Transcript Elastic Collisions

Bouncing and Momentum Change
• Bouncing Results in greater forces…..
Ball
Δp=FΔt
p
-p
Bouncing and Momentum Change
• Bouncing Results in greater forces…..
Ball
Δp=FΔt
p
-p
Pelton Wheel (1888, Lester Pelton)
• Pelton wheel was
a modified water
wheel that
increased the
previous efficiency
(90.2% vs
previous 76.5%)
Impulse - Momentum
Example
A 1.3 kg ball is coming straight at a 75 kg soccer player at 13 m/s who
kicks it in the exact opposite direction at 22 m/s with an average force
of 1200 N. How long are his foot and the ball in contact?
answer: We’ll use Fnet t =  p. Since the
ball changes direction,
 p = m  v = m (vf - v0)
= 1.3 [22 - (-13)]
= (1.3 kg) (35 m/s)
= 45.5 kg · m /s.
Thus, t = 45.5 / 1200 = 0.0379 s, which is just
under 40 ms.
Angular Momentum
• Defined as the quantity of motion of
objects that are rotating about a fixed axis.
• Depends upon the
• Mass of the object (greater mass, greater L)
• Distribution of that mass (at large radius, greater L)
• Tangential velocity (greater VT, greater L)
Angular Momentum
Moment of Inertia
I=
2
mr
Moment of inertia is
dependent upon the shape
of the object rotating and
the distribution of its mass
Moment of inertia is like a
“rotational inertia”
Moment of inertia for some objects
Page 304
Moment of Inertia
Conservation of Momentum
• “For a closed, isolated
system, the total momentum
remains constant”
Closed Isolated System
A system is a collection
of two or more objects.
An isolated system is a
system which is free
from the influence of a
net external force which
alters the momentum of
the system . A closed
system is one that doesn’t
gain or lose mass.
Why does cannon move backward?????
Johnny Hart—died April 2007
The Sport of Curling
If curling stones have same mass….what happens
when one strikes a stationary one head on?
What do rockets “push
against?”
Spacecraft in outer space can accelerate by firing attached
rockets. Since there is nothing in space for the escaping gases
to push against, how can the hot gases escaping from
these rockets cause direction change?
Because momentum of
“system” was zero before he
pulls trigger, then
momentum is also zero after
he pulls trigger. This then
means that the forward
momentum of man and
extinguisher must be equal,
but opposite, the momentum
of the gases exiting the
extinguisher.
Conservation of Angular
Momentum in Spin of Ice Skater
Why does this skater spin faster??
Why does pulling the arms in cause the
spin rate to increase?
Pulling the arms in
causes her moment
of inertia to
decrease. If the
angular momentum
remains constant,
then the angular
velocity must
increase for that to
happen.
What happens to wind speeds if tornado “shrinks” in diameter?????
Collisions
• Elastic Collisions -- collide without
permanently deforming (bounce)
• Inelastic Collisions -- collide, may
be deformed permanently, or stick
(generate heat in the process)
Types of Collisions
Remember, momentum remains constant……
Elastic Collision
Inelastic collision
Conservation of Momentum