1 PHY131H1F - Class 4 Today: • Vector Math • Velocity and Acceleration in 2- dimensions • Relative Velocity • Constant Acceleration in x and y • Projectile Motion • Motion on a Circular Path Clicker Question • A large, light beach ball is falling towards the beach on a windless day. The force of gravity on the ball, , is greater than the upward drag force from the air, . Which of the following directions is closest to the direction of the net force net = + on the ball? A.North B.East C.South D.West E.The net force makes an angle of 90° with respect to all four of these directions. East
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PHY131H1F - Class 4jharlow/teaching/phy131f15/lec04... · PHY131H1F - Class 4 Today: •Vector Math •Velocity and Acceleration in 2-dimensions •Relative Velocity •Constant Acceleration
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1
PHY131H1F - Class 4
Today:
• Vector Math
• Velocity and
Acceleration in 2-
dimensions
• Relative Velocity
• Constant Acceleration in
x and y
• Projectile Motion
• Motion on a Circular
Path
Clicker Question
• A large, light beach ball is falling
towards the beach on a windless day.
The force of gravity on the ball, 𝐹𝑔, is
greater than the upward drag force from
the air, 𝐹𝐷. Which of the following
directions is closest to the direction of
the net force 𝐹net = 𝐹𝑔 + 𝐹𝐷 on the ball?
A.North
B.East
C.South
D.West
E.The net force makes an angle of 90°
with respect to all four of these
directions.
𝐹𝐷
𝐹𝑔East
2
The Pointing Game
• There are three pairs of orthogonal directions in
this room.
• Within a pair, each direction is anti-parallel to the
other.
• Each pair is perpendicular to the other two pairs.
• Point with me:
North - South
West - East
Up - Down
Class 4 Preclass Quiz on MasteringPhysics
This was due this morning at 8:00am
92% answered correctly: In the study of physics, a scalar
is specified with a single number, but a vector is specified
using both a magnitude and direction.
Harlow note (based on student comments): A scalar can
have dimensions, or be dimensionless. ie: m = 5 kg. is a
scalar with dimensions of mass [M]. But π = 3.14 is a
dimensionless scalar (it is a ratio). Most vectors have
dimensions, but unit-vectors do not.
3
Class 4 Preclass Quiz on MasteringPhysics
88% answered correctly: The speed of the mouse
relative to the floor is the vector sum of the
mouse speed relative to the belt and the belt
speed relative to the floor (5 m/s)
88% answered correctly: Both bullets hit the
ground simultaneously!
84% answered correctly: When an object moves
in a circular path at a constant speed, the velocity
and acceleration vectors are perpendicular.
Student comments from Class 4 Preclass Quiz
“Was the mouse on the conveyor belt or was it on the edge
of the belt before it saw the cheese?”
Harlow answer: It was on the belt. The belt was the
mouse’s universe at that time – it didn’t care that the factory
floor was moving at a different velocity.
“I understand that in theory, both bullets should reach the
earth at the same time. However, doesn't the curvature of
the earth become a factor when firing a bullet over longer
distances? If so, would that not alter the height at which the
bullet being fired actually hits the ground?”
Harlow answer: YES! That’s a really good point! In fact,
due to the curvature of the earth, the fired bullet would take
slightly longer to hit the ground, because it would fall a
further distance.
4
Student comments from Class 4 Preclass Quiz
“I did this quiz without doing the assigned readings. I don't
even know what the assigned readings are, but I will find out
now and read it all posthaste.”
Harlow comment: Okay. Thank you for your honesty. Keep
in mind that you have defeated the entire purpose of the pre-
class quiz by doing this, though.
“Relative velocity seems like witchcraft. Please explain this
sorcery.”
“I just wonder what the rules are for this feedback section in
order to get the mark. Surely profanity is off the table, but
what about emoticons? Well, here goes nothing :P”
Student comments from Class 4 Preclass Quiz
“Confused why the section name includes 3-d motion, when
there isn't 3-d motion”
Harlow answer: 3-d motion exists. Most motion for this
course does happen in 2-D, which is a lot easier to draw.
But you should be aware that we live in a 3-D universe so, in
general, everything happens in 3-d.
“I like many people find frames of reference to be very
confusing. I watched the youtube video Frames of Reference
(1960) which helped me a lot with my understanding.
Surprisingly, this film was created in UofT many years ago.”
Harlow note: little known fact: Patterson Hume, one of the
two guys in that famous “Frames of Reference” video
https://youtu.be/aRDOqiqBUQY , was my grandmother’s first
cousin. (The taller one with straight hair.) I have hung out
with him on many family gatherings and he was a super-cool