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ConcepTest Clicker
Questions
Chapter 3
College Physics, 7th Edition
Wilson / Buffa / Lou
© 2010 Pearson Education, Inc.
ConcepTest 6.1 Rolling in the Rain
An open cart rolls along a
frictionless track while it is
raining. As it rolls, what
happens to the speed of the
cart as the rain collects in it?
(Assume that the rain falls
vertically into the box.)
a) speeds up
b) maintains constant speed
c) slows down
d) stops immediately
ConcepTest 6.1 Rolling in the Rain
An open cart rolls along a
frictionless track while it is
raining. As it rolls, what
happens to the speed of the
cart as the rain collects in it?
(Assume that the rain falls
vertically into the box.)
a) speeds up
b) maintains constant speed
c) slows down
d) stops immediately
Because the rain falls in vertically, it
adds no momentum to the box, thus
the box’s momentum is conserved.
However, because the mass of the
box slowly increases with the added
rain, its velocity has to decrease.
Follow-up: What happens to the cart when it stops raining?
Question 6.4 Collision Course
a) the car
A small car and a large truck
collide head-on and stick
together. Which one has the
larger momentum change?
b) the truck
c) they both have the same
momentum change
d) can’t tell without knowing the
final velocities
Question 6.4 Collision Course
a) the car
A small car and a large truck
collide head-on and stick
together. Which one has the
larger momentum change?
b) the truck
c) they both have the same
momentum change
d) can’t tell without knowing the
final velocities
Because the total momentum of the
because is conserved, that means that
Dp = 0 for the car and truck combined.
Therefore, Dpcar must be equal and
opposite to that of the truck (–Dptruck) in
order for the total momentum change
to be zero. Note that this conclusion
also follows from Newton’s Third Law.
Follow-up: Which one feels
the larger acceleration?
Question 6.5a Two Boxes I
Two boxes, one heavier than the
other, are initially at rest on a
horizontal frictionless surface.
The same constant force F acts
on each one for exactly 1 second.
Which box has more momentum
after the force acts ?
F
a) the heavier one
b) the lighter one
c) both the same
light
F
heavy
Question 6.5a Two Boxes I
Two boxes, one heavier than the
other, are initially at rest on a
horizontal frictionless surface.
The same constant force F acts
on each one for exactly 1 second.
Which box has more momentum
after the force acts ?
We know:
Dp ,
Fav =
Dt
so impulse Dp = Fav Dt.
In this case F and Dt are the
same for both boxes!
Both boxes will have the
same final momentum.
F
a) the heavier one
b) the lighter one
c) both the same
light
F
heavy
Question 6.5b Two Boxes II
In the previous question,
a) the heavier one
which box has the larger
b) the lighter one
velocity after the force acts?
c) both the same
Question 6.5b Two Boxes II
In the previous question,
a) the heavier one
which box has the larger
b) the lighter one
velocity after the force acts?
c) both the same
The force is related to the acceleration by Newton’s
Second Law (F = ma). The lighter box therefore has
the greater acceleration and will reach a higher
speed after the 1-second time interval.
Follow-up: Which box has gone a larger distance after the force acts?
Follow-up: Which box has gained more KE after the force acts?
Question 6.7 Impulse
A small beanbag and a bouncy
rubber ball are dropped from the
same height above the floor.
They both have the same mass.
Which one will impart the greater
impulse to the floor when it hits?
a) the beanbag
b) the rubber ball
c) both the same
Question 6.7 Impulse
A small beanbag and a bouncy
rubber ball are dropped from the
same height above the floor.
They both have the same mass.
Which one will impart the greater
a) the beanbag
b) the rubber ball
c) both the same
impulse to the floor when it hits?
Both objects reach the same speed at the floor. However, while
the beanbag comes to rest on the floor, the ball bounces back
up with nearly the same speed as it hit. Thus, the change in
momentum for the ball is greater, because of the rebound.
The impulse delivered by the ball is twice that of the beanbag.
For the beanbag:
For the rubber ball:
Dp = pf – pi = 0 – (–mv ) = mv
Dp = pf – pi = mv – (–mv ) = 2mv
Follow-up: Which one imparts the larger force to the floor?
Question 6.9a Going Bowling I
A bowling ball and a Ping-Pong ball
are rolling toward you with the same
momentum. If you exert the same
force to stop each one, which takes a
longer time to bring to rest?
a) the bowling ball
b) same time for both
c) the Ping-Pong ball
d) impossible to say
p
p
Question 6.9a Going Bowling I
A bowling ball and a Ping-Pong ball
are rolling toward you with the same
momentum. If you exert the same
force to stop each one, which takes a
longer time to bring to rest?
We know:
Dp
Fav =
Dt
a) the bowling ball
b) same time for both
c) the Ping-Pong ball
d) impossible to say
so Dp = Fav Dt
Here, F and Dp are the same for both balls!
It will take the same amount of time
to stop them.
p
p
Question 6.14a Recoil Speed I
Amy (150 lbs) and Gwen (50 lbs) are
standing on slippery ice and push off
each other. If Amy slides at 6 m/s, what
speed does Gwen have?
a) 2 m/s
b) 6 m/s
c) 9 m/s
d) 12 m/s
e) 18 m/s
150 lbs
50 lbs
Question 6.14a Recoil Speed I
Amy (150 lbs) and Gwen (50 lbs) are
standing on slippery ice and push off
each other. If Amy slides at 6 m/s, what
speed does Gwen have?
a) 2 m/s
b) 6 m/s
c) 9 m/s
d) 12 m/s
e) 18 m/s
The initial momentum is zero,
so the momenta of Amy and
Gwen must be equal and
opposite. Because p = mv,
then if Amy has three times
more mass, we see that
Gwen must have three times
more speed.
150 lbs
50 lbs
Question 6.14b Recoil Speed II
A cannon sits on a stationary
railroad flatcar with a total
mass of 1000 kg. When a 10-kg
cannonball is fired to the left at
a speed of 50 m/s, what is the
recoil speed of the flatcar?
a) 0 m/s
b) 0.5 m/s to the right
c) 1 m/s to the right
d) 20 m/s to the right
e) 50 m/s to the right
Question 6.14b Recoil Speed II
A cannon sits on a stationary
railroad flatcar with a total
mass of 1000 kg. When a 10-kg
cannonball is fired to the left at
a speed of 50 m/s, what is the
recoil speed of the flatcar?
Because the initial momentum of the
system was zero, the final total
momentum must also be zero. Thus, the
final momenta of the cannonball and the
flatcar must be equal and opposite.
pcannonball = (10 kg)(50 m/s) = 500 kg-m/s
pflatcar = 500 kg-m/s = (1000 kg)(0.5 m/s)
a) 0 m/s
b) 0.5 m/s to the right
c) 1 m/s to the right
d) 20 m/s to the right
e) 50 m/s to the right
Question 6.17 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?
a) the superball
b) the blob of clay
c) it doesn’t matter—they
will be equally effective
d) you are just too lazy to
throw anything
Question 6.17 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?
a) the superball
b) the blob of clay
c) it doesn’t matter—they
will be equally effective
d) you are just too lazy to
throw anything
The superball bounces off the door with almost no loss of
speed, so its Dp (and that of the door) is 2mv.
The clay sticks to the door and continues to move along with
it, so its Dp is less than that of the superball, and therefore it
imparts less Dp to the door.
Question 3.4a
A small cart is rolling at
constant velocity on a flat
track. It fires a ball straight
up into the air as it moves.
After it is fired, what happens
to the ball?
Firing Balls I
a) it depends on how fast the cart is
moving
b) it falls behind the cart
c) it falls in front of the cart
d) it falls right back into the cart
e) it remains at rest
Question 3.4a
A small cart is rolling at
constant velocity on a flat
track. It fires a ball straight
up into the air as it moves.
After it is fired, what happens
to the ball?
In the frame of reference of
the cart, the ball only has a
vertical component of
velocity. So it goes up and
comes back down. To a
ground observer, both the
cart and the ball have the
same horizontal velocity,
so the ball still returns into
the cart.
Firing Balls I
a) it depends on how fast the cart is
moving
b) it falls behind the cart
c) it falls in front of the cart
d) it falls right back into the cart
e) it remains at rest
when
viewed from
train
when
viewed from
ground
Question 3.4b
Now the cart is being pulled
along a horizontal track by an
external force (a weight
hanging over the table edge)
and accelerating. It fires a ball
straight out of the cannon as it
moves. After it is fired, what
happens to the ball?
Firing Balls II
a) it depends upon how much the
track is tilted
b) it falls behind the cart
c) it falls in front of the cart
d) it falls right back into the cart
e) it remains at rest
Question 3.4b
Now the cart is being pulled
along a horizontal track by an
external force (a weight
hanging over the table edge)
and accelerating. It fires a ball
straight out of the cannon as it
moves. After it is fired, what
happens to the ball?
Firing Balls II
a) it depends upon how much the
track is tilted
b) it falls behind the cart
c) it falls in front of the cart
d) it falls right back into the cart
e) it remains at rest
Now the acceleration of the cart is completely unrelated to the ball. In
fact, the ball does not have any horizontal acceleration at all (just like
the first question), so it will lag behind the accelerating cart once it is
shot out of the cannon.
Question 3.5
You drop a package from
a plane flying at constant
speed in a straight line.
Dropping a Package
a) quickly lag behind the plane
while falling
b) remain vertically under the
plane while falling
Without air resistance, the
c) move ahead of the plane while
falling
package will:
d) not fall at all
Question 3.5
You drop a package from
a plane flying at constant
speed in a straight line.
Dropping a Package
a) quickly lag behind the plane
while falling
b) remain vertically under the
plane while falling
Without air resistance, the
c) move ahead of the plane while
falling
package will:
d) not fall at all
Both the plane and the package have
the same horizontal velocity at the
moment of release. They will maintain
this velocity in the x-direction, so they
stay aligned.
Follow-up: what would happen if air resistance is present?
Question 3.6a
From the same height (and
at the same time), one ball
is dropped and another ball
is fired horizontally. Which
one will hit the ground
first?
Dropping the Ball I
a) the “dropped” ball
b) the “fired” ball
c) they both hit at the same time
d) it depends on how hard the ball
was fired
e) it depends on the initial height
Question 3.6a
From the same height (and
at the same time), one ball
is dropped and another ball
is fired horizontally. Which
one will hit the ground
first?
Dropping the Ball I
a) the “dropped” ball
b) the “fired” ball
c) they both hit at the same time
d) it depends on how hard the ball
was fired
e) it depends on the initial height
Both of the balls are falling vertically under the influence of
gravity. They both fall from the same height. Therefore, they will
hit the ground at the same time. The fact that one is moving
horizontally is irrelevant—remember that the x and y motions are
completely independent !!
Follow-up: is that also true if there is air resistance?
Question 3.6b
Dropping the Ball II
a) the “dropped” ball
In the previous problem,
b) the “fired” ball
which ball has the greater
c) neither—they both have the
same velocity on impact
velocity at ground level?
d) it depends on how hard the
ball was thrown
Question 3.6b
Dropping the Ball II
a) the “dropped” ball
In the previous problem,
b) the “fired” ball
which ball has the greater
c) neither—they both have the
same velocity on impact
velocity at ground level?
d) it depends on how hard the
ball was thrown
Both balls have the same vertical velocity
when they hit the ground (since they are
both acted on by gravity for the same
time). However, the “fired” ball also has a
horizontal velocity. When you add the two
components vectorially, the “fired” ball
has a larger net velocity when it hits the
ground.
Follow-up: what would you have to do to have them both reach the
same final velocity at ground level?
Question 3.6c
A projectile is launched
from the ground at an
angle of 30°. At what
point in its trajectory does
this projectile have the
least speed?
Dropping the Ball III
a) just after it is launched
b) at the highest point in its flight
c) just before it hits the ground
d) halfway between the ground and
the highest point
e) speed is always constant
Question 3.6c
A projectile is launched
from the ground at an
angle of 30º. At what
point in its trajectory does
this projectile have the
least speed?
Dropping the Ball III
a) just after it is launched
b) at the highest point in its flight
c) just before it hits the ground
d) halfway between the ground and
the highest point
e) speed is always constant
The speed is smallest at
the highest point of its
flight path because the
y-component of the
velocity is zero.
Question 3.7a
Punts I
Which of the
three punts
h
has the
longest hang
time?
a
b
d) all have the same hang time
c
Question 3.7a
Punts I
Which of the
three punts
h
has the
longest hang
time?
a
b
d) all have the same hang time
The time in the air is determined by the vertical motion!
Because all of the punts reach the same height, they all
stay in the air for the same time.
Follow-up: Which one had the greater initial velocity?
c
Question 3.7b
Punts II
A battleship simultaneously fires two shells at two enemy
submarines. The shells are launched with the same initial
velocity. If the shells follow the trajectories shown, which
submarine gets hit first ?
a
b
c) both at the same time
Question 3.7b
Punts II
A battleship simultaneously fires two shells at two enemy
submarines. The shells are launched with the same initial
velocity. If the shells follow the trajectories shown, which
submarine gets hit first ?
The flight time is fixed by the
motion in the y-direction. The
higher an object goes, the longer
it stays in flight. The shell hitting
submarine #2 goes less high,
therefore it stays in flight for less
time than the other shell. Thus,
submarine #2 is hit first.
a
b
c) both at the same time
Follow-up: which one traveled the greater distance?
Question 3.8
Cannon on the Moon
For a cannon on Earth, the cannonball would follow path 2.
Instead, if the same cannon were on the Moon, where g =
1.6 m/s2, which path would the cannonball take in the same
situation?
a
b
c
d
Question 3.8
Cannon on the Moon
For a cannon on Earth, the cannonball would follow path 2.
Instead, if the same cannon were on the Moon, where g =
1.6 m/s2, which path would the cannonball take in the same
situation?
The ball will spend more
time in flight because
gMoon < gEarth. With more
time, it can travel farther
a
b
c
in the horizontal
direction.
Follow-up: which path would it take in outer space?
d