Potential Energy and WET (Incline with friction)

[closer]

A 6.00 kg block is set into motion up an inclined plane with an initial speed of v0 = 8.50 m/s (Fig. P7.23). The block comes to rest after traveling 3.00 m along the plane, which is inclined at an angle of 30.0° to the horizontal.

(a) Determine the friction force exerted on the block (assumed to be constant).
(b) What is the coefficient of kinetic friction?

W_fk + W_g = -.5mv_i²
(f_k)(x) + (m)(g)(h) = (-.5)(m)(v_i²)
(f_k)(3) + (6)(9.8)(3sin30) = (-.5)(6)(8.5^2)
f_k = -101.65

Final answer is incorrect. Cannot move on to (b) without understanding (a). Any ideas?

 

[LowlyPion]

A 6.00 kg block is set into motion up an inclined plane with an initial speed of v0 = 8.50 m/s (Fig. P7.23). The block comes to rest after traveling 3.00 m along the plane, which is inclined at an angle of 30.0° to the horizontal.

(a) Determine the friction force exerted on the block (assumed to be constant).
(b) What is the coefficient of kinetic friction?

W_fk + W_g = -.5mv_i²
(f_k)(x) + (m)(g)(h) = (-.5)(m)(v_i²)
(f_k)(3) + (6)(9.8)(3sin30) = (-.5)(6)(8.5^2)
f_k = -101.65

Final answer is incorrect. Cannot move on to (b) without understanding (a). Any ideas?

Looks to me like the wrong sign.

KE = PE + Wf

 

[closer]

Tried positive 101.65; the answer is still incorrect.

 

[LowlyPion]

Tried positive 101.65; the answer is still incorrect.

When you change the sign there you don't get the same answer. That was my point.

 

[closer]

The change in KE is negative though.

 

[LowlyPion]

The change in KE is negative though.

It's the conservation of Energy.

Where does the KE go?

It goes into PE and to work done by friction.

KE = PE + Wf