Transcript Impulse, Momentum and Conservation of Momentum

Impulse, Momentum and
Conservation of Momentum
Newton Again!
 Newton observed objects colliding and
realized that two things dictate what it takes
to change the motion of an object.
Mass (how much matter)
 Velocity (how fast it’s going
in a given direction)

He Called This Momentum
p=
.
mv
Large Mass
The more mass
you have, the
more
momentum you
will have at a
given speed.
Homer Has Momentum
The faster Homer moves, the
more momentum he has for his
constant mass.
Conservation of Momentum
Let’s look at what happens to
momentum before, during and
after a collision/explosion.
Before collision
M=2000 kg, v = 5 m/s
After collision
The system has the
same total
momentum.
M = 500 kg, v = 0
Impulse “J”
 By rewriting his own 2nd law, Newton
defined impulse
F= m . a =m . v
t
 F . t = m . Δv
 J = F . t= mΔv = Δp =change in momentum
So Force
. time=ΔMomentum
Lets say Mini-me was trying to
stop Dr. Evil from running at
him.
Mimi-mi’s relatively small force
on Dr. Evil would have to be
exerted over a long period of time
to change the momentum of Dr.
Evil.
 So the safest way to stop an object is to
stretch out the time it takes to stop things so
you could use the smallest possible force.
That’s why if something is cushioned and
soft it is less likely to break.
Momentum is Conserved
TZ student defend school with potato
Conservation of Momentum
Newton’s Third Law:
The potato goes one way and the
gun “recoils” in the opposite
direction.
The gun exerts a force on the potato
and the potato exerts an equal but
opposite force on the gun, recoil.
pGun/Girl = pPotato
These forces produce equal but opposite
changes in momentum.
Since the girl is “attached” to the gun,
the combination of the girl /gun mass is
much greater than the mass of the
potato.
The girl and the gun recoil at a velocity
much smaller than the potato.
If pGun&Person = pPotato Then
Mgun/person.
vpotato
vgun/person=mpotatoe.
 If the mass on the left of the equation is large
compared to the right, then the velocity on the right
must be large compared to the left if they are to be
equal.
Action/Reaction
Conservation of
momentum is just a
consequence of
Newton’s third Law