Download L33-MomentumImpulseConsv

Class Notes
11 November
• Turn in / Pick up Homework
 #3: Why ? This is not a surface!
 #4: Force of gravity doesn’t change
 b) Bottom because PE is greatest
 #5: Use variables; Draw a sketch
 Start with EA + WNC = EB.
• Today:
 Chapter 7: Impulse and Momentum
Linear Momentum
• We will define linear momentum
as the mass times the velocity:
p = mv
Power and momentum are,
respectively, ___ quantities.
a.
b.
c.
d.
9
Vector / Vector
Scalar / Vector
Vector / Scalar
Scalar / Scalar
3
1
0
a.
b.
c.
d.
Linear Momentum
• We will define linear momentum
as the mass times the velocity:
p = mv
• A vector quantity!
• Units: [p] = kg·m/s
• Connection to force…
Connection to Newton
• F = ma = m(v/t)
=(mv/t) = p/t
• So Newton’s second law can be
rewritten using momentum.
• A net force causes a change in
momentum!
• Ex.-Impulsive Forces
Impulse = Ft = p
…but F is not constant!
F
F
t
t
Impulse = F
t = p
p
Impulse
Ft
F
F

F
t
t
t
Example: USC-Clemson
A 400. g football resting on the
ground is kicked so that it travels
the maximum distance. It lands
70.0 m away, and the contact
time was 48.0 ms. Estimate the
force applied. What impulse is
delivered?
F = 218N, p = 10.5 kg·m/[email protected]º
Forces and Momentum
• Consider one object:
F = p /t
If the net external force on an object is
zero then…
a. The momentum
must be zero
b. The momentum
may be zero
c. The momentum
must not be zero
d. It must accelerate
6
6
0
a.
b.
c.
0
d.
Forces and Momentum
• Consider one object:
F = p /t
if F = 0 then p = constant!
• Consider two objects:
For any system of particles
there may be a net force acting
on the system…
SF1=p1/t SF2=p2/t
• So when the net external force
acting on a system is zero the
total momentum of the system
does not change, which means,
the total momentum (of the
system) is conserved!
pi = pf
• So when the net external force
acting on a system is zero the
total momentum of the system
does not change, which means,
the total momentum (of the
system) is conserved!
pi + Jext = pf
Homework
• Read Chapter 7, sections 3-4
• Do problems on Worksheet #30
 Use impulse and momentum ideas
to solve
 Warning: Velocity is a vector!