9.) A 40-kg bookshelf requires a 200 N force to begin its motion. Once moving. It only requires a force of 50 N.
a. What is the normal force?
b. Determine the static coefficient of friction

Which of the following is an example of units for speed

An alpha particle with kinetic energy 12.5 mev makes a collision with lead nucleus, but it is not "aimed" at the center of the lead nucleus, and has an initial nonzero angular momentum (with respect to the stationary lead nucleus) of magnitude l = p0 b, where p0 is the magnitude of the initial momentum of the alpha particle and b=1.40×10−12 m. (assume that the lead nucleus remains stationary and that it may be treated as a point charge. the atomic number of lead is 82. the alpha particle is a helium nucleus, with atomic number 2.) what is the distance of closest approach?

What caused magnetic fields? what happens if you keep breaking a magnet in two until you reach a single atom?