Transcript Chapter1_2

Announcements:
- Get lab manual and look at first experiment before your first lab. Labs start week
of Jan. 25; room Olin 105
- Homework 1.1 due this Thursday, in class
- Bring i-clicker to class
- You are allowed 30 missed points in the i-clicker total score (~ 160 points)
• Homework TAs:
– Jillian Bjerke
– Colby Meador
– Calli Nguyen
(room 311)
• Lab TAs:
Samrat Dutta, Ashley Carlton, Maggie Baldwin, Calli Nguyen, Jack Own,
Zach Vance
(contact TAs if you have questions about lab or homework grading first)
PHY110 TUTOR SESSIONS
Tutor:
Jillian Bjerke & Maggie Baldwin
Session 1:
Session 2:
Session 3:
Mo, 4-6 pm
We, 4-6 pm
Th, 5-7 pm
(Jill)
(Jill)
(Maggie)
All tutorial session will be in Olin 101 (class room).
The tutor sessions in semesters past were very successful and received high marks from many
students.
All students are encouraged to take advantage of this opportunity.
There are also private tutors available, contact Judy Swicegood in the
Physics office (Olin 100)
Chapter 1: The laws of motion, Part I
First two chapters: Introduce the “language of physics”
Subsequent chapters: Explore objects and underlying physical concepts
- Reading assignment for today:
- Homework 1.2 (Meador Colby):
- due Thursday, Jan. 28 in class:
Exercises: 14, 15, 18, 19
Problem: 3, 4, 5, 6, 7
Chapter 1.2
Chapter 1.2
Falling balls:
Demos and Objects
Falling balls
Two falling balls
Throwing balls
Shooting a falling apple
(target)
Concepts
Gravity
Weight
Everything falls at the same
rate
Symmetry of up and down
motion
Vector components
i-clicker question-1:
Suppose that I throw a ball upward into the
air*. After the ball leaves my hand,
A. there is a force pushing the ball upward.
B. there is a force pushing the ball
downward.
C. there is no force acting on the ball.
D. it moves at constant velocity.
* We are ignoring friction, drag, buoyant force
Observations About Falling Balls
• A dropped ball:
– Begins a rest, but soon acquires downward speed
– Covers more and more distance each second
• A tossed ball:
– Rises to a certain height
– Comes briefly to a stop
– Begins to descend, much like a dropped ball
 There is a _________ force acting on the ball!
i-clicker question-2:
You are throwing a ball straight up in the air.
At the highest point, the ball’s
A. velocity and acceleration are zero.
B. velocity is nonzero but its acceleration is zero.
C. acceleration is nonzero, but its velocity is zero.
D. velocity and acceleration are both nonzero.
Brief Aside: Gravity and Newton’s Law of Universal Gravitation
Every particle in the Universe attracts every
other particle with a force of:

m m
F12  G  1 2 2
r
G… Gravitational constant G = 6.673·10-11 N·m2/kg2
m1, m2 …masses of particles 1 and 2
r… distance separating these particles
On earths surface:
 Gravity creates a downward force on any object.
 Object attracted directly to towards the center of the earth.
 Gravitational force is equal to the objects weight.
Weight = F = m·g
Mass, weight and gravity from a physicists point of view
• The mass of an object, m, does not change. It is measured
in kilogram.
• The weight of an object is given by W = m·g,
g: acceleration due to gravity
m: mass of the object
• Weight is a force!
• Gravity: attractive force between two objects.
(mainly between the earth and objects on earth)
On earth: g = 9.8 m/s2
On the moon: g = 1.6 m/s2
A Falling Ball
• Falling ball accelerates steadily downward
– Its acceleration is constant and downward
– Its velocity increases in the downward direction
• Falling from rest (stationary):
– Velocity starts at zero and increases downward
– Altitude decreases at an ever faster rate
How can we quantitatively describe motion?
Motion with constant acceleration
1. Velocity:
Velocity = starting velocity + acceleration x time
v = v0 + a • t
2. Position:
Position:
x = x0 + v0 t + 1/2 a • t2
x0… starting position
v0… starting velocity
A dropping ball
Quantitatively:
1. Acceleration is constant
a = g = - 9.8 m/s2
2. Velocity increases
v = v0 + g•t
But v0 = 0
because ball released
from rest so
v = - 9.8 m/s2 • t
3. Position changes rapidly
x = x0 + v0t + 1/2 gt2
with x0 = 0 and v0 = 0
we have
x = 0.5•(-9.8 m/s2 ) • t2
Black board examples
1. If it takes you 1.4 s to reach the water from a 10 m
platform, how fast will you go just before you enter the
water?
2. How far will you fall during a 5-second free fall?
A Falling Ball, Part 2
• A falling ball can start by heading upward!
–
–
–
–
–
–
Velocity starts in the upward direction
Velocity becomes less and less upward
Altitude increases at an ever slower rate
At some point, velocity is momentarily zero
Velocity becomes more and more downward
Altitude decreases at ever faster rate
i-clicker question-3:
You have the ability to throw a rock with a
speed of 30 m/s (67 mi/h).
In order to break the ice of a pond, do you
throw the rock up or down?*
a. Straight up
b. Straight down
c. Doesn’t matter
* We are ignoring friction, drag, buoyant force, same release height
Throwing a
ball upward
Note that the up and
down motions are
symmetric!!
It takes the same time
to go up as it does to
come back down.
Throwing a ball straight up and down
Going up
Going down
Throws and Arcs
• Gravity only affects
vertical motion
• A ball can coast
horizontally while
falling vertically
Bouncing Balls: Same Time up as Down
Vertical and horizontal components of a vector
(velocity, acceleration, force, …)
The vertical velocity gets smaller as the horizontal
component gets larger
i-clicker question-4:
i-clicker question-4:
An apple drops at the same time a shot is fired.
Do we aim
a) At the target?
b) Above the target?
c) Below the target?
i-clicker question-5:
Dropping balls
Which ball will hit the
ground first?
A. Straight drop
B. Straight out
C. Both at the same
time