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Example: Student Collision Example.
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.
Facts
Average mass of a person 68kg.
Students come to complete stop.
Choose student 1 as our system.
Lacking
Collision time $ \Delta t$
Force of collision on student 1 in x-direction.
Approximations & Assumptions
Approximate speed of person 5m/s^-1 (half speed of olympic sprinter)
On average student 1's body compresses a couple of cm on average - assume compression of 2.5cm.
Representations
$\Delta \vec{p}_{sys} = \vec{F}_{ext} \Delta t$
$\vec{p}_{sys,f} = \vec{p}_{sys,i} + \vec{F}_{ext} \Delta t$
$\vec{v}_{avg} = \dfrac{\vec{v}_{f} + \vec{v}_{i}}{2} = \dfrac{\Delta \vec{r}}{\Delta t}$
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.
$0 = M{V}_{ix} + {F}_{x,coll} \Delta t$
$ {F}_{x,coll} = \dfrac{-M{V}_{ix}}{\Delta t}$
negative means $-\hat{x}$ direction
Need to find collision time ${\Delta t}$
$\vec{v}_{avg} = \dfrac{\vec{v}_{f} + \vec{v}_{i}}{2} = \dfrac{\Delta \vec{r}}{\Delta t}$
In 1D: $\vec{v}_{avg} = \dfrac{\Delta x}{\Delta t} = \dfrac{\vec{v}_{f} + \vec{v}_{i}}{2}$
$ {\Delta t} = \dfrac{\Delta x}{{V}_{avg}} = \dfrac{\Delta x}{\dfrac{\vec{v}_{f} + \vec{v}_{i}}{2}} $
$ \dfrac{0.025}{\dfrac{5m/s + 0m/s}{2}} = 0.01s $
$ {F}_{x,coll} = \dfrac{-M{V}_{ix}}{\Delta t} = - {\dfrac{(68kg)(5m/s)}{0.01s}}$
$ = -34,000 \,N$