Answer:
1920s and 1930s,
Explanation:
it's worth reiterating that helmets were not mandatory. That wouldn't happen until 1943. During the 1920s and 1930s, variations of the leather helmet appeared, but in 1939, the game changed –or at least became safer– when John T. Riddel introduced the first plastic helmet
Answer:
Galena
Explanation:
Galena usually occurs in cubic crystals. If you hit a specimen of galena with a hammer it will shatter into small perfect cubic crystals. It has a metallic luster and a black to dark gray color and streak. Galena has a hardness of about 2.5 on the Mohs hardness scale which is about as hard as your fingernail.
D. physics
Physics is a field of study contributes ideas and concepts to biomechanics.
“Scientific models are often used in laboratories to represent Earth's processes. While field tests and observations are vital, physical, digital, and mathematical models are useful tools in the laboratory. Models provide insight into complex processes that might otherwise be difficult to grasp”
