What does it mean for a substance to have a high reactivity?

Atrain departs from its station at a constant acceleration of 5 m/s. what is the speed of the train at the end of 20s?

The focal length of a relaxed human eye is approximately 1.7 cm. when we focus our eyes on a close up object, we can change the refractive power of the eye by about 16 diopters. (a) does the refractive power of our eyes increase or decrease by 16 diopters when we focus closely? explain. (b) calculate the focal length of the eye when we focus closely.

Compare the horizontal displacement of the two balls as they travel to the ground. what do you observe?