How far above the earth's surface will the acceleration of gravity be half what it is at the surface?

The earth's gravitational pull would be halved when the distance above the earth would equal the radius of the earth. In this case, from the Earth's crust to the core is 1,64. If we want to half the gravitational pull on the Earth, we must consider the equation for gravitational acceleration: g = GM/r^2. G is the gravitational constant of the Earth, and the mass of the Earth. r is the radius of the earth. If g is to be halved, r^2 must be doubled. Therefore, one can use the square root of 2 to multiply the left side of the original equation. If the distance from the Earth's surface to its core is originally 6,378 km, we multiply it by .141414 (square root of 2) to come to 9,020 km. Subtract the original radius, and one comes to 2,642km. So 2,642 km above the Earth, the gravitational acceleration is halved of what it is on the surface.

  concave means having an outline or surface that curves inward like the interior of a circle or sphere. so the mirror curves inward.

Answer sample (c); // would be best because the percentage of the energy in an incident wave that remains in a reflected wave from this material is the smallest;