· 1.20 know and use the relationship between momentum, mass and velocity:
momentum = mass × velocity p = m × v p = m × v p = momentum (kgm/s) m = mass (kg) v = velocity (m/s)Tuesday, March 27, 2012
Investigating Momentum
Starter - Spot the mistakes!
· http://sites.google.com/site/winfailphysics/all-videos/roadrunner-human-canno...
· http://sites.google.com/site/winfailphysics/all-videos/roadrunner-spring-punch
· You know that these situations are wrong, but why are they wrong?! Guided discovery - Investigating Momentum
When 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 collision doesn'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
· http://sites.google.com/site/winfailphysics/all-videos/roadrunner-human-canno...
· http://sites.google.com/site/winfailphysics/all-videos/roadrunner-spring-punch
· You know that these situations are wrong, but why are they wrong?! Guided discovery - Investigating Momentum
When 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 collision doesn'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
Monday, March 5, 2012
7.10 to 7.12 Questions
1. What happens to the amount of ‘mother’ nuclei as time passes? = As time passes, they will decay causing the amount of the 'mother nuclei' to decrease. 2. What sort of radioisotope will decay the fastest - one with a long half life or one with a short half life? = The one with a short half life will decay the fastest.
3. Does half life tell us exactly when a particular nucleus in a radioisotope will decay? = No it does not. They will randomly decay.
4. What are the two definitions of half life? = 1. It is the time taken for the activity of a sample to half or 2. It is the time taken for the number of radioactive atoms in a sample to halve.
5. What does the activity of a source mean? = the activity of a source means the amount of atoms in a sample that decays per second.
6. What is the unit of activity? = Bq (Becquerel)
7. What will happen to the number of ‘mother’ nuclei after two half lives? = After two half lives, it will decrease to a quater of the original number.
8. What will happen to the activity of a source after two half lives? = Similarly, after two half lives, the activity will decrease to a quarter of the original number. 7.10 to 7.12 calculation questions 12 January 2012 10:24
1. A radioisotope has a half life of 12 years. What fraction of the radioisotope will be left after 60 years? = (1/2)^5 = 1/32
2. 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? = 120 / 6 = 20 minutes
3. 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? = (119 - 7)/4 = 28 Bq
4. 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? =
3. Does half life tell us exactly when a particular nucleus in a radioisotope will decay? = No it does not. They will randomly decay.
4. What are the two definitions of half life? = 1. It is the time taken for the activity of a sample to half or 2. It is the time taken for the number of radioactive atoms in a sample to halve.
5. What does the activity of a source mean? = the activity of a source means the amount of atoms in a sample that decays per second.
6. What is the unit of activity? = Bq (Becquerel)
7. What will happen to the number of ‘mother’ nuclei after two half lives? = After two half lives, it will decrease to a quater of the original number.
8. What will happen to the activity of a source after two half lives? = Similarly, after two half lives, the activity will decrease to a quarter of the original number. 7.10 to 7.12 calculation questions 12 January 2012 10:24
1. A radioisotope has a half life of 12 years. What fraction of the radioisotope will be left after 60 years? = (1/2)^5 = 1/32
2. 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? = 120 / 6 = 20 minutes
3. 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? = (119 - 7)/4 = 28 Bq
4. 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? =
7.10 to 7.12
7.10 to 7.12 starter · 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-241 which 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 to 7.12 · 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 Half-life of Different Isotopes
· http://youtu.be/S-goxH05LbY PhET animation - alpha decay Website http://phet.colorado.edu/en/simulation/alpha-decay
PhET animation - beta decay Website http://phet.colorado.edu/en/simulation/beta-decay 
· 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-241 which 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 to 7.12 · 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 Half-life of Different Isotopes
· http://youtu.be/S-goxH05LbY PhET animation - alpha decay Website http://phet.colorado.edu/en/simulation/alpha-decay
PhET animation - beta decay Website http://phet.colorado.edu/en/simulation/beta-decay
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 understand how to complete balanced nuclear equations Did you spot the deliberate mistake on this animation? Answer The symbol for Neptunium is Np not NP! 7.6 and 7.7 Plenary 
· 7.7 understand how to complete balanced nuclear equations Did you spot the deliberate mistake on this animation? Answer The symbol for Neptunium is Np not NP! 7.6 and 7.7 Plenary
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