Physics 20
Day 8: Word Problems with Gravity
•
Questions from last nights
homework?
Measurement
•
Find the value of the
measurements taken from
the scale below
•
Draw the scale for which
the following measurement
were taken
A) 0.7 cm
B) 23.7
o
C
3.2 x 10
2
g
5.4 x 10
2
cm
Physics terms
•
What distance has the skier traveled after 3 min?
•
What is the skier’s displacement after 3 min?
•
What is the skier’s average velocity for the first 2 min?
420 m
140 m
20 m/min
Conventions of direction and sign
•
Determine whether the vector values of initial
displacement, velocity and acceleration should
have positive or negative values
a)
An object slows down as
it moves east towards
you
b)
A ball is thrown
downward off a cliff
Dis.
Vel.
Accel
.

+

0


Problem Solving Strategy:
1)
Construct
an informative
diagram
of the physical
situation
.
2)
Identify
and list
the
given and unknown
information in
variable
form
.
3)
Identify
and list the
formula
(variable isolated before
numerical solution)
which will be used
to determine
unknown information from
known information
.
4)
Substitute
known values into the equation and
use
appropriate
algebraic steps to solve for the
unknown
information
.
5)
Check
your
numerical answer
to insure that it is
reasonable
and mathematically
correct.
6)
Sentence
with significant digits and units to properly
communicate your answer.
Word Problems
•
1. A car travels +3.0 x 10
3
m while
moving with an average velocity of
+15 m/s. What time is required for
this motion
•
An airplane reaches an air speed of
640 km/h after accelerating. If the
plane's average velocity was 164
m/s, what was the initial velocity of
the plane?
The time required is
2.0 x 10
2
s
The initial velocity was 150 m/s or 541 km/h
avg
d = v (t)
i f
avg
v + v
km m
v = 640 177.777
2 h s
i avg f
v = 2v  v
i
i
v = 2(164)  177.777
v = 150.223 m/s
a
D
t =
v
3
3 x 10
t =
15
Smile of the Day!
Daily Objectives:
1)
Interpret which formula to use for a given kinematics word
problem and understand the process of solving the problem.
(20 A1.3k)
2)
Be able to complete the mathematical component of solving
word problems.
(20 A1.3s)
Solving Word Problems:
Be able to solve a mixed set of word problems involving a
single uniform or accelerated motion in 1

dimension
(including vertical free fall).
Be able to recognize the different concepts in a written
problem and infer information about vertical motion cases (to
fall: v
i
= 0 m/s,
a =

9.81 m/s
2
).
Galileo’s Experiments
http://www.pbs.org/wgbh/nova/galileo/experi
ments.html
Gravity on the moon video clip….
Kinematics Equations and Free

Fall
1) When no forces are being applied, the acceleration is

9.81 m/s
2
2) A dropped object has an initial velocity of 0 m/s.
3) An object moving
horizontally
has an initial
vertical
velocity
of 0 m/s.
4) Displacement may be taken from the ground as a zero
position or the starting position of the object as the
zero position.
5) If an object leaves the ground at v m/s and rises against
gravity, it will have the same speed (but opposite
direction)
when it returns to the vertical level
of the
starting position.
•
A free

falling object is an object
which is falling under the sole
influence of gravity. That is to
say that any object which is
moving and being acted upon
only be the force of gravity is
said to be "in a state of
free
fall
."
•
Such an object will experience a
downward acceleration of 9.8
m/s/s (which is often
approximated to 10 m/s/s).
•
Whether the object is falling
downward or rising upward
towards its peak, if it is under
the sole influence of gravity, its
acceleration value is 9.8 m/s/s.
Example #1:
•
A coin is flipped into the air at 6.00 m/s.
How long does it
take to reach it's highest point, and if it continues moving for
2.50 s, what will it's velocity be?
Diagram:
Variables:
v
i
= 6.00 m/s
a =

9.81 m/s
2
v
f
= 0 m/s
t = ?
Formula:
a = (
v
f
–
v
i
) / t
Rearrange and Substitute:
Final Velocity after 2.50 s:
The final velocity of the coin will be 19.0 m/s downward.
Example #2:
•
A rock is dropped from a bridge and strikes the water
below in 3.50 s. How high is the bridge?
–
Although there appears to be insufficient information, for
any vertical motion where there is not contact force, (free
fall) we know that gravity contributes an acceleration of

9.81 m/s
2
. Also, when the rock is dropped, v
i
= 0
Diagram:
Variables:
Formula:
Substitution:
Answer:
Example #3:
•
A soccer ball is kicked directly upwards at 20
m/s. Ignoring air resistance, how long will it be
in the air?
Remember:
1)
formula with the unknown variable isolated.
2)
substitution of the known variables in
compatible units.
3)
answer with significant digits, units and a
sentence.
Homework:
Worksheet #3: 16

27
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