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--- 183_notes:examples:two_students_colliding [2014/09/25 15:45] – pwirving
+++ 183_notes:examples:two_students_colliding [2014/10/02 15:50] (current) – caballero
@@ Line 1: / Line 1: @@
-===== Example: Student Collision Example. =====
+===== Example: Colliding Students =====
 Two students are running to make it to class. They turn a corner and collide; coming to a complete stop. What force did they exert on each other.
@@ Line 31: / Line 31: @@
  $\vec{v}_{avg} = \dfrac{\vec{v}_{f} + \vec{v}_{i}}{2} = \dfrac{\Delta \vec{r}}{\Delta t}$
-{{183_notes:untitled.jpg|}}
+{{183_notes:untitled.jpg?400|}}
 === Solution ===
+We use the momentum principle to relate the momentum of the students to the force applied. We have chosen student 1 as our system.
  ${p}_{fx} = {p}_{ix} + {F}_{x,coll} \Delta t$
+We are told that the students come to a complete stop and so  ${p}_{fx}$  = 0.
  $0 = M{V}_{ix} + {F}_{x,coll} \Delta t$
+Rearranging the equation we relate  ${F}_{x,coll}$  to the remaining variables. We are trying to find force.
  ${F}_{x,coll} = \dfrac{-M{V}_{ix}}{\Delta t}$
-negative means  $-\hat{x}$  direction
+The negative sign means that the force is in  $-\hat{x}$  direction.
-Need to find collision time  ${\Delta t}$
+Now that we have the above relationship we must find the missing variables in order to solve for  ${F}_{x,coll}$ . First we need to find collision time  ${\Delta t}$ . We can relate the average velocity to displacement over time.
  $\vec{v}_{avg} = \dfrac{\vec{v}_{f} + \vec{v}_{i}}{2} = \dfrac{\Delta \vec{r}}{\Delta t}$
-In 1D:  $\vec{v}_{avg} = \dfrac{\Delta x}{\Delta t} = \dfrac{\vec{v}_{f} + \vec{v}_{i}}{2}$
+In 1D this looks like: $\vec{v}_{avg} = \dfrac{\Delta x}{\Delta t} = \dfrac{\vec{v}_{f} + \vec{v}_{i}}{2}$
+Relate these 3 equations together to solve for ${\Delta t}$
  ${\Delta t} = \dfrac{\Delta x}{{V}_{avg}} = \dfrac{\Delta x}{\dfrac{\vec{v}_{f} + \vec{v}_{i}}{2}}$
+Fill in the values for the variables from the assumptions and approximations you made previous.
  $\dfrac{0.025}{\dfrac{5m/s + 0m/s}{2}} = 0.01s$
+Having solved for  ${\Delta t}$  fill this value and the known value for mass and the approximated value for velocity into the equation that we arranged earlier to find  ${F}_{x,coll}$
  ${F}_{x,coll} = \dfrac{-M{V}_{ix}}{\Delta t} = - {\dfrac{(68kg)(5m/s)}{0.01s}}$