Chapter 3 Description of Motion
Why are there more images of the ball in Figure 4 than in Figure 3?
Δt represents a time interval t2 – t1 .
Suppose a strobe light flashes 50 times in one second. Calculate the time interval between two successive flashes.
If the motion of the ball shown in Figure 4 was made with a flash operating at 50 flashes per second what would the photograph look like?
When is a small value of Δt important in analyzing the motion of an object?
page 3-5 Displacement Vectors
What questions do you have on Figure 6?
page 3-7 A Coordinate System
Remind me to throw a ball in class.
Compare Figure 8 and Figure 10. What would be good labels for the axes in Figure 10?
page 3-8 Manipulation of Vectors
Review Chapter 2 if necessary.
page 3-9 Measuring the Length of a Vector
Measure the lengths of So S1 S2 and S3
Measure the length of So + S1 + S2 + S3.
How does this length compare the sum of the separate lengths you measured above?
Why?
page 3-11 Coordinate System and Coordinate Vectors
Where do you start the line as you draw a coordinate vector?
Analysis of Strobe Photographs
You know that the best description of the motion is produced by using a small value of Δt. What difficulty is created in doing graphical work with a small value of Δt?
Velocity
Why does a long displacement vector indicate a high speed?
(To be done in class) Who drives at least 4 hours when driving to Midland Lutheran College?
How far do you drive? What time is required for the trip?
Let’s calculate the average speed (total distance traveled/time required)
How does distance differ from displacement?
page 3-12
average velocity V = displacement / time required
Is V a scalar or a vector?
Figure 10 shows displacement vectors. What is the direction of the average velocity vector for each time interval?
I can get a little more picky than the book regarding the words speed and velocity. When is the average speed equal to the magnitude of the average velocity? (Hint: Consider the words distance and displacement.)
When is the average speed NOT equal to the magnitude of the average velocity?
What questions do you have after reading the last paragraph on pae 3-12b?
What is the magnitude of the average velocity for S3? The value of Δt is 0.1 seconds.
page 3-13b Acceleration
List two characteristics of the motion of an object that has constant velocity. Don’t use the word acceleration.
The measure of the rate of change of velocity is called acceleration. The average acceleration is found by dividing the finite quantities D V and D t.
aavg = D V/ D t
List two characteristics of the motion of an object that has a non-zero value for its acceleration.
Is it possible for your body to sense acceleration?
page 3-15a I think the author is stating that many people associate acceleration with only a change in speed. In fact, acceleration is present when the magnitude or direction of velocity is changing.
page 3-15b
The Acceleration Vector
What questions do you have on this page?
page 3-16
Does equation 4 produce the average acceleration vector?
What is the direction of the acceleration vector in Figure 22?
Projectile Motion
What error do you see in the derivation of a2?
page 3-17a We will do exercise 5 in class.
Uniform Circular Motion
Imagine a spot on the edge of a CDROM. Describe the path of the spot as the CDROM spins.
Demo I will swing a rubber stopper, attached to a string, in a circle above my head.
What is constant in the motion?
What is changing?
Is the acceleration of the stopper zero or non-zero?
In class, you will see a diagram that is similar to Figure 24.
Why doesn’t dividing S2 - S1 by D t2 change the direction of S2 - S1 ?
Make your own sketch of the subtraction of two more successive displacement vectors. What is the direction of the net vector?
Describe the direction of the acceleration vector for uniform circular motion.
*What is the meaning of the word "centripetal?" You may have to use a dictionary.
Magnitude of the Acceleration for Circular Motion
The PHY161 students may derive the result. PHY151 students are not responsible for the derivation.
aC = V2/r
Examples will be worked in class.
page 3-20 An Intuitive Discussion of Acceleration
If I let go of the string connected to the rubber stopper while it is moving in uniform circular motion, what is the future path of the rubber stopper?
Why do the acceleration vectors in Figure 27 have the same length?
From your knowledge to date in this course what is the relationship between the force acting on an object and the acceleration of the object?
page 3-21 Acceleration Due to Gravity
Most of the time we will ignore air resistance.
We may watch a video that shows that objects with different mass take the same time to fall the same distance when air resistance can be ignored? Name a place humans have been where air resistance plays no role in the motion of an object.
At sea level on the surface of the earth the approximate value of the acceleration due to gravity is 980 cm/s2. This value is often represented by the symbol, g. We will use this value for problems that involve motion near the surface of the earth. What is the direction of g?
Does g have any component in the horizontal direction?
Does g have any effect on the horizontal motion of an object?
Does g change its magnitude as you move away from the center of the earth?
Is the acceleration due to gravity 980 cm/s2 on the moon or mars?
There will be more descriptions of gravity in Chapter 8.
page 3-22 Answer the questions in Exercise 7.
Projectile Motion with Air Resistance
page 3-23 What is the effect of air resistance on the range of a projectile?
page 3-24 Instantaneous Velocity
Describe the difference in instantaneous velocity and average velocity.
From now on, when V is used it represents the instantaneous velocity.
page 3-26 Instantaneous Velocity from a Strobe Photograph
What questions do you have on Figure 33?
Equation 17 and Exercise 9 show two important equations of motion.
I will provide a handout that derives the four equations of motion.
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