B) Recoil Problem  When interacting objects start from rest
Ex 1) A 4.0 kg rifle fires a 5.0 x 10^{3 }kg bullet at a velocity of 500 m/s. What is the velocity acquired by the rifle? (no shooter)
Total Momentum Before =
0 = m_{1}V_{1} + m_{2}V_{2}
m_{1}v_{1} =  m_{2}V_{2}
Ex 1) A 4.0 kg rifle fires a 5.0 x 10^{3 }kg bullet at a velocity of 500. m/s. What is the velocity acquired by the rifle?
(4.0 kg)V =(5.0x10^{3} kg)500.m/s
(4.0 kg)V = 2.5 kgm/s
V = .63 m/s
Equal Mass
Total momentum after = 0 ??
Ex 2) Two disk magnets are arranged at rest on a frictionless horizontal surface.
When the string holding them together is cut, they move apart under a magnetic force of repulsion.
a) What is the total momentum of the magnets before the string is cut?
Total momentum before = 0 (Both objects begin at rest)
b) What is the total momentum of the magnets after the string is cut?
Total momentum after = 0 (Total mom. before = Total mom. after)
c) What is the velocity of the 1.0 kg mass when the velocity of the 2.0 kg magnet is 30. m/s?
m_{1 } = 2.0 kg m_{2 }= 1.0 kg V_{2 }=_{ } ? V_{1 }= 30. m/s
m_{1}v_{1} = m_{2}v_{2}
2.0 kg(30. m/s) = 1.0 kg(V)_{ }
V = 60. m/s
m_{1}v_{1} = m_{2}v_{2}
500 kg(V1) = 100 kg(15 m/s)
V_{1} = 3 m/s
Enrichment
Georgia Public Broadcasting Momentum Video
ŠTony Mangiacapre.,  All Rights Reserved
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