Motion

PH 338

Time 1 second. Stop & Reset

Time 10 seconds. Stop & Reset

What is the shortest amount of time you can measure?

Start & StopReset

Practice using a stopwatch

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

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

Road Trip!

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

• Example: 32 km for 2 hours, then 13 km for 1 hourTotal distance = 32 km + 13 km = 45 kmTotal time = 2 h + 1 hr = 3 hrAverage speed = 45 km ÷ 3 hr

= 15 km/hr

Traffic!

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 WestA 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

Dis

tanc

e (m

)

Time (min)5

50

Slope = slant of line

X

y

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. The object is

accelerating.

speed

Time

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 (sunrise) line means the object is moving at a constant speed

• A downward (negative) sloping line means the object is slowing down (decelerating)