge of system with one +6e sphere and another -4e sphere.

+2 e

Explanation: Net charge of a system is sum of charges on all the constituents of the system.

The system contains two spheres. Charge on one sphere is:

q₁ = +6 e

Charge on the second sphere is:

q₂ = -4 e

Net charge of the system is:

q = q₁ + q₂

q = +6 e + (-4 e) = + 2 e

Thus, the net charge of system is +2 e.

acceleration = (change in speed) / (time for the change)

change in speed = (20-120)= -100 m/s.

1.5 min = 90 sec

acceleration = -100/90 = -1.11 m/s^2.

we don't care about the 150kg or the 2,500 meters. they don't make any difference.

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speed of light in vacuum is 300,000 km/s

speed of light in other mediums is given by the expression v = c/n, where c is the speed of light in vacuum and n is the refractive index of medium.

for the first medium v = 200,000 km/s

  so, refractive index of medium = 300000/200000 = 1.5

    1.5 refractive index is usually for glass. so first medium is glass.

for the second medium v = 250,00 km/s

so, refractive index of medium = 300000/250000 = 1.2

    1.2 refractive index is usually for water. so second medium is water.

for the third medium v = 0 km/s

so, refractive index of medium = 300000/0 = ∞

    ∞ refractive index is usually for opaque mediums. so third medium is opaque medium.