<p>Can anyone tell me how to solve this problem?</p>

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

<p>hehe this problem isn't as hard as you think. First write the momentum equations. mass of tennis ball times initial velocity of tennis ball plus mass of moving target equals mass of tennis ball times velocity of tennis ball after collision plus mass of moving object times velocity of moving object after collision. Then, since there are two variables, you need to utilize the kinetic energy equation which is 1/2 m v squared. So you do the same thing: 1/2 mass of tennis ball times initial velocity of tennis ball squared ..... then u manipulate the equations to find your answer. good luck :)</p>

<p>Thank you zztao, but I am not sure how to use the KE equation
So far I have
1=tennis ball
2=movable object
m1v1i+m2v2i=m1v1f+m2v2f
m2(v2f-v2i)=m1(v1i-v1f)
m2=[m1(v1i-v1f)]/(v2f-v2i)
is it to be assumed that the movable object was at rest?
and, how do I use the KE equation to solve for the final velocity of the movable object?
Thanks, again</p>