Conservation of Momentum

Newton Collider

 

 

B) Recoil Problem - When interacting objects start from rest

 

 

 

"What is the total momentum before
cannon is fired?"

"Total momentum after clown is fired?"

Larger

 

 

 

"Total momentum
before=0. (at rest)

"Total momentum after
 = 0. (momenta are
equal but opposite.)"

 

 

Rifle Recoil
Dr. Dave's Hall of Fame

 

 

"In real life, it would be

foolish to shoot a rifle

 like this.  Why?"

 

 

 

"The recoil on that rifle

would throw the

shooter's shoulder out!"

 

 

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? (no shooter)

 

After rifle is fired

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

 

 

 

(4.0 kg)V =(5.0x10-3 kg)500.m/s

 

 

(4.0 kg)V = 2.5 kgm/s

 

V  =  .63 m/s

 

 

 

 

Momenta before/after= 0 before
 

Equal Mass

cars_explosion.mpg

W.F.U.

 

 

Total momentum after = 0 ??

carsExplosionUneq.mpg

W.F.U.

 

 

 

Yes, the slower cart has a greater mass

 

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?

 

 

| Larger |

 

 

 

 

 

Total momentum after = 0

(Total mom. before = Total mom. after)

 

 

 

c) What is the velocity of the .50 kg mass when the velocity of the 1.0 kg magnet is 30. m/s?

 



| Larger |

 

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

 

 

 

 

 

 

 

What is the velocity

of the cannon below?

 

 

m1v1 = m2v2

 

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

 

 

V1 = 3 m/s

 

 

 

 

Gravity, Friction Momentum

Chapter Review

http://www.wpclipart.com/

 

 

 

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