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![Page 1: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/1.jpg)
Chap. 4: Work and Energy
R i s h i k e s h V a i d y a
Theoretical Particle Physics
Office: 3265
Physics Group, B I T S Pilani
September 4, 2010
![Page 2: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/2.jpg)
Outline
1 Work Energy Theorem
2 Potential Energy
3 Non Conservative Force
4 Conservation Laws and Particle Collisions
![Page 3: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/3.jpg)
A Small Story and a Lesson
A worried father of a BITSian who was not
doing well, asked two of his son’s friends
to keep an eye on him.
Friend A (from Humanities) kept
Detailed record of his expenditure
His interactions, clubs, department
activities
His Attendance
Affluence of Inkahol
![Page 4: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/4.jpg)
A Small Story and a Lesson
A worried father of a BITSian who was not
doing well, asked two of his son’s friends
to keep an eye on him.
Friend B (from Physics)
Didn’t bother much about the details like
friend A and yet at the end of the
semester gave a better explanation of his
friend’s CGPA.
![Page 5: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/5.jpg)
A Small Story and a Lesson
Two complementary approaches to
mechanics
Take the bull by the horn: The
dynamical methods of Chap.2
Tame the Bull: Exploit conservation
laws as in Chap. 3 and 4
![Page 6: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/6.jpg)
The Fundamental Problem of Mechanics
Given all forces, solve
~F = m~a
to find ~r(t).
Problem!
What we know is F(x) and not F(t).
Solve half the problem!
md~v
dt= ~F(~r)
![Page 7: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/7.jpg)
Equation of Motion in One Dimension
md2x
dt2= F(x) or m
dv
dt= F(x)
Integrate with respect to x
m
∫ xb
xa
dv
dtdx =
∫ xb
xa
F(x)dx
Change variable from x to t using
differentials:
dx =
(dx
dt
)
dt = vdt
![Page 8: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/8.jpg)
Equation of Motion in One Dimension
md2x
dt2= F(x) or m
dv
dt= F(x)
Integrate with respect to x
m
∫ xb
xa
dv
dtdx=m
∫ tb
ta
dv
dtvdt
=m
∫ tb
ta
d
dt
(1
2v2
)
dt =1
2mv2
∣∣∣∣
ta
tb
=1
2mv2
b − 1
2mv2
a
![Page 9: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/9.jpg)
Equation of Motion in One Dimension
md2x
dt2= F(x) or m
dv
dt= F(x)
Work-Energy Theorem:
1
2mv2
b − 1
2mv2
a =
∫ xb
xa
F(x)dx
![Page 10: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/10.jpg)
Equation of Motion in One Dimension
md2x
dt2= F(x) or m
dv
dt= F(x)
Work-Energy Theorem: Motion in 3-D
1
2mv2
b − 1
2mv2
a =
∫ b
a
~F · d~r
![Page 11: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/11.jpg)
How Useful is Work-Energy Theorem
Evaluation of∫ b
a~F · d~r is tentamount to
knowing the trajectory beforehand !Two cases where Work-EnergyTheorem is useful.
◮ Conservative Forces: For many
interesting forces∫ b
a~F · d~r depends
only on end-points !◮ Constrained Motion: External
Constraints pre-determine the
trajectory.
![Page 12: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/12.jpg)
How Useful is Work-Energy Theorem
Regardless of whether forces are
conservative or not the Work-
Energy theorem is always true
![Page 13: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/13.jpg)
Prob. 4.2 A Block of mass M slides
along a horizontal table with speed v0. At
x = 0 it hits a spring with spring constant
k and begins to experience a friction force.
The coefficient of friction is variable and is
given by µ = bx, where b is constant. Find
the loss in mechanical energy when the
block has first come momentarily to rest.
![Page 14: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/14.jpg)
Prob. 4.2
![Page 15: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/15.jpg)
Example 4.6 A pendulum consists of a
light rigid rod of length l, pivoted at one
end and with mass attached at the other
end. The pendulum is released from rest
at an angle φ0. What is the velocity of m
when the rod is at angle φ.
![Page 16: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/16.jpg)
Example 4.6
![Page 17: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/17.jpg)
Prob.4.5 Mass m whirls on a frictionless
table, held to circular motion by a string
which passes through a hole in the table.
The string is slowly pulled through the
hole so that the radius of the circle
changes from �l1 to �l2. Show that the work
done in pulling the string equals the
increase in kinetic energy of the mass.
![Page 18: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/18.jpg)
Prob.4.5 Mass m whirls on a frictionless
table, held to circular motion by a string
which passes through a hole in the table.
The string is slowly pulled through the
hole so that the radius of the circle
changes from �l1 to �l2. Show that the work
done in pulling the string equals the
increase in kinetic energy of the mass.
The fundamental mystery here is ... how can
you pull radially and still end up changing the
angular velocity of the mass.
![Page 19: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/19.jpg)
Solution 4.5 For cicular motion:
Fr = mv21
�l1
= m �l1ω21
If Fr is increased, m will move to smaller r:
Fr(r) = mv2(r)
r= mrω2(r)
Evaluate:
W = −∫
�l2
�l1
Fr(r)dr = −∫
�l2
�l1
mrω2(r)dr
![Page 20: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/20.jpg)
~a = (r − rθ2)r + (rθ + 2rθ)︸ ︷︷ ︸
=0
θ
rω + 2rω=0 [θ = ω]1
ω
dω
dt=−2
1
r
dr
dt∫ ω(r)
ω1
dω
ω=−2
∫ r
�l1
dr
r⇒ ω(r) = �l
2
1
ω1
r2
W = −∫ �l2
�l1
Fr(r)dr = −∫ �l2
�l1
mrω2(r)dr
![Page 21: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/21.jpg)
W=−m�l
4
1ω21
∫�l2
�l1
dr
r3=
1
2m�l
4
1ω21
1
�l
2
2
− 1
�l
2
1
=1
2m
�l
4
1ω21
�l
2
2
− �l
2
1ω21
W=1
2m
[
(�l2ω2)2 − (�l1ω1)
2] [
ω2 �l
2
2 = ω1 �l
2
1
]
=K2 − K1
![Page 22: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/22.jpg)
Prob. 4.7 A ring of mass M, hangs from a
thread, and two beads of mass m slide on
it without friction. The beads are released
simultaneously from the top of the ring
and slide down opposite side. Show that
the ring will start to rise if m > 3M/2, and
find the angle at which this occurs.
![Page 23: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/23.jpg)
![Page 24: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/24.jpg)
Forces are different for cosθ > 2/3 and
cos θ < 2/3
![Page 25: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/25.jpg)
Forces are different for cosθ > 2/3 and
cos θ < 2/3
![Page 26: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/26.jpg)
cos θ, N and T vs. θ (m = 2M)
![Page 27: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/27.jpg)
N and T vs. θ: Magnified view
![Page 28: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/28.jpg)
2N cos θ vs. θ
![Page 29: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/29.jpg)
Accelaration of the ring vs. θ
![Page 30: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/30.jpg)
Prob. 4.10
A block of mass M on a horizontal
frictionless table is connected to a spring
(spring constant k). The block is set in
motion so that it oscillates about its
equilibrium position with a certain
amplitude A0. The period of motion is
T0 = 2π√
M/k.
![Page 31: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/31.jpg)
Prob. 4.10
(a) A lump of sticky putty of mass m is
dropped onto the block. the putty
sticks without bouncing. The putty hits
M at the instant when the velocity of
M is zero. Find1 The new period
2 The new amplitude3 The change in mechanical energy of the
system
(b) Repeat part a, assuming that the sticky
putty hits M at the instant when M has
its maximum velocity.
![Page 32: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/32.jpg)
Potential Energy
For a conservative Force Field:
∫ ~rb
~ra
~F · d~r= func. of (~rb) − func. of (~ra)
Wba =−U(~rb) + U(~ra)
Wba =Kb − Ka [Work − energyTh.]
Wba =−Ub + Ua = Kb − Ka
Ua + Ka = Ub + Kb = E
![Page 33: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/33.jpg)
What Does Potential Energy Tell us About Force?
Ub − Ua = −∫ xb
xa
F(x)dx
U(x + ∆x) − U(x) = ∆U
∆U=−∫ x+∆x
x
F(x)dx
∆U≈−F(x)(x + ∆x − x) = −F(x)∆x
F(x)≈−∆U
∆xor F(x)
lim ∆x→0= −dU
dx
![Page 34: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/34.jpg)
Potential Energy Determins Stability of a System
Harmonic Oscillator: U = kx2/2
![Page 35: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/35.jpg)
Potential Energy Determins Stability of a System
Simple Pendulum:
U(θ) = mg�l(1 − cos θ) and dUdθ
= mg�l sin θ
![Page 36: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/36.jpg)
Potential Energy Determins Stability of a System
Simple Pendulum:
U(θ) = mg�l(1 − cos θ) and dUdθ
= mg�l sin θ
![Page 37: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/37.jpg)
Potential Energy Determins Stability of a System
General Stability Criteria:
![Page 38: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/38.jpg)
Potential Energy Determins Stability of a System
General Stability Criteria:
![Page 39: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/39.jpg)
Potential Energy Determins Stability of a System
General Stability Criteria:
![Page 40: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/40.jpg)
Rock me Spin me yet I Stand Tall:
The Amazingly Stable Teeter-Toy
![Page 41: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/41.jpg)
Rock me Spin me yet I Stand Tall:
The Amazingly Stable Teeter-Toy
![Page 42: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/42.jpg)
Rock me Spin me yet I Stand Tall:
The Amazingly Stable Teeter-Toy
![Page 43: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/43.jpg)
Rock me Spin me yet I Stand Tall:
The Amazingly Stable Teeter-Toy
![Page 44: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/44.jpg)
A Sports Car
Stability requires low center of mass and
hence the peculiar design of a sports car.
![Page 45: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/45.jpg)
Non-Conservative Forces
~F = ~Fc + ~Fnc
Wtotalba =
∫ b
a
~F · d~r
Wtotalba =
∫ b
a
~Fc · d~r +
∫ b
a
~Fnc · d~r
Kb − Ka =−Ub + Ua + Wncba
Kb + Ub − (Ka + Ua) = Wncba
Eb − Ea = Wncba
![Page 46: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/46.jpg)
Power: Time rate of doing Work
P =dW
dt= ~F · d~r
dt= ~F ·~v
Units:
[S.I.] 1 W=1J/s [CGS]1 erg/s = 10−7W
[English] 1 hp=550 lb ft/s ≈ 746W
![Page 47: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/47.jpg)
Power: Time rate of doing Work
Typical Power Consumption
Man running upstairs: 1/2 to 1 hp for
30 s
A husky man can do work over a period
of 8 hours only at a rate of
0.2 hp ≈ 1000 Kcal
Per person energy use: India (0.7 kW),
Germany (6 kW), USA (11.4 kW)
![Page 48: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/48.jpg)
Prob. 4.18 A 160 lb man leaps into the
air from a crouching position. His center
of gravity rises 1.5 ft before he leaves the
ground, and it then rises 3 ft to the top of
his leap. What power does he develop
assuming that he pushes the ground with
constant force?
![Page 49: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/49.jpg)
![Page 50: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/50.jpg)
Sol.4.18
P=W/T (W = work done by N)
W=N · 1.5 (c.g. rises by 1.5ft)
N=mg + ma or N = 160 +160
32a
a=v2
2s= 64
[
v =√
2gs′ =√
2 · 32 · 3 = 8√
3]
N=480 lb W = 720 lb.ft T = v/a =√
3/8
P = W/T = 3325lb.ft/s ≈ 6hp
![Page 51: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/51.jpg)
Prob. 4.19
In the preceeding problem take
F = F0 cos ωt where F0 is the peak force,
and the contact with ground ends at
ωt = π/2. Find the peak power that the
man develops during the jump.
![Page 52: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/52.jpg)
Sol.4.19
P(t)=N(t)v(t) [N(t) = −F(t)]
N(t) − mg=ma(t)
m
∫ v(t)
0
dv=
∫ t
0
(F0 cos ωt − mg) dt
v(t)=F0
mωsin ωt − gt [F0, ω?]
x(t)=F0
mω2(1 − cos ωt) − 1
2gt2
![Page 53: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/53.jpg)
Sol.4.19
Boundary conditions:
v(t = π/2ω)=8√
3 =F0
mω− gπ
2ω
x(t = π/2ω)=1.5 ft =F0
mω2− gπ2
8ω2
![Page 54: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/54.jpg)
Sol.4.19
ω = 9.96s−1 F0 = 832 lb t =π
2ω= 0.16s
P(t)=F(t)v(t)
F0
2mω
F0 sin 2ωt︸ ︷︷ ︸
1
− 2mgωt cos ωt︸ ︷︷ ︸
2
A reasonable approximation: F0 >> mg
then 1st >> 2nd
![Page 55: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/55.jpg)
Sol.4.19
P(t)≈ F20
2mωsin 2ωt
For Pmax. :dP
dt=0
dP
dt=
F20
mcos 2ωt = 0
Pmax. = P|t= π4ω
=F2
0
2mωsin
(
2ωπ
4ω
)
=F2
0
2mω
![Page 56: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/56.jpg)
Sol.4.19
Check:
d2P
dt2
∣∣∣∣∣t= π
4ω
=−F2
0
m2ω sin 2ωt
∣∣∣∣∣t= π
4ω
< 0
![Page 57: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/57.jpg)
Sol.4.19
![Page 58: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/58.jpg)
Sol.4.19
![Page 59: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/59.jpg)
Prob. 4.20 A uniform rope of
mass λ per unit length is
coiled on a smooth horizontal
table. One end is pulled
straight up with constant
speed v0. (a) Find the force
exerted on the end of the rope as a
function of height y. (b) Compare
the power delivered to the rope
with the rate of change of the
rope’s total mechanical energy.
![Page 60: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/60.jpg)
Sol.4.20
Fext. =Mdv
dt− vrel.
dM
dt
F − Mg=v0
dM
dt[v = v0 vrel. = −v0]
F=λyg + λv20
P=Fv0 = λygv0 + λv30
E=1
2mv2
0 + mgy
2=
1
2λyv2
0 + λgy2
2dE
dt=
1
2λv3
0 + λyv0g
dEdt
is 12λv3
0 short of P because that is the amount
dissipated as heat.
![Page 61: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/61.jpg)
What do collisions teach us?
~F = m~a is a double-edge sword
Collisions studies gave most profound
knowledge about fundamental Physics
Constraints from energy and
momentum conservation severe enough
to extract vital information about
scattering
![Page 62: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/62.jpg)
LHC@CERN Geneva
![Page 63: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/63.jpg)
Fermilab@Illinois USA
![Page 64: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/64.jpg)
Classical Collisions
A + B → C + D
Mass is conserved:
mA + mB → mC + mD
Momentum is conserved:
pA + pB = pC + pD
K.E. may or may not be conserved
![Page 65: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/65.jpg)
Classical Collisions
A + B → C + D
Types of Collisionssticky: K.E. decreases
KA + KB > KC + KD
Explosive: K.E. increases
KA + KB < KC + KD
Elastic: K.E. is conserved
KA + KB = KC + KD
![Page 66: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/66.jpg)
Prob. 4.23 A small ball of
mass m is placed on top of the
a “superball” of mass M, and
the two balls are dropped to
the floor from height h.How
high does the small ball rise
after the collision? Assume
that the collision is perfecly
elastic, and that m << M.
![Page 67: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/67.jpg)
![Page 68: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/68.jpg)
Sol. 4.23 Let v1 and v2 be the initial
velocities of m and M before collision and
v′1 and v′
2, after collision. In a two body
one dimensional collision:
v′1 =
(m − M)v1 + 2Mv2
m + M
v′2 =
(M − m)v2 + 2mv1
m + M
Here: v1 = −√2gh, v2 =
√2gh, and
M >> m
![Page 69: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/69.jpg)
Sol. 4.23
v′1 =
3M · √2gh
M= 3
√
2gh
If m rises to height h′ after collision:
1
2m1v
′21 = m1gh′
h′ =v
′21
2g= 9h
![Page 70: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/70.jpg)
Problem 4.9 A “superball” of mass m bounces
back and forth between two surfaces with speed v0.Gravity is neglected and collisions are perfectly
elastic.
(a) Find the average force F on each wall.
(b) If one surface is moving uniformly toward the
other with speed V << v0, the bounce rate willincrease due to shorter distance between
collisions, and because the ball’s speed increaseswhen it bounces from the moving surface. Find
the F in terms of separation of surfaces, x.
(c) Show that work needed to push the surface
from l to x equals gain in kinetic energy of theball.
![Page 71: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/71.jpg)
Problem 4.29
![Page 72: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/72.jpg)
Sol. 4.29
(a) Momentum transfer to wall in 1
bounce: m(2v0)
No. of bounce per unit time: v0/2�l
F = 2mv0
v0
2�l
=mv2
0
�l
(b) F(x) = mv(x)2
x
Integrate a(x) = dv(x)dt
to find v(x).
![Page 73: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/73.jpg)
Sol. 4.29
(b) ∆v after each bounce ∆v = 2V
(Sling-shot effect)
This change happens in time ∆t = 2x/v
dv
dt=
Vv
x∫ v
v0
dv′
v′ =
∫ t
0
Vdt
x= −
∫ x
�l
dx′
x′
v(x) =v0 �l
x⇒ F =
mv(x)2
x=
m�l
2v20
x3
![Page 74: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/74.jpg)
Sol. 4.29
(c) Work done in moving the surface:
W=−∫ x
�l
F(x′) dx′ = −m�l
2v20
∫ x
�l
dx′
x′3
=m�l
2v20
2
[
1
x2− 1
�l
2
]
=m�l
2v20
2x2− 1
2mv2
0
=1
2mv(x)2 − 1
2mv2
0
=Change in K.E. of ball
![Page 75: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/75.jpg)
Sling-Shot (Gravity-Assist) and NASA’s Cassini
![Page 76: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/76.jpg)
Collisions and Center of Mass Coordinates
How does it help?
Total momentum in C-system is zero
Initial and final velocities lie in the
same plane
Each particle is scattered through the
same angle θ in the plane of scattering.
For elastic collisions, the speed of each
particle is same before & after the
collision
![Page 77: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/77.jpg)
Collisions and Center of Mass Coordinates
m1 & m2 having velocityv1 & v2.
V =m1v1 + m2v2
m1 + m2
![Page 78: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/78.jpg)
Collisions and Center of Mass Coordinates
v1c =v1 − V
=m2
m1 + m2
(v1 − v2)
v2c =v2 − V
=− m1
m1 + m2
(v1 − v2)
![Page 79: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/79.jpg)
Collisions and Center of Mass Coordinates
The momenta inC-system:
p1c =m1v1c
=m1m2
m1 + m2
(v1 − v2)
=µv
p2c =−µv
0=p1c + p2c
![Page 80: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/80.jpg)
C-System Plane of Scattering
m1 and m2 striking with
v1 & v2.
Initial velocities in L &
C-system.
![Page 81: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/81.jpg)
C-System Plane of Scattering
m1 and m2 striking with
v1 & v2. Final velocities in L &
C-system (The plane in
general is different)
![Page 82: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/82.jpg)
C-System Plane of Scattering
C-system plane of
scattering.
For elastic scattering,
v1c = v′1c and v2c = v′
2c.
Velocity vectors simple
rotate.
![Page 83: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/83.jpg)
If m2 is at rest
V=m1
m1 + m2
v1
v1c =v1 − V
=m2
m1 + m2
v1
v2c =−V
=− m1
m1 + m2
v1
![Page 84: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/84.jpg)
If m2 is at rest
V=m1
m1 + m2
v1
v1c =v1 − V
=m2
m1 + m2
v1
v2c =−V
=− m1
m1 + m2
v1
![Page 85: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/85.jpg)
Limitations on Lab scattering angle
λ = m1m2
= Vv1c
< 1
θ : 0 → π θ1 : 0 → π
![Page 86: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/86.jpg)
Limitations on Lab scattering angle
λ = m1m2
= Vv1c
= 1
θ : 0 → π θ1 : 0 → π/2
![Page 87: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/87.jpg)
Limitations on Lab scattering angle
λ = m1m2
= Vv1c
> 1
θ : 0 → π θ1 : 0 → θmax.1
![Page 88: Chap A](https://reader034.fdocuments.net/reader034/viewer/2022050801/55247c9e4a7959d4488b47b2/html5/thumbnails/88.jpg)
Look at solved example 4.19 to understand
limitations on laboratory scattering angle.
It will also help you solve problem 4.30