Conservation of Momentum

 

 

 

B) Recoil Problem - When interacting objects start from rest



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Rifle recoil
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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 =5.0 x 10-3 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.

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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.

 



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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

 

 

 

 

 

Ex) What is the velocity of the cannon?

 

 

m1v1 = m2v2

 

500 kg(V1) = 100 kg(15 m/s)

 

 

V1 = 3 m/s

 

 

 

 

 

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