The cosmic background radiation is A)
the flux of visible radiation in empty space, contributed by all visible stars in the universe.
B)
the radio noise generated by Earth-bound transmitters, spreading out into space since about 1920.
C)
the beam of atomic nuclei known as cosmic rays that continuously rain down upon the Earth from all directions in space.
D)
low-intensity radio noise, with a 3 K blackbody temperature, almost uniform in intensity in all directions.

Visualize the problem and identify special cases first examine the problem by drawing a picture and visualizing the motion. apply newton's 2nd law, ∑f⃗ =ma⃗ , to each body in your mind. don't worry about which quantities are given. think about the forces on each body: how are these consistent with the direction of the acceleration for that body? can you think of any special cases that you can solve quickly now and use to test your understanding later? one special case in this problem is if m2=0, in which case block 1 would simply fall freely under the acceleration of gravity: a⃗ 1=−gj^.