Transcript Question of the Day

Question of the Day
• Where will you be the evening of Tuesday,
March 15, 2016?
• Answer: Physics Fair
1
Question of the Day
• ________ is a measure of an object’s
location at a particular moment in time
relative to a reference point.
• Answer: Position
2
Question of the Day
• Why was the world wide web originally
created?
• Answer: To efficiently share physics data
around the world.
3
Question of the Day
• What is the slope of
the line?
• Answer: 2
16
12
8
4
0
0
2
4
6
8
10
4
Question of the Day
• What is the specific
equation for the line?
• Answer:
x = (2 m/s)∙t + 2 m
x (m)
16
12
8
4
0
0
2
4
6
8
10
t (s)
t (s)
5
Question of the Day
• The slope on an x vs. t graph represents
_______, which is an object’s _____ and
_________.
• Answer: velocity, speed, direction.
6
Question of the Day
• Which object is
going faster?
• What do the objects
have in common?
• What is different
about their motion?
• What happens at
t = 4 s?
x (m)
A
16
12
B
8
4
0
0
2
t (s)
4
6
8
10
7
Question of the Day
• Describe the
motion of
objects A & B.
• Moving? Faster?
Direction?
• Answer: A has v
= +2 m/s, B has
v = -3 m/s; B is
faster
v (m/s)
4
A
2
t (s)
0
-2
-4
B
0
2
4
6
8
10
8
Question of the Day
x (m)
v (m/s)
A
t (s)
B
t (s)
• Draw the corresponding v vs. t graph for objects
A & B.
9
Question of the Day
A
B
t=0
x=0
•
•
•
•
t=0
time intervals between data points are equal
Which object is going faster?
Which object has a head start?
Does this object stay ahead?
Are the objects ever at the same position at
the same time?
10
Question of the Day
A
B
t=0
t=0
x=0
• How many seconds of data are
represented?
• Compare the motion of A & B.
• Are A & B ever at the same place at the
same time? If so, when?
11
Question of the Day
• Gidget the Chihuahua
likes to go for pentagon
shaped walks. If Gidget
followed path ABCDE…
– What was Gidget’s total
distance traveled?
– What was Gidget’s total
displacement?
A
B
E
5m
D
C
• Answer: distance = 20 m,
Δx = 5m.
12
Question of the Day
• A cart is released from rest and allowed to
roll down a ramp. Describe its motion.
• Answer: vi = 0, but its vf ≠ 0! Therefore,
the cart undergoes a change in velocity.
13
Question of the Day
v (m/s)
v (m/s)
A
v (m/s)
C
B
t(s)
t(s)
t(s)
• Which of the above represents the motion
of an object with a velocity that is
constantly changing?
• Answer: B, A is constant velocity, C is
nonconstantly changing velocity
14
Question of the Day
v (m/s)
t(s)
• On the above graph…
– What does the slope represent?
– What are the units for slope on the above graph?
• Answer: the rate at which the velocity changes,
the acceleration; (m/s)/s = m/s2 (meters per
second, every second)
15
Question of the Day
v (m/s)
t(s)
• What is the general equation for this
graph?
• Answer: v = a ∙ t + vi
16
Question of the Day
v
t
• Is the acceleration positive, negative, or zero?
• Is the object speeding up, slowing down, or
maintaining a constant speed?
• Is the initial velocity (vi) positive, negative, or zero?
• Is the final velocity (vf) positive, negative, or zero?
• Answer: a = - , slowing down, vi = +, vf = 0
17
Question of the Day
• Brainstorm three real-life scenarios that
would demonstrate constant acceleration.
• Possible Answers: Tubing down a straight
hill, coasting on a bike down a straight hill,
rolling a laptop cart down the ramp on the
way to the lunch room, skydiving (initially),
dropping an object (any freefall), etc.
18
Question of the Day
v
v = +, slowing, a = v = 0, slowest moment, a = -
t
v = -, speeding, a = -
• Label where the velocity is +, -, and/or 0.
• Label where the velocity is slowing, speeding, or
remaining constant.
• Label where the acceleration is +, -, and/or 0.
19
Question of the Day
• A cart is launched up a ramp at 9 m/s. The
acceleration of the cart on the ramp is -3 m/s2.
– How long until the cart turns around?
– How long will it take the cart to return to its initial
position?
– What will be the cart’s velocity when it returns to its
initial position?
– What will be the cart’s velocity after 3.14159 s?
20
Question of the Day
• You release a marble down a stairway
railing. The marble’s velocity changes by
6 m/s/s.
– How fast will the marble be going after 3
seconds?
– How fast will the marble be going after
6.28318 seconds?
• Answer: t = 3 s, v = 18 m/s ; t = 6.28318 s,
v = 37.699 m/s
21
Question of the Day
m = 500 g
m = 250 g
m = 250 g
m = 250 g
vi = 0
vi = 0
vi = +
vi = 0
A
B
C
D
• Which cart will have the greatest acceleration?
• Which cart will have the least acceleration?
• Answer: Since mass and vi don’t affect the
acceleration, “D” will have the greatest
acceleration because it has the steepest ramp.
22
Question of the Day
v (m/s)
t(s)
• What is the displacement after 5 seconds?
• Answer: Δx = area between function and t
axis = ½ · (5 s) · [(5 s) · (3 m/s2)] = 37.5 m
23
Question of the Day
v (m/s)
t(s)
• What will be the object’s velocity after 30 s?
• How far will the object have traveled?
• Answer: vf = -300 m/s; Δx = -4500 m
24
Question of the Day
• A skydiver drops a golf ball from his balloon
capsule that has reached its maximum height
of 36,600 m. If there is no air resistance at
any point during the descent…
– How long will it take the ball to hit the ground?
– With what speed will the ball hit the ground?
– The speed of sound is about 340 m/s. How does
the final velocity compare to this?
• Answer: Δt = 86.4 s ; vf = 847 m/s = mach 2.5
25
Question of the Day
x
B
A
C
D
t
E
G
F
• At which point(s) is the object…
– moving the slowest? …moving the fastest?
– changing direction?
– decreasing speed? …increasing speed?
• Answer: slowest: B & F; fastest: A, D, or G;
changing direction: B & F; slowing: A B, DF;
speeding: BD, FG
26
Question of the Day
v (m/s)
t(s)
A
B
C
• The free fall acceleration on Jupiter is about 25 m/s2 and
on our Moon it is 1.6 m/s2. An object is thrown straight
up on Earth, Jupiter, and the Moon. Match the graph
with the location.
• Answer: Earth = B, Jupiter = A, Moon = C
27
Question of the Day
• Why does the ball do what the ball does?
• Answer: The ball is interacting with other
objects via forces.
28
Question of the Day
• Why is “atom” a misnomer (a false name)?
• Answer: Because “atom” means
undividable, and an atom can be divided,
or split, into more fundamental parts.
29
Question of the Day
• A large truck ran out of gas and a police car gives it a
push to the next gas station.
• How does the push of the car on the truck compare to
the push of the truck on the car…
– while speeding up to get to highway speed?
– once the car reaches the constant speed at which its driver
wishes to push the truck?
• Answer: According to Newton’s 3rd Law, Fa-b =-Fb-a
30
Question of the Day
• Which of the following best describes the
forces present as you throw a ball?
– Force of hand on ball > Force of ball on hand
– Force of hand on ball < Force of ball on hand
– Force of hand on ball = Force of ball on hand
– Not enough information is provided
• Answer: Fhand-ball = -Fball-hand
31
Question of the Day
Earth
• How many forces are acting upon the
person? What are they?
• Answer: Three forces, Fg-Earth, person, Fpbarbell, person, FN-chair, person
32
Question of the Day
• Your friend is jumping up and forward off the end
of a dock. How many forces are acting on your
friend? What are they? Are they balanced?
• Answer: Three forces, FN-dock, person, Fg-Earth, person,
Ff-dock, person; not balanced in order to provide
acceleration up and forward.
33
Question of the Day
Earth
• How many forces are acting on the
donkey? …on the person? What are
they?
• Answer: four forces on each, Fg, FN, FT, Ff
34
Question of the Day
• Draw a force diagram for the hover-puck
as it coasts along the surface of the table.
Label the force vectors with proper
notation.
• Answer: Fg-Earth, puck down, Fp-air, puck up
35
Question of the Day
• Draw a force diagram for the hover-puck
after it has left the table. Label the force
vectors with proper notation.
• Answer: Fg-Earth, puck down
36
37
Question of the Day
__ __ __.__ __ kg
• A toy helicopter hovers inside a sealed box. The
box is on a scale. Does the reading on the scale
change when the helicopter lifts off the ground?
• Answer: No, reading remains constant. Instead
of helicopter pushing down on box, helicopter
now pushes down on air, which pushes down on
box.
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39
Question of the Day
b
a
kick
•
A hover-puck moves at constant speed from “a” to “b”. At “b” it receives a kick in the direction
shown. Which path will it follow?
–
a.)
d.)
kick
kick
–
b.)
e.)
kick
–
c.)
kick
•
kick
Answer: b
40
Question of the Day
kick
• As the frictionless hover-puck is traveling in its
new direction…
– Describe its speed.
– Which of the following describes the forces upon it?
• a.) downward force of gravity
• b.) downward force of gravity, horizontal force in direction of
motion
• c.) downward force of gravity, upward force from air,
horizontal force in direction of motion
• d.) downward force of gravity, upward force from air
• e.) none (no forces act on the puck)
• Answer: speed will be constant; Fg-E, puck, Fp-air,
puck
41
Question of the Day
http://www.hq.nasa.gov/alsj/a15/a15v.1672206.mov
• Which will hit the ground first- hammer or feather?
• While the astronauts were on the moon, how did they keep
from falling off?
– Heavy boots?
– “Gravity Boots”?
-- Ropes?
-- Spikes on Boots?
-- Suction Cups?
• Answer: Hit sat same time; Same reason we don’t fall off
Earth, gravity pulls you toward the surface.
42
Question of the Day
20
kg
20
kg
100 kg
Earth
• Determine the value of FN-Earth, person.
• Answer: FN = (140 kg)(9.8 N/kg)=1372 N
43
Question of the Day
1
2
• Block 1 has a mass of 300 g and block 2 has a
mass of 150 g. The pulley is frictionless. If
block 1 is sliding at a constant velocity, then
what must be the coefficient of friction between
block 1 and the surface?
• Answer: µ = Ff/FN = 1.47 N / 2.94 N = 0.5
44
45
Question of the Day
• Draw a force diagram for the frictionless
hover-puck. Describe the hover-puck’s
motion.
• Answer: hover-puck will have a constant
acceleration to the right
46
Question of the Day
30
• The skier is keeping from sliding down the hill by
holding onto the rope.
– Draw a force diagram for the skier.
– Include vertical and horizontal reference lines
– Label any angles
• Answer: Fg, FN, and FT, the ΣF=0 since Δv=0
47
Question of the Day
• In a tug of war, which team exerts a
greater force on the other team, the
winning team or the losing team?
• Answer: The force of winning team on
losing team is equal, but opposite, to the
force of losing team on winning team. The
force each team exerts against ground
(and ground against them) is what
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determines who wins/loses.
Question of the Day
• Draw a force diagram for the frictionless
hover-puck.
• Describe its motion.
• Answer: Fg and Fp-air, puck only, forces are
unbalanced, therefore puck will accelerate.
49
Question of the Day
The only difference between each pair of objects are the values stated.
A
C
2000 kg
D
4000 kg
400 hp
B
200 hp
• Greater acceleration: a or b? …c or d?
• Answer: a > b, c > d
50
Question of the Day
• A rocket powered sled experiences a total
force of 100N. If the rocket sled has a
mass of 20 kg, what will be the resulting
acceleration?
• Answer: a = ΣF/m = 100 N / 20 kg = 5 m/s2
51
Question of the Day
• You traded in your rocket powered sled for a
rocket powered elevator. The rockets produce
an upward acceleration of 9.8 m/s2. Draw a
force diagram for a 50 kg person in the elevator.
• Answer: downward Fg of 490 N & upward FN of
980 N for a total upward Force of 490 N.
52
Question of the Day
• At the bottom of a 60 kg bungee jumper’s fall,
their velocity goes from -30 m/s to 0 m/s in 2.0
seconds. What tension in the bungee cord
causes this to happen (assume a constant
tension value)?
• Answer: Fg = 588 N, ΣF = 900 N, FT = 1488 N
53
Question of the Day
• If the elevator is moving down and slowing to a stop,
then the total force on the person is directed:
a.) up
b.) down
c.) zero total force (therefore no direction)
d.) cannot be determined
• Answer: “a”, up. Since v = - , a = + to slow to a stop.
54
Question of the Day
Original System
1.0 kg
1.0 N
• What would happen to the acceleration if…
– the pulling force was changed to 4.0 N?
– the mass was changed to 4.0 kg?
– the previous two changes were done at the same
time?
• Answer: 4 x the acceleration, ¼ x the
acceleration, the two changes would counteract
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each other  no change
Question of the Day
8 lb
16 lb
• An 8 lb bowling ball and a 16 lb bowling
ball have the same acceleration. How
must the total force on the two compare?
• Answer: Since 16 lb ball has 2x the mass,
it will require 2x the total force to have the
same acceleration.
56
Question of the Day
• A 16 g whiteboard marker is launched
down a length of PVC tubing. The end of
the marker has an area of 2.84 E -4 m2. A
lung-induced pressure of 2.5 psi (17,237
N/m2) is applied.
– What total force does the marker receive?
– What is the resulting acceleration?
– If the tube is 1.2 m long, how fast will it go?
• Answer: ΣF = 4.9 N, a = 306 m/s2 = 31g’s
– down 4 ft (1.2 m) tube, vf = 27 m/s = 60 mi/h
57
Question of the Day
• A 500 kg racecar goes from 0 to 30 m/s in
3.5 seconds. What total force must be
acting upon the racecar?
• Answer: ΣF = (500 kg)(8.57 m/s2) = 4285.7
N
58
Question of the Day
• You’re an Angry Bird. Draw the path that
you follow.
• Answer: parabolic trajectory.
59
Question of the Day
• Draw a force diagram for the Angry Bird at
this point in its trajectory.
• Answer: Fg only.
60
Question of the Day
• Based upon the bird’s force diagram, what
do you predict about the bird’s vertical
motion and the bird’s horizontal motion?
• Answer: const acc vert, const acc horiz 61
Question of the Day
A
B
C
D
• From fastest to slowest, rank the jumper’s…
– horizontal speed
– vertical speed
• Answer: horizontal, A=B=C=D; vertical,
D>A=C>B
62
Question of the Day
vx = 5 m/s
A
vx = 5 m/s
B
• Two rocks, A & B, are thrown from different
heights, as shown. Both are thrown horizontally.
Which rock will land farther away? Explain why.
• Answer: Rock A will land farther away since they
both have the same vx but A has more t to travel.63
Question of the Day
Throw
vx = 5 m/s
Drop
(vx = 0)
• One ball is thrown horizontally and the other is
dropped, both from the same height.
– Draw each ball’s trajectory.
– Which ball hits the ground first? Why?
• Answer: Same Δy, same Δt.
64
Question of the Day
x
X
• A truck moves at a constant velocity. Inside the cargo
compartment, a ball is held directly above an “x” painted
on the floor. The ball is dropped.
– Where does the ball hit the floor (on the x, in front, behind)?
– What path would a person in the cargo compartment see the ball
follow.
– A person with x-ray vision is watching all this happen from the
side of the road. What path would this person see the ball take?
• Answer: on the “x”, straight down, parabolic path forward
65
Question of the Day
Variable
Affect Time to Hit Ground?
vi-x
vi-y
ax
ay
Δx
Δy
mass
66
Question of the Day
• You throw a ball straight up in the air and then catch it on
its way back down. Between leaving and returning to
your hand…
– where is the ball going the fastest? …slowest?
– where is the acceleration the largest? …smallest?
– how will the time to the ball’s peak compare to the time to come
back down?
• Answer: fastest at immediately after thrown and before
caught, slowest at peak, acceleration is constant, time
up will be same as time down
67
Question of the Day
• Amazing Race:
http://www.youtube.com/watch?v=8cfeTZNcA3g
68
Question of the Day
• A satellite is in orbit around Earth. Draw a force
diagram for the positions shown.
• Answer: Only force is Fg, toward center of Earth
69
Question of the Day
v
A
•
A marble enters a frictionless groove, as pictured
above. “C” is the center of the circle. At position “A”,
which of the following forces are acting upon the
marble?
a.
b.
c.
d.
•
C
downward force of gravity
force exerted by the groove directed from A to C
force in the direction of motion
force from C to A
Answer: downward Fg & force by groove from A to C
70
Question of the Day
• It takes 4.3 s for a wind
turbine blade to complete
one revolution. Complete
the following table.
A
B
B
28 m
C
14 m
42 m
A
C
Angular
Velocity, ω
Linear
Velocity, v
• Answer: ω= 360°/4.3s = 83.7°/s at
A,B,C; vA = 88.0m/4.3s = 20.5 m/s,
vB = 176m/4.3s = 40.9 m/s, vC =
264m/4.3s = 61.4m/s (137 mph)
71
Question of the Day
elevator being lifted
up by cable at
constant velocity
•
Which of the following best describes the forces acting
upon the elevator?
a.
b.
c.
d.
•
upward force from cable > downward force of gravity
upward force from cable = downward force of gravity
upward force from cable < downward force of gravity
none of the above
Answer: b, forces balance each other, ΣF = 0, Δv = 0.
72
Question of the Day
M
M
2r
r
Δt = 1.0 s
Δt = 1.0 s
A
B
• Compare the angular velocity of A to B.
• Compare the linear velocity of A to B.
• Answer: A & B have equal angular
velocities, but B has 2x the linear velocity.
73
Question of the Day
i
ii
iii
iv
A
v
B
• An angry bird is drifting freely through outer space from
“A” to “B”. At “B”, it turns on (and keeps firing) a rocketpack that produces a constant thrust at 90° to its motion.
Which is the best representation of the bird’s resulting
path?
• Answer: “iv”, immediate parabolic path (the rocket
74
provides a constant unbalanced upward force)
Question of the Day
top
bottom
• A ball is attached to a string and is being swung in a
vertical circle. Draw a force diagram for the ball when it
is (a) at the top and (b) when it is at the bottom.
• Answer: At top Fg down, FT down (if even present), in
order for ΣF to be toward center of circle. At bottom  Fg
down, FT up, FT > Fg in order for ΣF to be toward the
center of the circle
75
Question of the Day
i
ii
iii
iv
v
C
A
B
• At “C”, the rocket-pack is turned off and the
thrust immediately goes to zero. Which is the
best representation of the bird’s resulting path?
• Answer: “ii”, returns to constant velocity in new
direction.
76
Question of the Day
• What would happen to the centripetal force if…
–
–
–
–
the mass doubled?
the velocity doubled?
the radius doubled?
all of the above were done at the same time?
• Answer: centripetal force would double, quadruple, half,
quadruple
77
Question of the Day
Part 1
•
A team of sled dogs exert a constant horizontal force on a heavy sled
on a flat field. As a result, the sled moves across the field at a constant
speed, vi. The constant horizontal force applied by the dogs…
a. has the same magnitude as the weight of the sled
b. is greater than the weight of the sled
c. has the same magnitude as the total force which resists the motion of the
sled
d. is greater than the total force which resists the motion of the sled
e. is greater than either the weight of the sled or the total force which resists
its motion
•
Answer: “c”, the horizontal forces must balance for constant velocity
78
Question of the Day
2M
M
2r
r
Δt = 1.0 s
Δt = 1.0s
A
B
• For objects A & B, compare their:
–
–
–
–
Linear velocity
Angular velocity
Centripetal acceleration
Centripetal force
• Answer: B has twice the linear velocity, they have equal
angular velocities, B has twice the centripetal
acceleration, B has four times the centripetal force
79
Question of the Day
Part 2
•
The team of sled dogs now doubles the amount of constant horizontal
force that they exert on the sled. As a result, the sled now moves…
a. with a constant speed that is double the speed “vi” in the previous question
b. with a constant speed that is greater than the speed “vi” in the previous
question, but not necessarily twice as fast
c. for a while with a speed that is constant and greater than the speed “vi” in
the previous question, then with a speed that increases thereafter
d. for a while with an increasing speed, then with a constant speed thereafter
e. with a continuously increasing speed
•
Answer: “e”, the horizontal forces are unbalanced → acceleration,
increasing speed
80
Question of the Day
a
overhead view
b
c
d
e
vi
•
A ball travels along a frictionless surface with constant velocity, vi. A
hammer applies a swift hit perpendicular to vi, as shown above.
– Which path will the ball follow after being hit?
– After being hit, the speed of the ball is:
a.
b.
c.
d.
e.
•
same as vi before being hit
vh, the speed that results from the hit, unrelated to the previous vi
equal to the arithmetic sum of vi and vh
smaller than either vi or vh
greater than vi or vh, but less than the arithmetic sum of the two speeds
Answer: path “d”, speed of the ball is “e”
81
Question of the Day
Part 3
•
If the team of sled dogs suddenly let their harnesses go slack and
stopped exerting a horizontal force on the sled, then the sled will…
a.
b.
c.
d.
e.
•
immediately come to a stop
continue moving at a constant speed for a while and then slow to a stop
immediately start slowing to a stop
continue at a constant speed
increase its speed for a while and then start slowing to a stop
Answer: “c”, the horizontal forces are unbalanced → acceleration,
decreasing speed
82
Question of the Day
• abc?
• Answer: xyz
83