Please help

[physics87]

I can't solve this problem:

Suppose a tennis ball moving to the right at 185 km/h hits a moveable target of unknown mass. After the one-dimensional, perfectly elastic collision, the tennis ball bounces to the left with a speed of 80.0 km/h. If the tennis ball's mass is 5.70x10^-2kg, what is the target's mass? (Hint: use the conservation of kinetic energy to solve for the second unknown quantity.)

Thank you very much!!

[dextercioby]

I can't solve this problem:

Suppose a tennis ball moving to the right at 185 km/h hits a moveable target of unknown mass. After the one-dimensional, perfectly elastic collision, the tennis ball bounces to the left with a speed of 80.0 km/h. If the tennis ball's mass is 5.70x10^-2kg, what is the target's mass? (Hint: use the conservation of kinetic energy to solve for the second unknown quantity.)

Thank you very much!!

The hint the problem gave u solves things just by half,since there's another equation u have to put in.The equation which results by a proper projection of the linear momentum conservation eq. along the axis of the movement.

[physics87]

thank you for your help, but i do not know how to combine these equations.
so far i have:
1=tennis ball
2=movable target
m1v1i+m2v2i=m1v1f+m2v2f
m2(v2f-v2i)=m1(v1i-v1f)
m2=[m1(v1i-v1f)]/(v2f-v2i)
Is the movable target initaily at rest?
And, how can I use the conservation of KE to solve for the final velocity of the movable target?
Thank you for all your help!