# Kinetic energy or momentum?

I'm really confused! After a collision, how would I know which formulas to use? I noticed this a few days ago, but when I use the conservation of energy to calculate the final speed of an object, the answer would be different from using conservation of momentum.

I don't know why I noticed this a few days ago (tomorrow is my exam), I've been using kinetic energy and momentum for 3 courses now >.>

Someone please help explain when I should use momentum vs kinetic energy..

Okay, here is an example:

a car of mass m collides with a stationary truck of mass M

the car's initial velocity is Vi. The car's final velocity is 0

find the truck's final velocity

so if I do momentum

p1i + p2i = p1f + p2f

p1i + 0 = 0 + p2f

mVi = MVf

(m/M)*Vi = Vf

But if I use kinetic energy

K1i + K2i = K1f + K2f

K1i + 0 = 0 + K2f

0.5*m*Vi^2 = 0.5*M*Vf^2

[(m/M)*Vi^2]^0.5 = Vf or Vf = Vi*((m/M)^0.5)

^0.5 is square root

Clearly, I got a different answer....

### 3 Answers

- OldPilotLv 78 years agoFavorite Answer
Momentum is always conserved

Kinetic Energy is not conserved (KE converts to other forms heat, sound,etc) in Inelastic Collisions. In an Inelastic Collision the objects stick together and move as a single mass (Example: a bullet hits a wood block and the bullet/block move as one mass. You need only balance momentum

Kinetic Energy is conserved in an Elastic Collision. In an Elastic Collision the objects bounce apart (Example: A steel ball bearing hits a steel block and bounces off. In this case you must balance momentum and kinetic energy

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- fennLv 44 years ago
pretend the mass is 2kg and the object is vacationing at 1m/s. KE = ½ x 2 x a million² = 1J Momentum = 2 x a million = 2kgm/s If the kinetic potential is more effective by technique of three hundred%, meaning it truly is more effective by technique of 3J, and the recent kinetic potential is a million+3 = 4J. the recent speed is given by technique of ½ x 2 x v² = 4 giving v= 2m/s. New momentum = 2x2 = 4kgm/s The momentum has doubled (from 2 to 4), that is a upward thrust of one hundred%. So the great answer is a million).

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- FiremanLv 78 years ago
Use work energy relation:-

=>W = ∆E

or Use law of energy conservation:-

=>ME = constant

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