# Homework Help: Need Help Please

1. Dec 3, 2004

### bongopukerat

A 90kg fullback moving south with a speed of 5m/s has a perfectly inelastic collision with a 95kg opponent running north at 3m/s.

a. Calculate the velocity of the players just after the tackle

b. calculate the decrease in total kinetic energy as a result of the collision.

2. Dec 3, 2004

### jamie_o

Perfectly inelastic means they stick together so they have a common final velocity. Use the principle of conservation of momentum and the kinetic energy equation for the 2nd one. Show me how you think you would do this question bearing in mind what I've said.

Last edited: Dec 3, 2004
3. Dec 3, 2004

### aviv87

Well, if they have a total inelastic collision then after the collision they move together, as one body, with the same velocity, right?
So all you've got to do is use the momentum conservation law to compare the total momentum before the collision and after it and then find the velocity.

About b, after you've calculate the new velocity then $$\Delta E_{k} = E_{k\ total\ after\ collision} - E_{k\ total\ before\ collision}$$

4. Dec 3, 2004

### futb0l

if u call north the positive direction then

$$m_1v_1 + m_2v_2 = (m_1 + m_2)v'$$

simple conservation of momentum...

$$90(-5) + 95(3) = 185v'$$

now u can get the velocity just after the collision...
and if u use the kinetic energy equation...

$$E_{before} = \frac{1}{2}m_1v_1^2 + \frac{1}{2}m_2v_2^2$$

...

$$E_{after} = \frac{1}{2}(m_1+m_2)v'^2$$

;)

Last edited by a moderator: Dec 3, 2004