1. The problem statement, all variables and given/known data A 20 kg wagon is released from rest from the top of a 15 m long plane, which is angled at 30° with the horizontal. Assuming there is friction between the ramp and the wagon, how is this frictional force affected if the angle of the incline is increased? 2. Relevant equations ∑Fy = n - mgcos30° = 0 ∑Fx = -ff (force of friction) + mg sin 30° = ma up = + down = - → = + ← = - ff (force of friction) = μ (coefficient of friction) * n (normal force) 3. The attempt at a solution I approached this problem by first finding the normal force when angle θ = 30°. Therefore, ∑Fy = n-mgcos30° = 0 ⇒n = mgcos30° ⇒n = 174 N Then I solved ∑Fy = 0 when angle θ is increased, for example when θ = 60° Therefore, ∑Fy = n-mgcos60° = 0 ⇒n = mgcos60° ⇒n = 100 N Then, I solved ∑Fx = mgsin30° - ff = ma when θ = 30° ⇒ff = mgsin30° - ma ⇒ff = 100 - 20a Then, I solved ∑Fx = mgsin60° - ff = ma when θ = 60° ⇒ff = mgsin60° - ma ⇒ff = 174 - 20a For example if I designate that "a" = 2 m/s^2, then ff = 100 - 20(2) ⇒60 N when θ = 30° and ff = 174 - 20(2) ⇒ 134 N when θ = 60°. Therefore, when angle of incline increases, friction force increases. However, my logic is wrong. The solution is that since ff = μ * n , and if angle of incline increases, friction force decreases, since ff = μ * 174 N when θ = 30° and ff = μ * 100 N when θ = 60°. I don't know why my approach is wrong...Any help would by greatly appreciated. Thanks.