ConcepTest 8.1 Crash Cars I
If all three collisions below are
totally inelastic, which one(s)
will bring the car on the left to
a complete halt?
1) I
2) II
3) I and II
4) II and III
5) all three
ConcepTest 8.1 Crash Cars I
If all three collisions below are
totally inelastic, which one(s)
will bring the car on the left to
a complete halt?
In case I, the solid wall
clearl stops the car
clearly
car.
In cases II and III, since
ptot
collision
t t = 0 before the collision,
then ptot must also be zero
after the collision,
collision which
means that
th t the
th car comes
to a halt in all three cases.
1) I
2) II
3) I and II
4) II and III
5) all three
ConcepTest 8.2 Crash Cars II
If all three collisions below are
1) I
totally inelastic, which one(s)
2) II
will cause the most damage
3)) III
(in terms of lost energy)?
4) II and III
5) all three
ConcepTest 8.2 Crash Cars II
If all three collisions below are
1) I
totally inelastic, which one(s)
2) II
will cause the most damage
3)) III
(in terms of lost energy)?
4) II and III
5) all three
The car on the left loses
the same KE in all 3 cases
cases,
but in case III
III, the car on
the right loses the most
KE because KE = 1/2 m v2
and the car in case III has
the largest
g
velocity
velocity.
y
ConcepTest 8.3 Motion of CM
Two equal
equal-mass
mass particles
(A and B) are located at
some distance from each
other Particle A is held
other.
stationary while B is
moved away at speed v.
What happens to the
center of mass of the
two-particle system?
1) it does not move
2)) it moves away
y from A with speed
p
v
3) it moves toward A with speed v
4) it moves away from A with speed 1/2 v
5) it moves toward A with speed 1/2 v
ConcepTest 8.3 Motion of CM
Two equal
equal-mass
mass particles
(A and B) are located at
some distance from each
other Particle A is held
other.
stationary while B is
moved away at speed v.
What happens to the
center of mass of the
two-particle system?
1) it does not move
2)) it moves away
y from A with speed
p
v
3) it moves toward A with speed v
4) it moves away from A with speed 1/2 v
5) it moves toward A with speed 1/2 v
Let’s say that A is at the origin (x = 0) and B is at
some position x.
x Then the center of mass is at x/2
because A and B have the same mass. If v = Δx/Δt
tells us how fast the position of B is changing,
then the position of the center of mass must be
changing like Δ(x/2)/Δt, which is simply 1/2 v.
ConcepTest 8.4 Momentum and KE
At time t = 0, a particle initially at
rest is subject to a resultant
force in the +x direction which
causes its momentum to
increase proportional to t 3. If
the value of the force at t = 0 is
zero the kinetic energy of the
zero,
particle subsequently increases
at a rate proportional to:
1) t
2)) t 2
3) t 3
4) t 5
5) t 6
ConcepTest 8.4 Momentum and KE
At time t = 0, a particle initially at
rest is subject to a resultant
force in the +x direction which
causes its momentum to
increase proportional to t 3. If
the value of the force at t = 0 is
zero the kinetic energy of the
zero,
particle subsequently increases
at a rate proportional to:
1) t
2)) t 2
3) t 3
4) t 5
5) t 6
The momentum of the particle is p = mv. Since p
increases proportional to t 3, this implies v
increases proportional to t 3. Kinetic energy is
½ mv2 so kinetic energy increases proportional to
(t 3)2 = t 6.
ConcepTest 8.5 Shut the Door!
You are lying in bed and you want to
shut your bedroom door. You have a
superball and a blob of clay (both with
the same mass) sitting next to you.
Which one would be more effective
to throw at your door to close it?
1) the
th superball
b ll
2) the blob of clay
3) it doesn’t
doesn t matter -- they
will be equally effective
4) you are just too lazy to
throw anything
ConcepTest 8.5 Shut the Door!
You are lying in bed and you want to
shut your bedroom door. You have a
superball and a blob of clay (both with
the same mass) sitting next to you.
Which one would be more effective
to throw at your door to close it?
1) the
th superball
b ll
2) the blob of clay
3) it doesn’t
doesn t matter -- they
will be equally effective
4) you are just too lazy to
throw anything
The superball bounces off the door with almost no loss of
speed, so its Δp (and that of the door) is 2mv
2mv.
The clay sticks to the door and continues to move along with
superball and therefore
it, so its Δp is less than that of the superball,
it imparts less Δp to the door
door.
ConcepTest 9.1
Bonnie and Klyde I
Bonnie sits on the outer rim of a
merry-go-round, and Klyde sits
midway between the center and the
rim The merry-go-round makes
rim.
one complete revolution every two
seconds.
Kl d ’ angular
Klyde’s
l velocity
l it is:
i
1)) same as Bonnie’s
2) twice Bonnie’s
3) half of Bonnie
Bonnie’s
s
4) 1/4 of Bonnie’s
5) four times Bonnie’s
ω
Klyde
Bonnie
ConcepTest 9.1
Bonnie and Klyde I
Bonnie sits on the outer rim of a
merry-go-round, and Klyde sits
midway between the center and the
rim The merry-go-round makes
rim.
one complete revolution every two
seconds.
Kl d ’ angular
Klyde’s
l velocity
l it is:
i
1)) same as Bonnie’s
2) twice Bonnie’s
3) half of Bonnie
Bonnie’s
s
4) 1/4 of Bonnie’s
5) four times Bonnie’s
The angular velocity ω of any point
ω
on a solid object rotating about a
fi d axis
fixed
i is
i the
th same.
same Both
B th Bonnie
B
i
& Klyde go around one revolution
Klyde
(2π radians) every two seconds.
Bonnie
ConcepTest 9.2
Bonnie and Klyde II
Bonnie
B
i sits
it on th
the outer
t rim
i off a merrygo-round, and Klyde sits midway
between the center and the rim. The
merry-go-round
d makes
k one revolution
l ti
every two seconds. Who has the larger
linear (tangential) velocity?
1) Klyde
Kl d
2) Bonnie
3) both the same
4) linear velocity is
zero for both of them
ω
Klyde
Bonnie
ConcepTest 9.2
Bonnie and Klyde II
Bonnie sits on the outer rim of a merrymerry
go-round, and Klyde sits midway
between the center and the rim. The
merry go round makes one revolution
merry-go-round
every two seconds. Who has the larger
linear (tangential) velocity?
1) Klyde
Kl d
2) Bonnie
3) both the same
4) linear velocity is zero
for both of them
Their linear speeds v will be
ω
different since v = Rω
Rω and
Bonnie is located further out
Klyde
(larger radius R) than Klyde.
1
VKlyde = VBonnie
2
Follow--up: Who has the larger centripetal acceleration?
Follow
Bonnie