# Kinetic energy question

1. Jan 11, 2010

### richardnumber

kinetic energy question!!

hey,

I have the following question to complete, and dont have the slightest clue where to start! :(

A sphere of mass 1kg, moving at 10 m/s, overtakes another sphere of
mass 5 kg moving in the same line at 3m/s. Find the loss of kinetic
energy during impact, and show that the direction of motion of the
first sphere is reversed. Assume the coefficient of restitution e = .75

should i use the following forumla?: 2(m1 + m2)E = m1m2(u1-u2)2(1-e2)

any help would be greatly appreciated,

Rich

2. Relevant equations

3. The attempt at a solution

2. Jan 11, 2010

### ideasrule

Re: kinetic energy question!!

Start by doing the first question: the loss in kinetic energy. Since the coefficient of restitution is 0.75, 25% of the initial kinetic energy is lost, correct?

Then write out the conservation of momentum & conservation of energy equations, remembering that 25% of the original kinetic energy is lost.

3. Jan 11, 2010

### Staff: Mentor

Re: kinetic energy question!!

I don't recognize that formula. In any case, it's usually best to derive your own formula from first principles.

Hint: What's the definition of the coefficient of restitution? That will give you one equation. Conservation of momentum will give you the other.

4. Jan 11, 2010

### Redbelly98

Staff Emeritus
Re: kinetic energy question!!

Just a friendly correction, that isn't what is meant by coefficient of restitution; it involves the relative velocities of the objects before and after the collision. Presumably it's in the OP's textbook or lecture notes so I will reiterate Doc Al's advice and let the OP look it up.