What is the difference between each distance traveled and displacement travled

An open cart is moving along a straight frictionless horizontal track. when rain starts falling vertically into the cart, what happens to the speed of the cart?

What does si stand for,when referring to a system of measurement

Abeam of light shining from water to air strikes the surface at an angle greater than 48 degrees and produces total internal reflection. if the incident beam is at a 50 degree angle what angle is the reflected beam 13 25 50 100