measuring motion ch5.1 8th
DESCRIPTION
Information obtained from: Holt Science and Technology: Physical Science. New York: Henry Holt & Co, 2007. Print.TRANSCRIPT
Chapter 5 Section 1
Describe the motion of an object by the position of the object in relation to a reference point.
Identify the 2 factors that determine speed. Explain the difference between speed and
velocity. Analyze the relationship between velocity and
acceleration. Demonstrate that changes in motion can be
measured and represented on a graph.
Between the 16th and 18th centuries
Basis for modern science: Ideas from:
Copernicus Galileo Descartes Sir Isaac Newton (1642-1727)
Describe your position in the classroom using a reference point and a set of reference directions. Write your answer on a sheet of paper…
Watch an object in motion…
You are observing the object in relation to another object that appears to stay in place.
The object that appears to stay in place: Reference Point
When an object changes in position over time relative to a reference point: Motion
North, South, East, or West
Earth’s surface, trees, buildings…
Moving objects can also be reference points Ex: Hot-air balloon relative to a flying bird
http://image.tutorvista.com/content/motion/frame-of-reference-example.jpeg
Speed: the distance traveled by an object divided by the time taken to travel that distance.
SI Unit for speed: meters per second (m/s)
Other common units: km/h, ft/s, mi/h
Most of the time, objects do not travel at a constant speed
It is useful to calculate average speed
average speed = total distance/total time
What is your average speed if you take 0.5h to jog 4,000 m?
If the average speed of a car is 110km/h, how long will it take the car to travel 715km?
8,000 m/h
6.5h
Distance vs. Time
Distance is plotted on the y-axis
Time is plotted on the x-axis
Straight, diagonal line indicates constant speed
Slope of the line is the average speed
http://www.golfranger.co.uk/images/distance_time.gif
Page 120 of text. Calculating average speed.
Go through Step 1 & 2 together
Try #1-3 ‘Now it’s Your Turn’
Velocity: the speed of an object in a particular direction
Velocity must include a reference direction (different from speed!)
Example: 600km/h south, not 600km/h.
Rate of change of an object’s position
An object’s velocity is constant only if it’s speed and direction do not change
Constant velocity is always motion in a straight line
An object’s velocity changes if it’s speed or direction changes
Change in velocity: Car traveling 60mi/h north speeds up to
70mi/h north
Change in velocity: Car traveling 60 mi/h north, stays at the
same speed but begins to travel east
Combine velocities to find the resultant velocity
Text example pg. 121 Fig. 4 A person’s resultant velocity on a moving bus
15 m/s east + 1 m/s east – 16 m/s east 15 m/s east – 1 m/s west = 14 m/s east
Consider these statements (true/false?) If you slow down on your bicycle, you
accelerate. (T/F?)
If you ride your bicycle at a constant speed, you cannot accelerate. (T/F?)
Changing the speed & changing direction of your bicycle are both examples of acceleration. (T/F?)
Acceleration: the rate at which velocity changes
Velocity changes if speed changes, direction changes, or if both change.
Therefore, an object accelerates if any of these changes occur
Positive acceleration: increase in velocity
Negative acceleration: decrease in velocity
How fast velocity changes = acceleration
Average acceleration = final velocity – starting velocity / time is takes to change velocity
a = v2 - v1
t
Units: meters/sec/sec or m/s²
http://www.arocspeed.com/images/img-acceleration-bolt.jpg
Try these… You are riding your bike at 9 km/h. Ten
minutes later, your speed is 6km/h.
You ride your bike around the block at a constant speed of 11 km/h.
You ride your bike in a straight line at a constant speed of 10 km/h.
Calculating acceleration: A plane passes over point A at a velocity of
240 m/s north. Forty seconds later, it passes over point B at a velocity of 260 m/s north. What is the plane’s average acceleration?
http://www.antonine-education.co.uk/Physics_AS/Module_2/Topic_1/graph_1.gif
Y-axis: velocity (m/s) X-axis: time (s)
Increasing velocity with time: segment 0-A
Constant speed: A-B Decreasing velocity with time: B-C
http://www.revisioncentre.co.uk/gcse/maths/travelgraph2.gif
http://www.science-class.net/Notes/images_7th_notes/dist_time_graph.gif
An object traveling in a circular motion is always changing direction.
Because its velocity is always changing, it is accelerating!
Centripetal acceleration
What distinguishes the measurement of speed from that of velocity and acceleration?
What is centripetal acceleration?
How do you calculate average speed? Acceleration?