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PHYS 101: Physics for the Life Sciences Lecture 18:

Momentum and Collisions

Recall:

V1f=[m1−m2]

[m1+m2]⋅V1i+[2m2]

[m1+m2]⋅V2i

V2f=[m2−m1]

[m1+m2]⋅V2i+[2m1]

[m1+m2]⋅V1i

If m1 = m2 then

V1f=V2i

and

V2f=V1i

Example:

A car of mass 1800 kg is stopped at a traffic light, the car is struck from behind by a 900

kg car, they become entangled(locked together). The 900 kg car was travelling at 20 m/s

before the collision. Calculate the velocity of the entangled cars after the collision. Note:

this is a perfectly inelastic collision.

Pi=m1V1i+m2V2i

Pi=(900)(20)+(1800)(0)

Pi=18000

Pf=(m1+m2)Vf

Pf=(900+1800)Vf

Pf=2700 ⋅Vf

Pi=Pf

18000=2700 ⋅Vf

Vf=6.67 m/s