Motion: Changing Position

Motion Most of physics is concerned with motion To understand the hows (forces) and whys (energy) of motion, we need to be able to describe motion How many basic spatial dimensions? Three (up/down, left/right, in/out) Time is often considered a 4th dimension (large distances are measured in light years)

Definition Motion is any change in position. As you can’t go backward in time, there is no negative motion. Moving back to where you came from is negative direction, but not negative motion.

Describing Motion Albert Einstein demonstrated that all motion is relative (must be compared to something) To accurately and completely describe motion, a frame of reference must be chosen A frame of reference is a system of objects that are not moving with respect to one another Example: A flea is standing on a dog’s ear. The dog is running. Is the flea moving? Answer: Depends on the frame of reference chosen Relative Motion – movement in relation to a frame of reference

Describing Motion If an object is in motion, then its position has changed relative to a frame of reference Distance – how far an object travels on a path Scalar – quantity that only has a magnitude When adding scalars, direction doesn’t matter Displacement – straight-line distance from start point to end point Vector – quantity that has magnitude AND direction When adding vectors, direction matters

Speed Speed: The rate at which something moves over a given distance. General formula for speed: Speed = distance / time Abbreviations commonly used: d = distance t = time v = speed s = d/t d s t

How far does a police car go if it travels 40 mph for 6 hours? D(m) ? S (m/h) T (h) 40mph X 6 h = 240 miles

Velocity Velocity describes speed and direction of motion It is a vector quantity Direction is very important Velocity = displacement time Ex: A car might have an average speed of 65 mi/h but this is not the car’s velocity. If the car was heading east, the velocity would be 65 mi/h east Ex: If a car maintains a constant speed of 200 km/h around an circular track, the speed never changes, but the velocity is constantly changing!

Average Speed Average speed is the total distance traveled by an object divided by the total time taken to travel that distance.  It takes 3 hours to drive 120 miles. (We stopped for gas along the way.) Average speed? 120 miles = 40 miles/hour 3 hours

Instantaneous Speed Instantaneous speed is an object's speed at any given instant.

Describing Motion If you walked east 500 m and then west 500 m, you have walked a distance of 1,000 m (500 m + 500 m) But east and west are opposites of one another so, consider east to be positive (+) and west to negative (-) This means the displacement is 0 m (+500 m + -500 m). (You’re back where you started.) Displacement of 2 or more vectors was found by a process called vector addition which gives the resultant vector

Velocity is a vector quantity that includes both speed and direction. The car is driving 50 km/h east.

VECTORS A vector represents a speed or a force (a push or a pull) of a certain magnitude (size) and direction. For example an vector pointing is said to be pointing east.

Vectors Magnitude: Tells how large the quantity is. Scalar quantities have magnitude only. (Ex: 5) Vector quantities both magnitude and direction. (Ex: 5 northeast)