A 20 kg wagon is pulled with a force of 90 N. Ground friction provides 20 N of force in the opposite direction. What is the weight of the wagon? And why?
20 N
200 N
70 N

Students in an introductory physics lab are performing an experiment with a parallel-plate capacitor made of two circular aluminum plates, each 11 cm in diameter, separated by 1.0 cm. how much charge can be added to each of the plates before a spark jumps between the two plates? for such flat electrodes, assume that value of 3×106n/c of the field causes a spark.