For a van de graaff generator to acquire a charge on its surface what must happen?
a. positive and negative charges must be exactly equal in proportion to each other and remain so for a period of time.
b. an uncharged object must be placed nearby.
c. the metal surface must stay neutral for as long as possible.
d. positive and negative charges must be separated inside the device and one type of charge must be carried upwards to the metal sphere.

On a horizontal frictionless surface, a small block with mass 0.200 kg has a collision with a block of mass 0.400 kg. immediately after the collision, the 0.200 kg block is moving at 12.0 m/s in the direction 30° north of east and the 0.400 kg block is moving at 11.2 m/s in the direction 53.1° south of east. use coordinates where the +x-axis is east and the +y-axis is north.(a) what is the total kinetic energy of the two blocks after the collision (in ) what is the x-component of the total momentum of the two blocks after the collision? (indicate the direction with the sign of your ) what is the y-component of the total momentum of the two blocks after the collision? (indicate the direction with the sign of your answer.)