A running man has half the kinetic energy of a boy half his mass. The man speeds up by 1.70 m/s and then has the same kinetic energy as the boy. What is the speed (in meters/second) of the boy?

I feel like this question is lacking information, but clearly its not since its a homework question.
Any hints would be really appreciated.

Question

A running man has half the kinetic energy of a boy half his mass. The man speeds up by 1.70 m/s and then has the same kinetic energy as the boy. What is the speed (in meters/second) of the boy?

I feel like this question is lacking information, but clearly its not since its a homework question.
Any hints would be really appreciated.

irishboy123 · Answer

you need to build a few equations.  

(1) if the KE of the man to start with = 1/2 M V^2 and the boy runs at U, we know what?  we can write an equation in V, U and M.  recap, the boy's mass is M/2.

anonymous · Answer

Ohhh, I see. I did that, but I wasn't multiplying my answer of 4.106 by 2 to get the answer for the boys mass, which is 8.21. Thank you @IrishBoy123 !

irishboy123 · Answer

if you want to check answer, tell me and i'll work it out too...

anonymous · Answer

Sure, I was doing:

Man's velocity = v
Boys velocity = V
Mass of the man = M

$$\frac{ 1 }{ 2 } mv ^{2} = (\frac{ 1 }{ 2 } )(\frac{ 1 }{ 2 }) mV ^{2}$$
$$v ^{2} = 0.25V ^{2}$$

$$\frac{ 1 }{ 2 } M (v+1.7) ^{2} = (\frac{ 1 }{ 2 }) (\frac{ 1 }{ 2 }) MV ^{2}$$

($$(\frac{ (v+1.7) }{ v })^{2} = 2V$$

$$v + 1.7 = 1.414v$$
$$0.414v = 1.7$$
$$v=\frac{ 1.7 }{ 0.414 } = 4.106$$
$$4.106 \times 2 = 8.21 (answer)$$

I know the answer is right because our homework is a computer system that immediately tells us. :D 
Sorry if I skipped a lot of algebra.