1) A 5kg box is pulled by a force F horizontally (no angle). There is friction between the box and horizontal surface. Mu_k = .25 and Mu_s = .5. If F=20N find the magnitude of the friction force and the acceleration of the block. I used the formula |f_k| = mu_k * |Normal force| f_k = .25 * 9.8 * 5 f_k = 12.25 N The correct answer is that the friction is 20 N, meaning that the block didn't move. Where did I go wrong? 2) A 5kg sled is pulled an an angle 5 degree above the horizontal. Friction between the sled and the snow is described by mu_k = .1 and mu_s = .15. What is acceleration of the sled? The correct answer is 2.99 m/s^2 This is what I did. I took the 20 N and multiply by cos (5 degree) to get 19.92. |f_k| = mu_k * N f_k = .1*5*9.8 f_k = 4.9 (19.92 - 4.9) / 5 = a a = 3.004 Did I just round my answer differently or did I do something wrong? 3) A 2kg object is under the influences of several forces. Assume that the net force is constant. At t=0, the object's velocity is 3i + 2j and at t=4s its velocity is 11i - 14j . Find the magnitude and direction of net force acting on the object. Please explain this to me. I have no idea how to start. Thanks in advance for your help.