A student moves a horseshoe magnet by hand across a tabletop to varying distances from an electromagnet, as shown in the diagram above. The student records observations of how much force it takes to hold the horseshoe magnet in place at each distance. Observations

• 20 cm: no force required
• 10 cm: small force required; easy to hold in place
• 5 cm: large force required; very difficult to hold in place

What do the student's observations demonstrate?

A. The electromagnet's magnetic field becomes stronger as more magnetic material is added to it.

B. The electromagnet's magnetic field is stronger as more turns are added to the wire on the nail.

C. The electromagnet's magnetic field becomes stronger as a small amount of strength is added to the system by the battery.

D. The electromagnet's magnetic field is stronger close to it than far from it.

The inside surface of a cylindrical-shaped cave of inner diameter 1.0 m is continuously covered with a very thin layer of water. the cave is very long and it is open on both ends. the water on the cave surface is at a constant temperature of 15.5 °c. the cave is constantly exposed to wind such that 15.5 °c air flows through the cave at 4.5 m/s. the kinematic viscosity of the air is 14.66 x 10-6 m2/s and the molecular diffusion coefficient of water vapor in the air is 0.239 x 10-4 m2/s. because the cave diameter is so large, the flow of wind down the length of the cave, in the x direction, can be treated like it is external flow and the cave surface can be approximated as flat where appropriate. calculate the x value, in a) the transition to turbulent flow occurs at rex meters, where the air flow transitions from laminar to turbulent along the inside surface of the cave b) calculate the x value, in meters, where the bulk steady state concentration of water vapor in the air flowing in the cave is 10% of the saturation concentration. assume the air at the surface of the water layer is 100% saturated with water vapor. assume the wind entering the cave contained no moisture before it entered the cave. take into account the transition from laminar to turbulent flow when solving part b

Which statements did both aristotle and ptolemy assume? check all that apply.

If a person whirls an object horizontally in a counter clockwise direction and the string breaks at point p as shown, what path will the rock follow?