Conservation of Energy: Finding the Speed of a Falling Ball

[sireh]

Homework Statement

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The coefficient of friction between the block of mass m₁ = 3.00kg and the surface in Figure P7.22 is μ_k = 0.400. The system starts from rest. What is the speed of the ball of mass m₂ = 5.00 kg when it has fallen a distance of h = 1.50m?

Homework Equations

This is in the chapter on conservation of energy, so
U_i + K_i + ΔE_mech = U_f + K_f

The Attempt at a Solution

Using downward direction as positive so g is positive

m₂gh - fh = 1/2(m₁ + m₂)v²
(5.00kg)(9.8m/s²)(1.5m) - (-11.76N)(1.5m) = 1/2(3.00kg + 5.00kg)v²
91.14Nm = 4kg * v²
22.785Nm²/s² = v²
4.77m/s = v

That's not the answer that I'm seeing on the answer sheet. I'm not sure what I did wrong.

 

[TSny]

m₂gh - fh = 1/2(m₁ + m₂)v²
(5.00kg)(9.8m/s²)(1.5m) - (-11.76N)(1.5m) = 1/2(3.00kg + 5.00kg)v²

Think conceptually and decide if you want to have two minus signs here.