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MOMENTUM
IMPULSE
CONSERVATION OF MOMENTUM
One-dimensional motion
p  mv
impulse  F t
 F  0  p  0
J  F t  m v  m u
+
v>0
v<0
m
m
p=mv>0






p=mv<0
mass of object m [kg]
velocity of object v [m.s-1]
momentum of a moving object p [kg.m.s-1]
impulse acting on object impulse  J [N.s or kg.m.s-1]
force acting on object F [N]
time interval of force acting on object t [s]
v1 = u
force F acts time t
v2 = v
m
m
initial momentum p1
final momentum p2
J  F t  m v  m u

Impulse = Change in momentum of object

initial velocity of object u, final velocity after a time interval t is v

Launching a rocket
momentum of rocket
procket
FRG force on rocket by gas
FGR force on gas by rocket
momentum of exhaust gases
pgases
Equation Mindmap eq04:
Doing Physics on Line
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Newton’s third law
FRG  FGR
FRG t  FGR t
Forces act for time interval t 
impulse:
Impulse = Change in momentum:
pR  pG
(mv)rocket  (mv)gases
pR  pG  0
Momentum is conserved:

If net force acting on object is zero  impulse is zero  no change in momentum of
object  Law of Conservation of Linear Momentum
 F  0  p  0  p1  p2 initial momentum = final momentum
In an isolated system (zero net force acting) then in any collisions or explosions
within the system, momentum is conserved psystem  0
Beta decay - electron e- or positron e+ released from a nucleus had a continuous range
of energies. Why?
no.  particles detected

210Bi
83

210Po
84
+ e- +  e
KE of  particles (MeV)

Correct explanation proposed by Pauli (1930) – a third neutral particle called a
neutrino must also be produced in the decay for energy and momentum to be
conserved. For each beta emission, the total energy carried away from the decaying
nucleus would be shared between the beta particle and the neutral particle emitted
with it - it would be expected that the beta particles emitted would have a range of
energies depending on the energies of the neutral particles emitted with them. In 1934,
Italian physicist Enrico Fermi named Pauli’s particle the neutrino (), meaning
“little neutral one” in Italian, and formulated a theory of  decay using this particle.
Fermi’s theory successfully explained all experimental observations.
Equation Mindmap eq04:
Doing Physics on Line
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
Neutron a constituent of the nucleus
unknown
radiation
protons
ionization
chamber
polonium
 particle
source
Be
paraffin
block
Joliot’s Experiment
Chadwick (1932) proposed that the radiation emitted from the Be target after it was
bombarded by  particles was a new type of neutral particle – the neutron (originally
proposed by Rutherford). He then applied the conservation of energy and momentum laws
to his experimental results and showed that the particles emitted from the Be had to be neutral
with about the same mass as the proton. Chadwick had indeed discovered the neutron.
4
He2 + 9Be4 
12
C6 + 1n0
Following Chadwick’s discovery of the neutron, a new model of the nucleus was proposed the nucleus consists of protons and neutrons. Together these particles are called the nucleons
– particles that make up the nucleus: nucleon is a generic term for a proton or a neutron
Equation Mindmap eq04:
Doing Physics on Line
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