A "screen" or even just a set of parallel bars are highly reflective to electromagnetic waves as long as the open spaces are small compared to the wavelengths.
"Grid" dishes work fine ... with less weight and less wind resistance ... for frequencies below about 3 GHz. (Wavelengths of at least 10 cm.)
(I even worked on a microwave system in South America where huge grid dishes were used on a 90-mile link.)
The velocity of the object. because the greater the movement in particles the greater would be the kinetic energy.
The area under a velocity time graph is distance travelled (as integrating velocity gives distance), so we can imagine the v/t as a line, giving a triangle with 2 sides of 13.8 and 18. The area of a triangle is bh/2, so 9*13.8 = 124.2m which is how far the car travelled in that time.
60.8cm is 0.608m, and now we can divide to work out the number of revolutions:
124.2/0.608 = 204.28 = 204 revolutions to 3sf (number of sf in the question assuming 18 is in fact 18.0, so the number you should give your answer to)
The uneven heating of the earth
