Motion PH 338

Time 1 second. Stop & Reset Practice using a stopwatch Start & Stop Time 1 second. Stop & Reset Time 10 seconds. Stop & Reset What is the shortest amount of time you can measure? Reset

1. What is motion? A way of describing how fast something is moving Our definition: Motion is a change in position.

2. What is a reference point? A point motion is measured against Usually a stationary object

3. How do you know if an object is in motion? An object is in motion if it changes position relative to a reference point

4. What is the difference between distance and displacement? Distance is the total length of the actual path between two points. Displacement is the length and direction of a straight line between starting and ending points. Displacement and distance can be the same if the direction is in one direction only.

What is my displacement?

5. What is speed? The distance an object travels per unit of time. miles per hour miles/hour

6. How do you calculate speed? Divide the distance the object traveled by the amount of time it takes to travel that distance.

…some useful speeds Growing fingernails = 4 cm/year Walking speed = 3 miles/hr or 1.3 m/s Driving speed = 60 miles/hr or 26 m/s Earth’s revolution = 30 km/s Speed of light = 300,000 km/s

S = D S = 120 miles S = 60 miles/hour T 2 hours Road Trip! Example: The white car travels120 miles in 2 hours. What is the car’s speed? S = D S = 120 miles S = 60 miles/hour T 2 hours

7. Why would you calculate average speed? Because most moving objects do not move at a constant speed. Think about biking uphill and downhill

…or think of traffic

8. How do you calculate average speed? Average speed = Total Distance Traveled Total Time of Travel

Traffic! Example: 32 km for 2 hours, then 13 km for 1 hour Total distance = 32 km + 13 km = 45 km Total time = 2 h + 1 hr = 3 hr Average speed = 45 km ÷ 3 hr = 15 km/hr

9. What is instantaneous speed? The rate at which an object is moving at a given instant in time. …or what your speedometer says!

10. What is velocity? Speed in a given direction Ex: 20 miles/hour West A storm is moving 25 km/h north. Should you prepare?

11. How can you describe changes in velocity? Change in velocity may be due to change in speed, change in direction, or both!

Acceleration Acceleration is when an object speeds up, slows down or changes direction. Acceleration is a change in velocity.

12. How do you graph speed? By a distance versus time graph! X-axis = time Y-axis = distance Slope = slant of line y Distance (m) 50 X 5 Time (min)

13. How can you interpret graphs of distance versus time? The slope represents speed Slope = slant of line = speed! The steeper the slope the faster the speed! Steep = Fast! Shallow = Slow!

What if it is flat? Flat = Not moving!

Speed-Time Graphs Speed-Time Graphs look much like Distance-Time graphs. Be sure to read the labels! Time is plotted on the X-axis. Speed or velocity is plotted on the Y-axis

A straight horizontal line on a speed-time graph means the speed is constant. This means the speed is not changing over time. How is this different from a distance-time graph?

A positive slope represents an increasing speed A positive slope represents an increasing speed. The object is accelerating. speed Time

A negative slope represents a decreasing speed A negative slope represents a decreasing speed. The object is decelerating. Speed Time

Match the part of the graph to the description Steady acceleration ________ Steady speed _________ Gradual acceleration _________ Steady deceleration _________ Summary: The steeper the line, the greater the acceleration A horizontal line means the object is moving at a constant speed A downward (negative) sloping line means the object is slowing down (decelerating)

Summary The steeper the line, the greater the acceleration. A horizontal (sunrise) line means the object is moving at a constant speed A downward (negative) sloping line means the object is slowing down (decelerating)