Which statements describe dark matter? Select two options.

It is invisible.
It is the same as dark energy.
It is well understood by scientists.
It does not emit electromagnetic radiation.
It can be observed by shifts in red and blue light.

Describe the path of a ray that approaches a mirror parallel to the principal axis.

According to newton’s first law of motion, which force is expected to cause a body to accelerate?

Asheet of steel 1.5 mm thick has nitrogen atmospheres on both sides at 1200oc and is permitted to achieve a steady-state diffusion condition. the diffusion coefficient for nitrogen in steel at this temperature is 6 x 10- 11 m2 /s, and the diffusion flux is found to be 1.2 x 10-7 kg/m2 -s. also, it is known that the concentration of nitrogen in the steel at the high-pressure surface is 4 kg/m3 . how far into the sheet from this high-pressure side will the concentration be 2.0 kg/m3 ? assume a linear concentration profile.