Dynamics

This is cool!

OK I get 8.93 feet in one second. What do you get XV

Do you have a formula at all?

conservation of linear momentum
 

I don't know how to do this, I'm really sorry :(

Initial momentum of the system = 0.

Crate acceleration along ramp = g*3/5   (component of g along ramp)

Velocity of crate (along ramp) at B given by  v^2 = 2*(g*3/5)*L   (L = length of ramp).

Horizontal component of crate velocity v_crate = -v*4/5   (with horizontal velocity positive to the right).

Momentum of system when crate reaches B:    m_crate*v_crate + m_ramp*v_ramp = 0

See if you can take it from here.

I understood what you wrote Alan, 

but the book answer confusses me, 

why did he put the kinatic energy of the ramp in the equation of conservation of energy ?! 

it suppose to be for the box !