A young girl with mass 40.0 kg is sliding on a horizontal, frictionless surface with an initial momentum that is due east and that has magnitude 90.0 kg · m/s. Starting at ?t? = 0, a net force with magnitude ?F? = (8.20 N/s)?t? and direction due west is applied to the girl. (a) At what value of? ? does the girl have a westward momentum of magnitude 60.0 kg · m/s? (b) How much work has been done on the girl by the force in the time interval from ?t? = 0 to the time calculated in part (a)? (c) What is the magnitude of the acceleration of the girl at the time calculated in part (a)?

Making Sense of the Universe: Chapter 3 4.1 Describing Motion: Examples from Daily Life How do we describe motion Terms used to describe motion: velocity, acceleration, and momentum. Speed – how far something will go in a certain amount of time. Velocity – the speed and direction of something. Acceleration – when there is a change in speed or direction. Acceleration is not just an increased speed, in science you are accelerating when you are slowing down and speeding up. Also, when you turn around means a change in direction, which therefore means a change in velocity. Galileo pointed out that gravity accelerates all objects by the same amount of time. Air resistance would change the rate of a feather and a rock fall