Problem Solving Day: Geometry, movement,

and Free-fall.

Test schedule

• First mid-term in 2 weeks! 7-10PM; Feb 8, Eiesland Hall.

• Review and practice a little each day!!!

• EMAIL ME THIS WEEK if you have class or athletic conflicts. For other conflicts, please note rules on the syllabus!

• Topics: Chapters 1—3 including:• Units, conversion, and

estimation.• Horizontal kinematics and

free-fall.• Graphing x, v, and a vs. time• Projectile motion (we’ll study

it this week).

Last year’s equation sheet included with exam.

Understanding problem solving and PRACTICE are really what are going to help you do well. Make sure you practice a little every day and sort out your

misunderstandings:

you won’t do well if you wait then cram.

SOH CAH TOA!For right triangles…

adjacentoppositetan

hypotenuseadjacentcos

hypotenuseoppositesin

=

=

=

θ

θ

θs oh

adjacentoppositetan

hypotenuseadjacentcos

hypotenuseoppositesin

=

=

=

θ

θ

θ

hac

adjacentoppositetan

hypotenuseadjacentcos

hypotenuseoppositesin

=

=

=

θ

θ

θ

t oa

soh

cah

toa

Opposite

Adjacent

Hypotenuse

θ

Pythagorean Theoremc = hypotenusea2 + b2 = c2

You’re in Paris, far away from the Eiffel tower. Your friends want to walk to the tower. You know it is 300m tall, and it appears to have

an angular size of ~20 degrees. You want to impress your friends by telling them how far it is, so you remember Physics 101.1 and whip

out your calculator. About how far is the Eiffel Tower?

adjacentoppositetan

hypotenuseadjacentcos

hypotenuseoppositesin

=

=

=

θ

θ

θs oh

adjacentoppositetan

hypotenuseadjacentcos

hypotenuseoppositesin

=

=

=

θ

θ

θ

hac

adjacentoppositetan

hypotenuseadjacentcos

hypotenuseoppositesin

=

=

=

θ

θ

θ

t oa

θ

A. 110 mB. 800 mC. 1100 mD. 5 kmQ11

No talking please!

Example: Surveying the River (speak to your neighbor)

100 m

river

A surveyor wants to measure the distance across a river. Starting directly across from a big tree on the

opposite bank, he walks 100 m along the riverbank to establish a baseline. Then, he sights across to the tree. The angle from his baseline to the tree is 35 degrees.

How wide is the river? Draw a picture.

Q12

A. 1 mB. 20 mC. 70 m D. 100 m

E. 1 km

You can easily measure the size of anything you know the distance to!

adjacentoppositetan

hypotenuseadjacentcos

hypotenuseoppositesin

=

=

=

θ

θ

θ

t oa

θ

Physics Life Hack!

Your hand at arm’s length gives excellent indicator of angular size!

θ =

D

h

You can easily measure the size of anything you know the distance to!

Your hand at arm’s length gives excellent indicator of angular size!

adjacentoppositetan

hypotenuseadjacentcos

hypotenuseoppositesin

=

=

=

θ

θ

θ

toa

θ

D

hTry this when you go home tonight: estimate the size of something in

town or in your room.

Let’s solve problems!• We’ll breeze through the topics we’ve covered

• For practice and diagnosis :).

• No chatting please.

1. Dimensional analysis, unit conversion.

2. Estimation.3. Graphing.4. “Multi-phase” problems.

Dimensional analysis

Δx = v0t + ½ at21 2 3

A. m m/s m/s2

B. m m m/sC. All are m/sD. All are meters

1 2 3

Q13

What are the SI units of each piece of this equation?

Unit conversion

A. 1.0B. 3.28C. 91.44D. 328.0E. 64.6

Q14

How many cm are in 3 feet?

1 m = 3.28 ft

Estimation: Humans to the Moon

The moon is about 2.4 x 105 miles from Earth. If you stacked up adult humans, each standing

on the next one’s head, estimate how many would you need to stack to reach the Moon.

1 mile = 1604 m

A. 100 peopleB. 105 peopleC. 108 peopleD. 1010 peopleE. 1012 peopleQ15

Graphing

Q16

A driver is traveling west. She comes to a stop at a stop sign and then continues traveling in the same direction. If the positive direction is taken

to be east, which velocity vs. time graph could describes this motion?

Multi-stage problems• Make a plan.

• Identify important EVENTS and INFORMATION.

• What variables do you need to deal with?

• What is actually being asked?

• Follow problem-solving strategies for each part of the problem!

Multi-stage problems• Make a plan.

• Identify important EVENTS and INFORMATION.

• What variables do you need to deal with?

• Kinematics: always x, v, a, t

• What is actually being asked?

• Follow problem-solving strategies for each part of the problem!

A jogger running at 6 mph drops her keys, and then comes to a stop within 2 seconds. She then walks back to get her keys.

How far does she need to walk back to get the keys [assume constant acceleration]?

Practice your tips:1. Identify important parts and information.2. Draw it.3. Make axes.4. KNOWNS and UNKNOWNS.5. Pick the right formula.

Practice Resources

• Book: odd numbered problems have answers in the back!

• Office hours

• T.A. office hours.

http://sarahspolaor.faculty.wvu.edu/home/physics-101/practice-problems