Chapter 11: Motion What are some things we need to know to describe something or someone’s motion?

How fast am I walking? How fast would I be walking according to someone who is just sitting in one spot looking at the Earth? Roughly 1,000 mph! How fast would I be walking according to someone sitting on the Sun? Nearly 67,000 mph! What about compared to someone at the center of our Milky Way galaxy? Roughly 420,000 mph! So how fast am I really walking? 3, 1,003, 67,003, or 420,003 mph? All speeds are correct….it all just depends on your frame of reference

Frame of Reference A system of objects that are NOT moving with respect to one another What is the frame of reference when you say that I am walking across the classroom at 3 mph? Sitting stationary in your desk How can 2 people look at the same object, and only one of them see the object as moving? Choosing a meaningful frame of reference allow you to describe motion in a clear and meaningful way What is the frame of reference for the person in the red shirt? The train What would a cop’s frame of reference be for catching you speeding? the ground

Mamma Meacham’s place is 83 miles from Lake Crystal “83 miles” tells you what? The distance from Lake Crystal to Mamma Meacham’s Mamma Meacham makes unreal taco salad and your in the mood for some taco salad…..do you have enough information to get to Mamma Meacham’s? If not, why not? You have no idea which direction to go! Distance only tells you the length of a path taken between 2 points Distance is a scalar quantity: fully described by a magnitude (number value) and no direction involved

So if distance alone can’t get us to Momma Meacham’s then what can? Mamma Meacham lives 67.5 miles northeast of Lake Crystal What is different about this description? Involves direction! This is known as displacement Displacement is the direction from the starting point and the length of a straight line from start to finish Vector quantity: involves magnitude & direction

Distance vs. Displacement Is it possible for distance and displacement to have the same magnitude? Yes, but only if the path taken from start to finish is a straight line Because most things in life do not travel in perfectly straight lines, it is most likely that displacement will have a smaller value than distance

Vectors A vector is a quantity that has both magnitude and direction. It is represented by an arrow. The length of the vector represents the magnitude and the arrow indicates the direction of the vector. Remember, displacement is a vector....distance is not Blue and orange vectors have same magnitude but different direction. Blue and green vectors have same direction but different magnitude. Blue and purple vectors have same magnitude and direction so they are equal. Two vectors are equal if they have the same direction and magnitude (length).

Combining Vectors When motion is in a straight line, vectors add and subtract easily 4 km 2 km 4 km + 2 km = 6 km 4 km 2 km 4 km – 2 km = 2 km

Displacement that isn’t along a straight path (p. 331) Home School Distance? 7 blocks Displacement? 5 blocks Resultant vector