Transcript Momentum and Impulse

Momentum and Impulse
Momentum:
A measure of how difficult it is to stop
a moving object.
Momentum = mass x velocity
p = mv
m
Unit for momentum: kg
s
Can a VW bug have
the same
momentum as an
ocean liner?
Can a bullet have the
same momentum as
an Army tank?
If the boulder and
the boy
have the same
momentum,
will the boulder
crush the
boy?
Hint: Which would
have the
larger speed?
How do you change the momentum of an
object?
PUSH on it for a period
of TIME.
Impulse: the product of
the force exerted on
an object and the time
interval during which
it acts.
Impulse = Force x time
Impulse = Ft
Example: Wall exerts
a force of 10,000 N.
The contact time is
0.01 s.
-----------------------------Impulse = F t
= 100 N-s
The impulse given to an object is equal to the
change in momentum of the object.
Ft = D mv = mvf – mvo
Impulse
Change in momentum
The same change in momentum may be the
result of a SMALL force exerted for a LONG
time, or a LARGE force exerted for a SHORT
time.
Example
A boy pushed against a crate at rest with a
net force of 20 N for 4 seconds. What
impulse did he impart on the crate?
Impulse = Ft = 20 x 4 = 80 Ns
What momentum did the crate have after the
push?
Impulse = change in momentum
Change in momentum = 80 Ns
Example
A boy pushed on a 8 kg crate at rest with a
net force of 20 N for 4 seconds. How fast
was the crate moving afterwards?
Ft = Dmv = mvf - mvo
vf = Ft / m
vf = 20 x 4 / 8
vf = 10 m/s
Conservation of Momentum
System- a collection of objects that is being
observed.
Closed system- no objects enter or leave the
system
Isolated system- no external forces act on any
object in the system. The objects can exert
forces on one another.
In a closed, isolated system, the total momentum
will remain the same- it will be CONSERVED.
Recoil
Is this a closed, isolated system?
Momentum before = momentum after
Momentum
Before = 0
------------Momentum
After = 0
------------After firing,
the equal but
opposite
momenta
cancel.
Conservation of Momentum in
Collisions
•
•
•
Elastic collision- A collision in which objects
collide and bounce off each other with no
energy loss. (occurs very rarely)
Inelastic collision- A collision in which objects
collide and some of the kinetic energy is
transformed into heat energy. (almost all
collisions)
Completely inelastic collision- the objects
stick together after the collision- will result in
the maximum kinetic energy loss- in fact,
sometimes ALL kinetic energy is lost if the
objects stop completely.
Momentum before = momentum after
m1v1
=
(m1 + m2) v
Three Examples of conservation of momentum in collisions with equations:
m1v1o = m2v2f
m1v1o + m2v2o
m1v1o - m2v2o = - m1v1f + m2v2f
= m1v1f + m2v2f
Recoil:
m1v1 = m2v2
Totally Inelastic collisions:
m1v1 + m2v2 = (m1 + m2)vf
Other collisions
m1v1o + m2v2o = m1v1f + m2fv2f