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General Physics 1 • C5 – Applying Momentum Conservation
General Physics 1
C5 Applying Momentum Conservation
General Physics 2
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General Physics 3
Tips on Maximizing your homeworkscores
Review the instructions on setting up a problem From todays lecture new framework From the first lecture slides: find, given, box answers, etc.
If I need to read the question in the book while Im reviewingyour HW, youll lose points, especially on WH
Units! Direction for vector quantities! Do corrections! Collaborate with other classmates! Get help
Office hours T, TH 10 am- 12 pm, virtual S 8 9 pm Schedule an appointment, send me an email Tutoring S Th evenings 8 11 pm in RB 150
General Physics 4
Momentum conservation withoutisolation
In chapter 4 we learned that in orderfor momentum to be conserved, thesystem had to be isolated from externalinteractions.
Consider a cart collision Its obviously not isolated from external
interactions LR gravitational interaction, contact
compression interaction
General Physics 5
Gravity and compression interactionscancel
The gravitational interaction and thecompression interaction cancel
The system ACTS like its isolated
General Physics 6
Degrees of Isolation
floats in space functionally isolated collision momentarily isolated
General Physics 7
Floats in space
Both the system and the frame floatfreely in space and respondgravitationally to the same distantbodies
Examples two stars in deep space Earth and earths satellite
General Physics 8
Functionally Isolated
A system whose external interactions cancel The most common systems
Gravity and compression interactions cancel
Examples Why are these functionallyisolated? Books on a table Cars driving down the road Hockey pucks on ice Billiard balls on pool table
General Physics 9
Momentarily isolated - collision
A collision is any process where two systemobjects interact for a short time And delivers an impulse much larger than any
other interaction during that time External interactions dont have time to
transfer significant momentum to the system. Examples
Collisions Explosions
General Physics 10
Are these systems isolated? How?
Space shuttle and its contents Two billiard balls that collide Two magnetic hockey pucks that
change each others paths in a hockeygame
Exploding fireworks Two cars skidding on a road hit each
other
General Physics 11
Problem solving framework
Translate the problem into mathematicalsymbols Pictures are VERY important
Build conceptual model linking physicsequations
Work out algebraic solutio Evaluate (check) your results to see if they
make sense
Review p. 86 91!!!
General Physics 12
Translate
Given: Find: Draw a picture Define reference frame axes
General Physics 13
Conceptual Model
What theories/principles apply Simplifications/Assumptions Select equations to develop a coherent
and reasonable plan to solve problem Most important step
Requires 90% of thinking to accuratelysolve problem!
General Physics 14
Algebra
Solve equations symbolically Dont plug numbers in until the last step Keep track of units
Great way to check if answers arereasonable
General Physics 15
Evaluate
Do the numbers make sense? Are the units correct? Are the signs (direction) and magnitudes
correct?
General Physics 16
Model diagrams
Helping diagram Master equation Physical principle why it applies Identify symbols that are zero or that cancel Circle unknown symbols Identify other needed equations Repeat until solved
General Physics 17
Master equation for conservation ofmomentum
pi = pf
General Physics 18
Conservation of Momentum
The total momentum of a system is constant. For collisions, use
In component form, the total momenta in eachdirection are independently conserved
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General Physics 19
Exploding spaceship example C5.1
A 10,000-kg spaceship in deep spacetravels at 40 kms toward theAndromeda galaxy. Its enginessuddenly explode, blowing the shipapart into two hunks. The front (8000-kg) hunk continues moving towardAndromeda at 60 km/s. What is thevelocity (magnitude and direction) ofthe back hunk?
General Physics 20
Solution
General Physics 21
Auto Collision
A 1000-kg car traveling west at 20 m/scollides with an 800-kg car travelingnorth at 16 m/s. The collision locks thecars together. What is the velocity ofthe two-car unit just after the collision?Include magnitude and direction angle.
General Physics 22
Solution - 1
General Physics 23
Solution - 2
General Physics 24
Examples
Kim holds a 2.0 kg air rifle loosely andfires a bullet of mass 1.0 g. The muzzlevelocity of the bullet is 150 m/s. Whatis the recoil speed of the rifle?
General Physics 25
Clowns on skateboards
Suppose two clowns on skateboards are standingmotionless on a smooth surface 2.0 m apart. ClownB tosses a 5.0 kg medicine ball to clown A. Themedicine ball is traveling with a speed of 10. m/s. What is the resulting velocity of clown B after throwing
the ball? (Be sure to indicate which direction is positive.)What is the resulting velocity of clown A after catching the ball?
