Kinetic energy of inelastic collision problem

[physicspupil]

Homework Statement

A particle of mass m1 and velocity u1 collides with a particle of mass m2 at rest. The two particles stick together. What fraction of the original kinetic energy is lost in the collision?

Homework Equations

Conservation of momentum law

The Attempt at a Solution

I think this one is right but was hoping somebody could check for me... It seemed too easy, so I just wanted to make sure I'm understanding it correctly.

m1u1 = (m1 + m2)u by conservation of momentum, so u = m1u1/(m1+m2)

Ti = ½ m u1^2 and Tf = ½ (m1+m2)u^2 = m1^2 u1^2/2(m1+m2) in terms of u1


Then I found Ti – Tf = (m1m2 u1^2)/2(m1 + m2) and then found ratio of this with Ti


I got m2/(m1 + m2). Is this correct? Thanks! :)

 

[rock.freak667]

I got $\frac{m_1}{m_1+ m_2}$ and worked it the same basic way you did.

 

[D H]

At least one of you two is obviously wrong. How much energy is lost when you throw a 1 gram spitball at a 75 kg man? What happens when a 75 kg man runs into a 1 gram spitball suspended from the ceiling?

 

[physicspupil]

Hi, rock.freak667, isn't your answer the amount of the original remaining (not lost)? I'm not sure, I'll check my work again. Man's kinetic energy isn't effected much but spitball's is - I think that's what makes me uncomfortable about these problem w/o numbers - harder to get an intuitive feel for them but making up concrete examples helps. Thanks to both of you.