· You know that these situations are wrong, but why are they wrong?!Guided discovery - Investigating Momentum14 March 201207:20When we collide two gliders on the
air track, what happens?
Situation 1: Elastic collision with a stationary glider
Initial
Initial speed of LH glider = ul = 1m/s
Initial speed of RH glider = ur = 0m/s
Final
Final speed of LH glider = vl = 0m/s
Final speed of RH glider = vr = 1m/s
We can represent this graphically as
Initial
Final
Conclusion
· It appears that the speed is "transferred" to the RH glider
Situation 2: Inelastic collision with a stationary glider
Initial
Initial speed of LH glider = ul = 1m/s
Initial speed of RH glider = ur = 0m/s
Final
Final speed of LH glider = vl = 0.5m/s
Final speed of RH glider = vr = 0.5m/s
We can represent this graphically as
Initial
Final
Conclusion
· Speed is conserved in the collision· Total Initial speed = Total Final speed
Situation 3: Head on collision
Initial
Initial speed of LH glider = ul = 1m/s
Initial speed of RH glider = ur = -1m/s
Final
Final speed of LH glider = vl = 0m/s
Final speed of RH glider = vr = 0m/s
We can represent this graphically as
Initial
Final
Conclusion
· Velocity is conserved in the collision· Total Initial velocity = Total Final velocity
Situation 4: Head on collision with different masses
Initial
Initial speed of LH glider = ul = 1m/s
Initial speed of RH glider = ur = -1m/s
Final
Final speed of LH glider = vl = 0m/s
Final speed of RH glider = vr = 0m/s
Problem!
Our previous conclusion that
o Velocity is conserved in the collisiondoesn't hold for this situation!
Why do they move off to the left?
Because the RH glider has twice the mass
What could I change about the LH glider to make both gliders stop after the collision?
o Double the mass (obvious)o Double the initial velocity
We can represent this graphically as
Initial
Final
So something is conserved in the collision, but what is it?
What does the area of the rectangles represent?!
Time to label our axes!
Final Conclusion
· The area of the rectangles are mass x velocity· Momentum = mass x velocity· So momentum is conserved in collisions
Saturday, April 21, 2012
Investigating Momentum
Thursday, March 29, 2012
7.19 and 7.20
·7.19 understand that a chain reaction can be set up if the neutrons produced by one fission strike other U-235 nuclei· 7.20 understand the role played by the control rods and moderator when the fission process is used as an energy source to generate electricity<<Fission for energy.pptm>><<DJFPh110chain2.swf>><<chain reaction animation.swf>><<reaction with control rods animation.swf>>7.17 to 7.20 Plenary Answers13 January 201214:171. What is the process of splitting large nuclei called?Fission
2. How is 235U made to decay inside a nuclear reactor?It is bombarded by a neutron to turn it into 236U which is unstable and rapidly decays
3.What are the generic names for the products of a fission reaction?Two daughter nuclei and either two or three neutrons are produced
4.In what form is energy produced during a fission reaction?Energy is released in the form of Kinetic Energy of the products of the reaction - the two daughter nuclei and the neutrons
5. For a stable chain reaction to occur how many neutrons must, on average, collide with another nucleus of 235U?Exactly one. If the value is less than this the reaction will eventually finish. If the value is more than this the reaction will very rapidly accelerate to explosive rates - utilised in nuclear weapons
6.What is the purpose of the moderator?The moderator slows down the neutrons produced by fission so that they can be captured by other 235U nuclei and thus sustain the chain reaction
7. What do the control rods do?Control rods absorb excess neutrons in the reactor and thereby provide a method for controlling the chain reaction
7.17 and 7.18
A fuel that doesn’t burn. What is it?Answers
· Uranium. When Uranium atoms split into two (fission) they release energy which can be captured in a nuclear power station. This is a nuclear reaction and is fundamentally different to burning (combustion is a chemical reaction)7.17 and 7.18 starter 213 January 201214:02How many protons and neutrons are there in
· 23592U?· 23692U?Answers
· 23592U = 92 protons; 143 neutrons.This radioisotope of Uranium is commonly used as a fuel for nuclear power stations
· 23692U = 92 protons; 144 neutronsThis radioisotope of Uranium is highly unstable and is artificially created in nuclear power stations where it undergoes fission
7.17 and 7.1812 January 201210:32· 7.17 understand that a nucleus of U-235 can be split (the process of fission) by collision with a neutron, and that this process releases energy in the form of kinetic energy of the fission products· 7.18 recall that the fission of U-235 produces two daughter nuclei and a small number of neutrons<<Nuclear Fission.pptm>><<U235 fission animation.swf>>PhET animation - nuclear fission31 January 201213:34<<nuclear-fission_en.jar>>WebsiteEmbed code for your blog<div style="position: relative; width: 300px; height: 225px;"><a href="http://phet.colorado.edu/sims/nuclear-physics/nuclear-fission_en.jnlp" style="text-decoration: none;"><img src="" alt="Nuclear Fission" style="border: none;" width="300" height="225"/><div style="position: absolute; width: 200px; height: 80px; left: 50px; top: 72px; background-color: #FFF; opacity: 0.6; filter: alpha(opacity = 60);"></div><table style="position: absolute; width: 200px; height: 80px; left: 50px; top: 72px;"><tr><td style="text-align: center; color: #000; font-size: 24px; font-family: Arial,sans-serif;">Click to Run</td></tr></table></a></div>
Saturday, March 3, 2012
7.10 to 7.12 questions
1.What happens to the amount of ‘mother’ nuclei as time passes? - They will decay, possibly into another element2. What sort of radioisotope will decay the fastest - one with a long half life or one with a short half life? - One with short half life3. Does half life tell us exactly when a particular nucleus in a radioisotope will decay? - No, it is a random process.4. What are the two definitions of half life? 1) Time taken for the activity of a sample to halve; 2) The time taken for the number of radioactive atoms in a sample to halve5. What does the activity of a source mean? - The amount of atoms in a sample that decays per second6. What is the unit of activity? - Becquerel (Bq)7. What will happen to the number of ‘mother’ nuclei after two half lives? - It will decreased to 1/4 of the original number8. What will happen to the activity of a source after two half lives? - It decreases to 1/4 of the original count7.10 to 7.12 calculation questions (working in notebook)1. A radioisotope has a half life of 12 years. What fraction of the radioisotope will be left after 60 years? - 1/322. If the activity of a sample falls to 1/64th of its original level after 2 hours, what is the half life of the sample? - 20 minutes3. The background radiation in a laboratory is 7 Bq. The count rate from a radioisotope is measured and it has a reading of 119 Bq. If the half life of the radioisotope is 10 minutes, what will be the reading 20 minutes later? - 28 Bq4. Potassium decays into argon. The half life of potassium is 1.3 billion years. A sample of rock from Mars is found to contain three argon atoms for every atom of potassium. How old is the rock? 2.6 Billion Years
7.10 to 7.12
·Smoke detectors use 241Am to emit alpha particles which pass through a small air gap before being detected. If smoke particles are present they interrupt the beam of alpha particles and this triggers the alarm to go off· Tomorrow, will the 241Am still be as radioactive?· Next year, will the 241Am still be as radioactive?· In a thousand years, will the 241Am still be as radioactive?Answers
· To answer the questions, we need to know the half life of Americium-241which is 432 years· Tomorrow and even next year its activity will hardly have changed at all (sensible for a smoke detector - you don't want it to suddenly stop working!)· In a thousand years its activity will have dropped to about a quarter
·7.10 understand that the activity of a radioactive source decreases over a period of time and is measured in becquerels· 7.11 recall the term ‘half-life’ and understand that it is different for different radioactive isotopes· 7.12 use the concept of half-life to carry out simple calculations on activity
PhET animation - alpha decay31 January 2012
13:34<<alpha-decay_en.jar>>
PhET animation - beta decay31 January 201213:34<<beta-decay_en.jar>>
7.6 and 7.7
·7.6 describe the effects on the atomic and mass numbers of a nucleus of the emission of each of the three main types of radiation·7.7 understa
nd how to complete balanced nuclear equations
AnswerThe symbol for Neptunium is Np not NP!
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