Download Law of conservation of linear momentum

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Gymnázium, Brno, Slovanské náměstí 7
III/2 – Inovace a zkvalitnění výuky prostřednictvím ICT
Inovace výuky na GSN prostřednictvím ICT
CZ.1.07/1.5.00/34.0940
RNDr. Bc. Miroslav Hruška
Dynamics of particles (dynamika hmotných bodů)
Law of conservation of linear momentum
VY_32_INOVACE_FY.3.06
20. 10. 2013
Žáci středních škol s CLIL výukou fyziky v angličtině
Linear momentum of a system, isolated system, Newton’s
third law of motion, conservation of linear momentum,
reactive engine
Prezentace slouží k uvedení do problematiky zákona
zachování hybnosti. Je vhodné ji použít při probírání
nového učiva.
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Dynamics of particles
6. Law of conservation of linear
momentum
Revision




What is the most important quantity of
dynamics? Is it a vector, or a scalar?
What is its unit?
What are the names of Newton’s laws
of motion?
What does Newton’s first law of
motion say?
What does inertia mean?
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Revision – answers




Force; it is a vector; unit is newton
Law of inertia, law of acceleration, law of
action and reaction
Law of inertia: A body is at rest or in uniform
rectilinear motion unless made to change its
state by external forces
Inertia is a property of matter by which it
remains at rest or continues moving
uniformly in a straight line unless acted on
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by an external force
Linear momentum of a system



System (of bodies) (soustava těles) =
usually more than one body
Linear momentum of a system of two
bodies with linear momenta p1 and p2
is p = p1 + p2 (addition of vectors)
Note: p can be even a zero vector (o)
when p2 = –p1!
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Exercise I

A wagon (1) with a mass of 10 tonnes
is going at 10 m/s to the north. A
wagon (2) with a mass of 15 tonnes is
going at 8 m/s to the south. Find the
linear momentum of the system of
the wagons.
Solution I


p1 = m1v1 = 100 t.m/s (↑)
p2 = 120 t.m/s (↓)
p = 20 t.m/s (↓)
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Laws of conservation


Isolated system = system that exchanges
neither energy nor matter with its
surroundings (s okolím)
Isolated system obeys
laws of conservation of (zákony zachování)
energy
electric charge
mass
linear momentum
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Conservation of linear momentum

Law of conservation of linear momentum:
“The linear momentum of an isolated
system is (a) constant: p = const.”

If a system of two bodies is isolated, and
bodies exert forces (F1,2 and F2,1) on each
other, then
F1,2 equals the net force acting on body 2
F2,1 equals the net force acting on body 1
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Conservation of linear
 momentum

Have a look: F2 ,1  F1 ,2


p1
p2

t
t


p1   p2

 
p1  p2  o
 

(p1  p2 )  o
 
p1  p2  const.
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Exercise II

Have a look at the Exercise I.
If the wagons collide and connect,
what will their velocity be?
Solution II

Linear momentum of the system is
p = 20 t.m/s (↓). After collision
momentum doesn’t change!
Direction is southward, and magnitude
v = p ÷ (m1 + m2) = 20/25 m/s = 0.8 m/s
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A cannon and a cannon ball

After shot: opposite directions, and
equal magnitudes of linear momenta:
MV = mv

v
m
http://www.youtube.com/watch?v=Tsql5Wv0TiE 1 min
M

V
Zdroj: archiv autora
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Action and reaction around us
•reactive engines
http://www.youtube.com/watch?v=p1TqwAKwMuM
•walking
http://www.youtube.com/watch?v=rQXis2u7F0E
http://www.youtube.com/watch?v=Fbb3631ew_4
•swimming
•paddling (canoeing)
http://www.youtube.com/watch?v=qH3HnhEaeok
•inline skating etc.
http://www.youtube.com/watch?v=sWv6em9EZGE
http://www.youtube.com/watch?v=ZHPvR7DKISU
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That is all for now
Thank you for your attention

Materiál je určen pro bezplatné používání pro potřeby výuky
a vzdělávání na všech typech škol a školských zařízeních.
Jakékoliv další využití podléhá autorskému zákonu.
Zdroje:


Bednařík, M., Široká, M. Fyzika pro gymnázia, Mechanika.
Dotisk 3. vyd. Praha: Prometheus, 2004. 288 s. ISBN 807196-176-0
Hornby, A. S. Oxford Advanced Learner’s Dictionary of
Current English. 5. vyd. Oxford: Oxford University Press,
1995. 1428 s. ISBN 0-19-431423-5
Autor nezaručuje funkčnost webových odkazů!
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