Lily A. Physics

Tuesday, March 27, 2012

Investigating Momentum

________________________________________
From: Matt Baker
Sent: Tuesday, March 20, 2012 6:37 PM
To: Andrew Koomenjoe Nyaga; Arisara Amrapala; Boondaree Chang; Chrischawit Chomsoonthorn; Christopher Lo; Connor Blair Sailes; Frazer Allen Briggs; Huei-Yu Daniel Lo; Isabel Catriona McDonald; Kavin Supatravanij; Luke Michael Gebbie; Lydia Anna Foley; Morrakot Sae-Huang; Puchawin Borirackujarean; Qing Tang; Sanyam Grewal; Sebastien Grimm; Soo Hyun Lee; Tatiksha Singh; Usa Wongsanguan; Yanida Areekul; Yi-Lin Huang
Subject: Investigating Momentum

Starter - Spot the mistakes!

13 March 2012

16:35
· 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

14 March 2012

07:20

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

[cid:image001.jpg@01CD0767.BCDE8CE0]

Final

Final speed of LH glider = vl = 0m/s

Final speed of RH glider = vr = 1m/s

[cid:image002.jpg@01CD0767.BCDE8CE0]

We can represent this graphically as

Initial

[cid:image003.png@01CD0767.BCDE8CE0]

Final

[cid:image004.png@01CD0767.BCDE8CE0]

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

[cid:image005.jpg@01CD0767.BCDE8CE0]

Final

Final speed of LH glider = vl = 0.5m/s

Final speed of RH glider = vr = 0.5m/s

[cid:image006.jpg@01CD0767.BCDE8CE0]

We can represent this graphically as

Initial

[cid:image003.png@01CD0767.BCDE8CE0]

Final

[cid:image007.png@01CD0767.BCDE8CE0]

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

[cid:image008.png@01CD0767.BCDE8CE0]

Final

Final speed of LH glider = vl = 0m/s

Final speed of RH glider = vr = 0m/s

[cid:image009.png@01CD0767.BCDE8CE0]

We can represent this graphically as

Initial

[cid:image010.png@01CD0767.BCDE8CE0]

Final

[cid:image011.png@01CD0767.BCDE8CE0]

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

[cid:image012.png@01CD0767.BCDE8CE0]

Final

Final speed of LH glider = vl = 0m/s

Final speed of RH glider = vr = 0m/s

[cid:image013.png@01CD0767.BCDE8CE0]

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

[cid:image014.png@01CD0767.BCDE8CE0]

Final

[cid:image015.png@01CD0767.BCDE8CE0]

So something is conserved in the collision, but what is it?

What does the area of the rectangles represent?!

Time to label our axes!

[cid:image016.png@01CD0767.BCDE8CE0]

Final Conclusion
· The area of the rectangles are mass x velocity
· Momentum = mass x velocity
· So momentum is conserved in collisions

1.22 Plenary questions and answers

See the full gallery on Posterous

1.22

· 1.22 use the conservation of momentum to calculate the mass, velocity or momentum of objects

Momentum conserved in collisions
· http://www.youtube.com/watch?v=b6QzJSUKzQM
· and http://www.walter-fendt.de/ph14e/collision.htm

Momentum conserved in explosions

Example - Pearson, p.41

Consider final momentum

Truck, plasticine and pellet

p = m x v

p = (0.1+0.002) x 0.8

p = 0.0816kgm/s

total final momentum = pf = 0.0816kgm/s

Principle of Conservation of Momentum tells us:

total initial momentum = total final momentum

Σpi = Σpf

so total initial momentum = pi = 0.0816kgm/s

Consider initial momentum

Truck and plasticine

p = m x v

p = 0.1 x 0

p = 0kgm/s

Pellet

p = m x v

0.0816 = 0.002 x v

v = 40.8m/s

1.22 animation

14 March 2012

15:49

Website:

http://www.walter-fendt.de/ph14e/collision.htm

newtons_cradle[1].swf Download this file

AirTrack simulation.swf Download this file

See the full gallery on Posterous

1.20

· 1.20 know and use the relationship between momentum, mass and velocity:

momentum = mass × velocity

p = m × v

p = momentum (kgm/s)

m = mass (kg)

v = velocity (m/s)

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

See the full gallery on Posterous

P8 objectives sheet

IGCSE Physics Student Objectives for P8.docx Download this file

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? =