Uniform Motion

Frame of Reference Motion is described relative to a stationary point. But what is stationary? You? The Earth? The Galaxy? Your Frame of Reference depends on what you define as stationary. (The “stationary” point doesn’t actually have to be stationary)

Sign Conventions Think of the + and – directions on the x-axis and y-axis Up and right are + Down and left are – North and East are + South and West are – N + - + W E - S

Vector vs. Scalar Scalar – only has magnitude (22 kg, 14 m) Vector – has both magnitude and direction (72 m North, 18 m/s at 54°) Change either magnitude or direction and you change the vector.

Distance vs. Displacement Distance (d) is how much ground has passed beneath your feet. Displacement (Δs) is how far you are from where you started, and in what direction you moved. Which is a vector? Displacement

Mr. Avis walks 3 meters east, 2 meters south, 3 meters west and finally 2 meters north. What is his displacement? What total distance did he walk? Zero 10 m

Speed vs. Velocity Speed is a scalar quantity (10 m/s) Velocity is a vector (speed and direction) (10 m/s North) Speed: Velocity:

Average vs. Instantaneous Average velocity is total distance/total time. Instantaneous velocity is the velocity right now.

Acceleration Acceleration is a change in velocity Units are m/s2 change magnitude change direction Units are m/s2 Acceleration is a vector

Speeding Up vs. Slowing Down When the sign on v and a are the same (+a & +v or –a & -v) the object is speeding up. When the sign on v and a are different the object is slowing down.