121. Physics Tuesday January 22, 2008.
The bubble chamber picture of the rst omega-minus. An incoming K- meson interacts with a proton in the liquid hydrogen of the bubble chamber and produces an omega-minus, a K0 and a K+ meson which all decay into other particles. Neutral particles which produce no tracks in the chamber are shown by dashed lines. The presence and properties of the neutral particles are established by analysis of the tracks of their charged decay products and application of the laws of conservation of mass and energy.
Credit: Brookhaven National Laboratory, NY, USA http://www.bnl.gov/bnlweb/history/Omega-minus.asp
Frank L. H. Wolfs
Department of Physics and Astronomy, University of Rochester
Physics 121. Tuesday January 22, 2008.
Today's topics:
Physics 121 website, and homework Introduction to PRS Chapter 2: Describing One Dimensional Motion:
Position Velocity Acceleration
Frank L. H. Wolfs
Department of Physics and Astronomy, University of Rochester
Physics 121. Course Information.
Our web site provides up-to-date course information:
All course handouts are posted in pdf format on our web site. The web site contains links to workshop and laboratory information. The web site contains a link to our homework system and homework solutions (the solutions are password protected). The web site contains links to practice exams and their solutions. All lecture presentations, including audio recordings, will be available on the web site shortly before or after class. http://teacher.pas.rochester.edu/phy121/Phy121HomePage.html
Frank L. H. Wolfs
Department of Physics and Astronomy, University of Rochester
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Physics 121. Course Information.
Additional comments:
Homework set # 0 is now available; it is due on Saturday 1/26 at 8.30 am. This set is to make you familiar with WeBWorK and does not count toward your nal grade. Make sure you verify that you can log in to WeBWorK. Homework set # 1 will be available later this week and is due on Saturday 2/2 at 8.30 am. This set will be the rst set that counts towards your nal grade.
Frank L. H. Wolfs
Department of Physics and Astronomy, University of Rochester
Introduction to the Personal Response System (PRS).
Frank L. H. Wolfs
Department of Physics and Astronomy, University of Rochester
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.
Frank L. H. Wolfs Department of Physics and Astronomy, University of Rochester
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Describing motion in one dimension.
When we limit ourselves to pure translational motion, we in general can describe the motion in terms of three parameters:
The position x(t): units m. The velocity v(t): units m/s. The acceleration a(t): units m/s2.
x < 0 x = 0 x > 0
To specify the position x of an object we need to dene the origin (the point where x = 0 m) of our coordinate system.
Frank L. H. Wolfs Department of Physics and Astronomy, University of Rochester
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.
Frank L. H. Wolfs Department of Physics and Astronomy, University of Rochester
Position.
Do we understand position versus time graphs and do we really understand the difference between distance traveled and displacement? Lets see: Concept Tests 2.1 and 2.2.
Frank L. H. Wolfs
Department of Physics and Astronomy, University of Rochester
3
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 dened 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.
Frank L. H. Wolfs Department of Physics and Astronomy, University of Rochester
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 is does not depend on the sign of the position. The speed and velocity of an object are often confused. The speed of an objective the is magnitude of the velocity of the object. It is thus always positive!
Frank L. H. Wolfs
Department of Physics and Astronomy, University of Rochester
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..
Frank L. H. Wolfs Department of Physics and Astronomy, University of Rochester
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Velocity.
Understanding velocity. Do you get it?
Concept Tests 2.3, 2.6, and 2.7
Frank L. H. Wolfs
Department of Physics and Astronomy, University of Rochester
Acceleration.
The acceleration of an object is dened 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.
Frank L. H. Wolfs
Department of Physics and Astronomy, University of Rochester
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!
Frank L. H. Wolfs Department of Physics and Astronomy, University of Rochester
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Constant acceleration.
Many important physics effects involve motion with constant acceleration. Constant acceleration implies that
a =
dv = constant dt
In this case, the velocity will have a linear dependence on time:
v(t) = v 0 + a t
Frank L. H. Wolfs Department of Physics and Astronomy, University of Rochester
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:
x(t) = x 0 + v 0 t +
1 2 at 2
An important case of constant acceleration, is the vertical motion of objects under the inuence of the gravitational force.
Frank L. H. Wolfs Department of Physics and Astronomy, University of Rochester
Constant acceleration.
In order to fully dene 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.
a =
dv = constant dt
v(t) = v 0 + a t
1 2 at 2
x(t) = x 0 + v...