Differences

This shows you the differences between two versions of the page.

--- 183_notes:examples:holding_block_against_a_wall [2014/09/16 06:52] – created pwirving
+++ 183_notes:examples:holding_block_against_a_wall [2014/09/22 04:16] (current) – pwirving
@@ Line 6: / Line 6: @@
 === Facts ====
+The metal block has a mass of 3 kg
+Horizontal force applied to metal block of 40N in positive x-direction
+Coefficient of friction for the metal-wall pair of materials is 0.6 for both static and sliding friction.
 === Lacking ===
+$\vec{F}_{net}$ in the x-direction
+$\vec{F}_{net}$ in the y-direction
 === Approximations & Assumptions ===
+Assume applied horizontal force is constant.
 === Representations ===
+{{183_notes:block_on_wall.jpg|}}
+$\Delta \vec{p} = \vec{F}_{net} \Delta t$
 ==== Solution ====
-You need to identify whether the momentum in the y direction is negative (if it is, that would mean it was slipping down the wall).
+You need to identify whether the momentum in the y direction is negative (if it is, that would mean it was slipping down the wall, if it was positive it would mean it is slipping up the wall).
 Start by computing the change in momentum for both the x direction and the y direction.
-$ x: \Delta p_x = (F_head - F_N) \Delta t = 0 $
+$ x: \Delta p_x = (F_{hand} - F_N) \Delta t = 0 $
+$ y: \Delta p_y = (F_N - mg) \Delta t, \,\,assuming\, it\, slides $
+Combining these two equations
+$ (F_{hand} - F_N) \Delta t = 0 $
+$ F_{hand} \Delta t - F_N \Delta t = 0  \,\,\,\,\,\,\,Multiply\, out.$
+$ F_{hand} \Delta t = F_N \Delta t  \,\,\,\,\,\,\,\,\,Make\, equal\, to\, each\, other.$
+$ F_{hand} = F_N  \,\,\,\,\,\,\,\,\,\,\,\,Cancel\, \Delta t. $
+Substituting in we get:
+$ \Delta p_y = (F_{head} - mg) \Delta t = (0.6(40N) - (3 kg)(9.8 N/kg)) \Delta t $
+$ \Delta p_y = (-5.4 N) \Delta t $
+Since there is a nonzero impulse in the -y direction, the block will slip downward with increasing speed.