Which x-t graph goes with the v-t graph? (When t = 0 s, x = –10 m.) A B C D 10/2/15 1Oregon State University PH 211, Class #3

Do these two graphs agree? Work with a neighbor and decide. If these graphs agree, prepare to defend their accuracy. If they don’t agree, choose one to “fix” so that it agrees with the other. 10/2/15 2Oregon State University PH 211, Class #3

10/2/15 Oregon State University PH 211, Class #33 Two cars are traveling in the same direction in parallel lanes on a straight, level freeway. You are standing by the side of the road, observing them. As they pass your position, they happen to be alongside one another, but Car B is just passing Car A, too. From that moment on, Car B slows at a steady rate, while Car A maintains a constant speed. Draw both cars’ motions together on graphs of position, velocity and acceleration.

Which statement is true? 1.At time t B, both cars have the same velocity. 2.Both cars speed up all the time. 3.Both cars have the same velocity some time before t B. 4.Somewhere on the graph, both cars have the same acceleration 10/2/15 4Oregon State University PH 211, Class #3

10/2/15 Oregon State University PH 211, Class #35 Same situation as before: As they pass your position, the two cars happen to be alongside one another, but Car A is just passing Car B, too. Then, from that moment on, Car A maintains a constant speed, while Car B goes steadily faster. Draw both cars’ motions together on graphs of position, velocity and acceleration.

10/2/15 Oregon State University PH 211, Class #36 A closer look at acceleration What are the SI units of acceleration? m/s 2 Where does this come from? From the definition: a =  v/  t, which has units of (m/s)/s, or m/s 2. What does this mean? Suppose a car is moving eastward, initially at 30 m/s (about 65 mph) but is accelerating to the west at, say, 10 (m/s)/s. What is happening? As each second goes by, what is its velocity?

10/2/15 Oregon State University PH 211, Class #37 An object has a constant negative acceleration. 1.Its velocity must be positive. 2.Its velocity must be negative. 3.Its speed must be decreasing. 4.Its velocity is becoming more negative (or “less positive”). 5.Its velocity can never become zero. (What common, everyday event would demonstrate which of these is the true statement?)