Physics 121: Electricity & Magnetism – Lecture 2 Electric Charge
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![Page 1: Physics 121 Today's topics: Physics 121 website, workshop signup, and homework Review of course survey Introduction to PRS Chapter 2: Describing One Dimensional.](https://reader038.fdocuments.in/reader038/viewer/2022110207/56649d445503460f94a20e09/html5/thumbnails/1.jpg)
Physics 121
• Today's topics:
• Physics 121 website, workshop signup, and homework
• Review of course survey
• Introduction to PRS
• Chapter 2: Describing One Dimensional Motion:
• Position
• Velocity
• Acceleration
![Page 2: Physics 121 Today's topics: Physics 121 website, workshop signup, and homework Review of course survey Introduction to PRS Chapter 2: Describing One Dimensional.](https://reader038.fdocuments.in/reader038/viewer/2022110207/56649d445503460f94a20e09/html5/thumbnails/2.jpg)
Physics 121, Spring 2005
• Frank Wolfs Adaptations
• Peer Instruction (PRS)
![Page 3: Physics 121 Today's topics: Physics 121 website, workshop signup, and homework Review of course survey Introduction to PRS Chapter 2: Describing One Dimensional.](https://reader038.fdocuments.in/reader038/viewer/2022110207/56649d445503460f94a20e09/html5/thumbnails/3.jpg)
Physics 121
• Additional comments:
• Homework: every problem ending in 1 (i.e., 1,11,21,31…) and every other odd problem from the general problems section at the end of the problems section (e.g., from chapter 2 problems 69, 73, 77, 81, 85, 89). After walking to school both ways up hill in three feet of snow in the summer I did all odd problems in the text. All of these problems have answers in the back.
• Homeworks will be due Fridays at 2:30 p.m. (homework locker in B&L at bottom of stairs). No late homeworks accepted, except if you perish from the bird flu!
• Chapter 2 due this Friday. I bet you can’t wait to get out of class and start!!! Hold your horses though, we need to finish the lecture.
![Page 4: Physics 121 Today's topics: Physics 121 website, workshop signup, and homework Review of course survey Introduction to PRS Chapter 2: Describing One Dimensional.](https://reader038.fdocuments.in/reader038/viewer/2022110207/56649d445503460f94a20e09/html5/thumbnails/4.jpg)
Physics 121Summary Survey: Purpose of the Lecture
is to is not toBe inspiring/stimulating Cover all you are expected to knowClarify the book Practice problemsExplain confusing issues Introduce the materialMake you think criticallyGive you lots to think aboutSpark further interest in the material
![Page 5: Physics 121 Today's topics: Physics 121 website, workshop signup, and homework Review of course survey Introduction to PRS Chapter 2: Describing One Dimensional.](https://reader038.fdocuments.in/reader038/viewer/2022110207/56649d445503460f94a20e09/html5/thumbnails/5.jpg)
Physics 121Summary Survey: Purpose of the Book
is to is not toProvide background for lectures Clarify the lecturesBe a resource for detailed explanationsBe a reference and study guideOffer practice problemsTeach problem solving
![Page 6: Physics 121 Today's topics: Physics 121 website, workshop signup, and homework Review of course survey Introduction to PRS Chapter 2: Describing One Dimensional.](https://reader038.fdocuments.in/reader038/viewer/2022110207/56649d445503460f94a20e09/html5/thumbnails/6.jpg)
Introduction to PRS
• Each PRS has a unique ID that will be matched to your name.
• For most questions there will be several possible answers. You can record your answer by pressing the corresponding key on your PRS.
• You can indicate your level of confidence by pressing H or L before pressing the answer key.
• Quizzes will be recorded for credit. Most concept tests are not.
• Let’s try it out!
Low confidence
High confidence
![Page 7: Physics 121 Today's topics: Physics 121 website, workshop signup, and homework Review of course survey Introduction to PRS Chapter 2: Describing One Dimensional.](https://reader038.fdocuments.in/reader038/viewer/2022110207/56649d445503460f94a20e09/html5/thumbnails/7.jpg)
Describing Motion in One Dimension
• There are different ways in which we can describe the motion of an object.
• In Chapter 2 we will focus on describing motion along a straight line, or one-dimensional motion.
• The direction of motion is not limited to the horizontal direction, but for example can also be in the vertical direction (e.g. free fall).
• One dimensional motion can be simply translational but may also include rotational motion.
![Page 8: Physics 121 Today's topics: Physics 121 website, workshop signup, and homework Review of course survey Introduction to PRS Chapter 2: Describing One Dimensional.](https://reader038.fdocuments.in/reader038/viewer/2022110207/56649d445503460f94a20e09/html5/thumbnails/8.jpg)
Describing Motion in One Dimension
• When we limit ourselves to pure translational motion, we in general can describe the motion in terms of three scalar parameters:
• The position x(t): units m.
• The velocity v(t): units m/s.
• The acceleration a(t): units m/s2.
• To specify the position x of an object we need to define the origin (the point where x = 0 m) of our coordinate system.
x < 0 x > 0x = 0
![Page 9: Physics 121 Today's topics: Physics 121 website, workshop signup, and homework Review of course survey Introduction to PRS Chapter 2: Describing One Dimensional.](https://reader038.fdocuments.in/reader038/viewer/2022110207/56649d445503460f94a20e09/html5/thumbnails/9.jpg)
Position
• Two terms often confused in in describing the motion of an object are:
• Distance traveled: the total distance the object moved during the motion from its starting point to its end point. Note: depends on the exact path followed and is always positive.
• Displacement: the change in the position of an object. Note: depends only on its starting point and its end point.
![Page 10: Physics 121 Today's topics: Physics 121 website, workshop signup, and homework Review of course survey Introduction to PRS Chapter 2: Describing One Dimensional.](https://reader038.fdocuments.in/reader038/viewer/2022110207/56649d445503460f94a20e09/html5/thumbnails/10.jpg)
Position
• Do we understand position versus time graphs and do we really understand the difference between distance traveled and displacement?
• Let’s see: Concept Tests 2.1 and 2.2.
![Page 11: Physics 121 Today's topics: Physics 121 website, workshop signup, and homework Review of course survey Introduction to PRS Chapter 2: Describing One Dimensional.](https://reader038.fdocuments.in/reader038/viewer/2022110207/56649d445503460f94a20e09/html5/thumbnails/11.jpg)
Velocity
• All information about the motion of an object is in principle contained in the time dependence of its position x(t).
• Often it is useful to talk about the velocity v of the object, which is defined as the ratio of the change in position, x, and the change in time t.
• The velocity calculated in this manner is the average velocity over the time interval t.
![Page 12: Physics 121 Today's topics: Physics 121 website, workshop signup, and homework Review of course survey Introduction to PRS Chapter 2: Describing One Dimensional.](https://reader038.fdocuments.in/reader038/viewer/2022110207/56649d445503460f94a20e09/html5/thumbnails/12.jpg)
Velocity
• Some remarks about velocity:
• Velocity can be positive and negative. In our choice of coordinate system, a positive velocity corresponds to motion towards the right, and negative velocity corresponds to motion towards the left.
• The sign of the velocity does not depend on the sign of the position.
• The speed and velocity of an object are often confused. The speed of an objective is the magnitude of the velocity of the object. It is thus always positive!
![Page 13: Physics 121 Today's topics: Physics 121 website, workshop signup, and homework Review of course survey Introduction to PRS Chapter 2: Describing One Dimensional.](https://reader038.fdocuments.in/reader038/viewer/2022110207/56649d445503460f94a20e09/html5/thumbnails/13.jpg)
Frank L. H. Wolfs Department of Physics and Astronomy, University of Rochester
NASCAR trivia: Dayton Testing
• Average Speeds • 1. Dale Jarrett, No. 88 Ford, 48.269 seconds, 186.455 mph
2. Jeff Gordon, No. 24 Chevrolet, 48.456, 185.7363. Randy LaJoie, No. 98 Chevrolet, 48.588, 185.231
• Average Velocities• 0
![Page 14: Physics 121 Today's topics: Physics 121 website, workshop signup, and homework Review of course survey Introduction to PRS Chapter 2: Describing One Dimensional.](https://reader038.fdocuments.in/reader038/viewer/2022110207/56649d445503460f94a20e09/html5/thumbnails/14.jpg)
Velocity
• When the time interval t decreases, the average velocity approaches the instantaneous velocity.
• The velocity of the object is related to the slope of the position versus time graph:
• A positive slope correspond to a positive velocity; a negative slope corresponds to a negative velocity.
• When the slope increases, the velocity increases..
t
txttxLim
dt
dxtv
t
)()()(
0
![Page 15: Physics 121 Today's topics: Physics 121 website, workshop signup, and homework Review of course survey Introduction to PRS Chapter 2: Describing One Dimensional.](https://reader038.fdocuments.in/reader038/viewer/2022110207/56649d445503460f94a20e09/html5/thumbnails/15.jpg)
Velocity
• Concept Tests 2.3 and 2.4: (marathon runner and object dropping)
• considering the motion of two objects simultaneously.
![Page 16: Physics 121 Today's topics: Physics 121 website, workshop signup, and homework Review of course survey Introduction to PRS Chapter 2: Describing One Dimensional.](https://reader038.fdocuments.in/reader038/viewer/2022110207/56649d445503460f94a20e09/html5/thumbnails/16.jpg)
Acceleration
• The acceleration of an object is defined at the ratio of the change of the velocity of an object, v, and the change in time t.
• The acceleration calculated in this manner is the average acceleration over the time interval t.
• The acceleration can be positive or negative depending on whether v > 0 m/s or v < 0 m/s.
t
tvttvLim
dt
dvta
t
)()()(
0
![Page 17: Physics 121 Today's topics: Physics 121 website, workshop signup, and homework Review of course survey Introduction to PRS Chapter 2: Describing One Dimensional.](https://reader038.fdocuments.in/reader038/viewer/2022110207/56649d445503460f94a20e09/html5/thumbnails/17.jpg)
Acceleration
• Some remarks about acceleration:
• Acceleration can be positive and negative.
• In our every day life, we often use the term acceleration when we speed up and deceleration when we slow down. This leads to the assumption that a positive acceleration implies an increase in speed while a negative acceleration corresponds to a decrease in speed. THIS IS ABSOLUTELY WRONG!!!!!!!!!
• Negative acceleration implies that v < 0 m/s. This can be achieved in a number of different ways:
• If v1 = 30 m/s and v2 = 10 m/s: a reduction in speed!
• If v1 = -10 m/s and v2 = -30 m/s: an increase in speed!
![Page 18: Physics 121 Today's topics: Physics 121 website, workshop signup, and homework Review of course survey Introduction to PRS Chapter 2: Describing One Dimensional.](https://reader038.fdocuments.in/reader038/viewer/2022110207/56649d445503460f94a20e09/html5/thumbnails/18.jpg)
Constant Acceleration
• Many important physics effects involve motion with constant acceleration.
• Constant acceleration implies that
• In this case, the velocity will have a linear dependence on time:
a =
dvdt
= constant
v(t) = v0 + at
![Page 19: Physics 121 Today's topics: Physics 121 website, workshop signup, and homework Review of course survey Introduction to PRS Chapter 2: Describing One Dimensional.](https://reader038.fdocuments.in/reader038/viewer/2022110207/56649d445503460f94a20e09/html5/thumbnails/19.jpg)
Constant Acceleration
• When the velocity of an object has a linear dependence on t, the position of the object will have a quadratic dependence on t:
• An important case of constant acceleration, is the vertical motion of objects under the influence of the gravitational force.
x(t) = x 0 + v0 t +
12
a t2
![Page 20: Physics 121 Today's topics: Physics 121 website, workshop signup, and homework Review of course survey Introduction to PRS Chapter 2: Describing One Dimensional.](https://reader038.fdocuments.in/reader038/viewer/2022110207/56649d445503460f94a20e09/html5/thumbnails/20.jpg)
Constant Acceleration
• In order to fully define the motion of an object if we know the acceleration, we need to have more information:
• We need to know v0, which is the velocity of the object at time t = 0 s.
• We need to know x0, which is the position of the object at time t = 0 s.
• Make sure the signs are consistent!
a =
dvdt
= constant
v(t) = v0 + at
x(t) = x 0 + v0 t +
12
a t2
![Page 21: Physics 121 Today's topics: Physics 121 website, workshop signup, and homework Review of course survey Introduction to PRS Chapter 2: Describing One Dimensional.](https://reader038.fdocuments.in/reader038/viewer/2022110207/56649d445503460f94a20e09/html5/thumbnails/21.jpg)
Constant Gravitational Acceleration• Objects moving in the vertical
direction close to the surface of the earth experience a constant gravitational acceleration due to the gravitational force between the earth and the object.
• In the absence of other forces, such as the drag force due to the air, all objects will experience the same acceleration, independent of their mass or shape.
• The gravitational acceleration is g = –9.8 m/s2. The minus sign indicated that the acceleration is directed downwards.
![Page 22: Physics 121 Today's topics: Physics 121 website, workshop signup, and homework Review of course survey Introduction to PRS Chapter 2: Describing One Dimensional.](https://reader038.fdocuments.in/reader038/viewer/2022110207/56649d445503460f94a20e09/html5/thumbnails/22.jpg)
Constant Gravitational Acceleration
Note: velocity and acceleration donot have to be direction in the samedirection!
![Page 23: Physics 121 Today's topics: Physics 121 website, workshop signup, and homework Review of course survey Introduction to PRS Chapter 2: Describing One Dimensional.](https://reader038.fdocuments.in/reader038/viewer/2022110207/56649d445503460f94a20e09/html5/thumbnails/23.jpg)
Measuring the Gravitational Acceleration
• There are many different ways in which we can measure the gravitational acceleration. In order to describe vertical motion we usually use y coordinates.
• Let’s consider an object falling from rest, from the origin of our coordinate system.
• The initial conditions are such that v0 = 0 m/s and y0 = 0 m.
• We conclude that y/t2 equals g/2 and measuring the time required to fall a distance y allows us to calculate g. Let’s do it!
a =
dvdt
= - g
v(t) = v0 -g t
y(t) = y 0 + v0 t -
12
g t2
![Page 24: Physics 121 Today's topics: Physics 121 website, workshop signup, and homework Review of course survey Introduction to PRS Chapter 2: Describing One Dimensional.](https://reader038.fdocuments.in/reader038/viewer/2022110207/56649d445503460f94a20e09/html5/thumbnails/24.jpg)
Quality Control?
• From top to ground t=4 s• States can reach 250 feet
222
2
/816
)250)(/254.0(222
1/00
sms
ftftcm
t
yg
gtsmmy
![Page 25: Physics 121 Today's topics: Physics 121 website, workshop signup, and homework Review of course survey Introduction to PRS Chapter 2: Describing One Dimensional.](https://reader038.fdocuments.in/reader038/viewer/2022110207/56649d445503460f94a20e09/html5/thumbnails/25.jpg)
Understanding the Gravitational Acceleration
• In order to probe your current understanding of gravitational motion let’s work on a few questions.
• Note: these questions will not involve a group discussion.
• Concept Tests 2.5, 2.6, and 2.7 (up and down objects, train, parallel trains)
![Page 26: Physics 121 Today's topics: Physics 121 website, workshop signup, and homework Review of course survey Introduction to PRS Chapter 2: Describing One Dimensional.](https://reader038.fdocuments.in/reader038/viewer/2022110207/56649d445503460f94a20e09/html5/thumbnails/26.jpg)
We are done for today!Please review Chapter 3 before the next class.
Postcard from Mars and TitanCredit: Mars Exploration Rover Mission, JPL, NASA, European Space Agency