Rifle recoil - Dr. Dave's Hall of Fame
B) Recoil Problem - When interacting objects start from rest
In real life, it would be foolish to shoot a rifle like this. Why?
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?
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?
| Rifle | Bullet |
| mR = 4.0 kg Rifle | mB =5.0 x 10-3 kg |
| VR = ? | VB = 500. m/s |
Total Momentum Before =
Total Momenta After
Interaction
0 = m1V1 + m2V2
Subtract m2V2 from both sides
m1v1 = - m2V2
In problem Solving, remove negative sign and use
| ** m1v1 = m2v2 ** |
(Not in Reference Table)
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?
| Rifle | Bullet |
| mR = 4.0 kg | mB =5.0x10-3 kg |
| VR = ? | VB = 500. m/s |
| ** m1v1 = m2v2 ** |
(4 kg)V =(5.0 x 10-3 kg)500 m/s
(4 kg)V =2.5 kgm/s
V = .63 m/s
Momenta before/after= 0 before
1
2 W.F.U.
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?
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)
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.
b) What is the total momentum of the magnets after the string is cut?
Total momentum after = 0
(Total mom. before = Total mom. after)
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.
c) What is the velocity of the .50 kg mass when the velocity of the 1.0 kg magnet is 30. m/s?
m1 = 1.0 kg
m2 = .50 kg
V2 = ?
V1 = 30. m/s
m1v1 = m2v2
1.0 kg(30. m/s) = .50 kg(V)
V = 60. m/s