Measuring Motion Ch5.1 8th

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?