Motion Key Ideas

Reference point The position of an object must be defined in relation to some standard reference point. This means that you pick a point (any point) and every movement is measured from that point, in a positive or negative direction.

Constant Speed An object which is not speeding up or slowing down is moving at a constant speed. Examples: the speed of light always moves at - 299,792,458 m/s (in a vacuum like outer space) a train once it is moving at 50mph This motorcycle has a constant horizontal speed (it moves the same distance sideways each unit of time.)

Distance vs Time Graph Constant Speed An object moving at constant speed (not speeding up or slowing down) will have a linear relationship, like this: To calculate speed simply find the slope. 1)Pick any two points on the graph 2)Subtract the distance values (this is the rise) 3)Subtract the time values (this is the run) 4)Divide the rise by the run for speed! = 20m = 5s 20 ÷ 5 = 4m/s

Speed vs Time Graph for Constant Speed Constant speed is a flat horizontal line since speed is not changing. Reading the graph: This object is moving at a constant speed of 20m/s

Average Speed (sometimes called “rate”) Everyday objects do not normally maintain constant motion, the speed of an object can change while it is traveling. In order to make life simple we use “Average Speed” to describe the motion of an object. To Calculate Average Speed: Distance Taveled ÷ TimeTaken or Distance Time Common Units: mph = miles per hour cm/s = centimeters per second Km/h = Kilometers per hour

If a car goes 15 meters in 3 seconds, what is its average speed? 15m/3s = 5 m/s If a bike takes 4 hours to travel 2 miles, what is its average speed? 2mile/4hr = 0.5mph If a snail travels 10 inches in 120 seconds, what is its average speed in inches per MINUTE? 120s = 2 minutes so… 10in/2min = 5 in per min

Calculating Average Speed from a TABLE To find the average speed from a table, you still only need to divide distance traveled by time elapsed. D = Final Distance-Initial Distance = D f -D i T Final time - Initial time t f - t i For example average speed from 2 to 6 seconds would be 35 m - 5 m = 30 m = 7.5 m/s 6 s - 2 s 4 s TimeDistance 0 s0 m 2 s5 m 4 s15 m 6 s35 m

Rearranging the formula or you can remember “dirt” d=rt distance=rate x time How far will a car travel in 7 hours if it is moving at an average speed of 60 miles per hour? D=st 60 mph x 7 hr= 420 miles How long will it take you to drive to LA, 90 miles away, if you are traveling at an average speed of 45 miles per hour (due to traffic)? T=d/s t= 90 miles/ 45 mph = 2 hrs

Velocity Velocity is the speed and direction of an object. 30 mph To find velocity you use the average speed and state the direction, so the velocity of this car is 30 mph to the west.

Constant Velocity If an object has constant velocity then its speed AND direction are staying the same. Constant Velocity means the object is moving in a straight line at constant speed!!

Rearranging the formula (again) Instead of “s” for speed you may have “v” for velocity, it works the same! How far will a car travel in 7 hours if it is moving at an average speed of 60 miles per hour? D=vt 60 mph x 7 hr= 420 miles How long will it take you to drive to LA, 90 miles away, if you are traveling at an average speed of 45 miles per hour (due to traffic)? T=d/v t= 90 miles/ 45 mph = 2 hrs V

Is the rate the object’s velocity changes. An object is accelerating if it is speeding up, slowing down, AND/OR changing direction Acceleration Speeding Up Slowing Down Changing Direction This base jumper is accelerating in the downward direction.

Objects that are Speeding up Distance vs. Time Graph For every unit of time the object moves an increasingly far distance. For example in this graph the car between 2 s and 4 s goes about 35 m (from 15 to 50 m) and between 6 s and 8 s the car goes 80m (from 100m to 180m). The car is going faster! It is accelerating. The Distance vs. Time graph is nonlinear (not a straight line.) The distance vs. time graph is an upward facing parabola.

Objects that are Slowing Down Distance vs. Time Graph For every unit of time the object travels a smaller distance. For example in this graph the car between 5 s and 10 s moved about 35 m (from 45 to 80 m) and between 15 s and 20 s the car moves 15 m (from 105 m to 120 m). The car is slowing down! Slowing down is another type acceleration. The Distance vs. Time graph is nonlinear (not a straight line.) The distance vs. time graph for a slowing down object is an sideways facing parabola.

Speed vs. Time Graphs Constant speed- the speed on the graph stays the same. (it is a flat horizontal line. The slope of the line is zero Speeding Up- the speed on the graph stays the same. (the slope of the line is positive) Slowing Down- the speed on the graph decreases. (the slope of the line is negative)