a body of mass 4kg is on the point of slipping down the plane inclined at 30degrees. what force parallel to the plane will just move it up the plane?

Awave is produced in a rope. the wave has a speed of 33 m/s and a frequency of 22 hz.

Modern physics a photon emitted from an excited hydrogen atom has an energy of 3.02 electron volts. which electron energy-level transition would produce this photon? a. n=1 to n=6 b. n=2 to n=6 c. n=6 to n=1 d. n=6 to n=2 i chose b but the correct answer is d can someone tell me why? and what's the difference?

Which diagram correctly demonstrates the various forces acting on a ball moving horizontally with some speed?