Transcript Momentum

momentum

inertia in motion

inertia
 the tendency an object has to resist a change in
motion

inertia is measured by mass
 the more mass an object has, the greater it
resists a change in motion
momentum = mass • velocity
p = mv
units for momentum
mass is measured in kg
 velocity is measured in m/s


momentum is therefore measured in
kg•m/s
find the momentum of

a 1.5 kg runaway skateboard rolling
down the street at 4 m/s

a .5 kg fish swimming away from a shark
at 18 m/s

a 20,500 g horse galloping at 12 m/s in
a meadow
Which has greater momentum?

a .5 kg shooting star moving in space at
8,000 m/s
or

a 1,000 kg space shuttle cruising at 4 m/s

If the momentum of an object changes either
the mass, the velocity, or both have to
change.

If the mass remains unchanged, the velocity
must change—which produces an
acceleration.

What produces an acceleration?
a force!
a force produces an acceleration
when exerted on a mass
Newton’s Second Law of Motion
impulse

the change in momentum
impulse = F∆t

The greater the impulse exerted on an object,
the greater will be the change in momentum
∆(mv) = F∆t
average force

the force exerted on an object changes
from instant to instant, so when we talk
about the force used to change the
momentum of an object we are actually
mentioning the average force
Increasing Momentum

to gain the greatest change in
momentum, it makes sense to apply the
greatest force for the longest period of
time

Example:
 following through while shooting a basketball
or swinging a baseball bat
 If
you are driving a car that’s out
of control and you had to decide
to crash into a concrete wall or
stack of hay, which would you
choose?
the stack of hay!
….but why?
∆(mv) = F∆t
by
extending the time during
which your momentum is
brought to zero, the force of
impact is reduced

Example:
 catching a baseball with your bare hands
What
advantage do
bumpers on a car provide
in a collision?
What
about air bags?
∆(mv) = F∆t
 if
the time of impact in a collision is
increased by 4 times, how much does
the force of impact change?
 if
the time of impact in a collision is
decreased by 100 times, how much
does the force of impact change?
by
shortening the amount of
time in an impulse given to an
object, the force of impact is
greatly increased

Example:
 breaking wood slats in karate class
 quick jabs in a fight