MCAT Physics Review 1

8/18/2019 MCAT Physics Review 1

1/46

MCAT Physics Review

Grant Hart

[email protected]


2/46

Historical areas of emphasis --

probably similar in the future

Mechanics 25%

Fluid Mechanics 20%

Waves, Optics, Sound 20%

Electricity & Magnetism 10%

Nuclear & Atomic Physics 15%

Tools 10%


3/46

Important Ideas about the

Chemical and Physical Foundations

part of the MCAT

• The problems are not complicated. Theyusually involve just one or two concepts,

but you may have to dig a little in the

reading material to find what you need.

– You may also have to apply some common

sense to what you read.

– The majority of what you read is probably not

going to be relevant to the questions.


4/46

Important Ideas about the MCAT

• The problems are almost all conceptual

and can be answered with fairly basicphysics. The reading may involve more

complicated ideas, but the questions are

based evaluating based on simple

physics.


5/46

Important Ideas about the MCAT

• Most of the time if you have to do more

than add or multiply a couple of numberstogether, you are probably on the wrong

track.


6/46

Suggestions for doing well

1. Read everything carefully.

2. There is a lot of unused information in

the reading. Don’t worry if you don’t use

it.3. If you are weak in a topic, don’t just pass

it over. There are several techniques to

improve your chances when you guess.


7/46


8/46

If you are familiar with the topic:

1. Simplify.

2. Round your numbers.

3. Calculate.

You cannot use a calculator, so anycalculations will necessarily be simple. You

can use scratch paper if you need to.

4. Check for reasonableness. This is oftena very good way to eliminate answers!


9/46

How to Prepare

• Study the prime areas:

– Mechanics/E&M,circuits/Fluids/Radioactivity/

Waves/Optics

• Understand the concepts – complicated

problems are not the MCAT way. The

context may be complicated, but the

problem itself is not.


10/46

How to Prepare

• Know the important equations. They are

generally closely related to the basicconcepts.

– Memorize the ones that are related to basic

concepts. Secondary equations won’t help you! – Often they are used as ratio-type problems. For

example, some quantity is known to be inversely

proportional to the temperature, so when youincrease the temperature by a factor of 1.5, that

quantity decreases by a factor of 1.5.


11/46

How to Prepare• You should know (to 1 significant figure)

some important physical constants:• 6.63 10 J∙s 7 10 J∙s

• 1.6 10 C 2 10 C

• 2.99 10

m/s 3 10

m/s• 9.8 m/s2 10 m/s2

• Know how to read graphs and tables.

There will be a number of them on theexam!


12/46

From the MCAT instructions:

• Neither the passage-based questions nor

the independent questions test your abilityto memorize scientific facts. Rather, both

types of questions assess knowledge of

basic physical and biological science

concepts and your facility at problem

solving at using these concepts.


13/46

Format of Physical Science Section

• 95 Minutes

• 59 questions. About 1/4 will be on physicsand 3/4 on chemistry, biochemistry andbiology. They may be mixed together in

the same reading.• 10 readings of about 250 words each with

4-7 questions about each one. All will be in

the context of biology (defined loosely.)• 15 questions unrelated to any reading.


14/46

Exam Preparation• The purpose of this class is not to teach you

physics – you should know most of what youneed to know already.

• The purpose of this class is to help you organize

that material in your mind so you can get morepoints on the exam.

• It is essential that you practice thinking physics,

that is the only way to recognize when theprinciples come up in the reading.


15/46

How to approach a Physics

problem

1. Read

• Passage• Problems

• Answers – are they reasonable?

2. Organize your thoughts• Visualize and sketch it.

• Decide what physics principles are

important.

• Note given any needed information.


16/46


problem3. Simplify the problem

• Ignore extraneous information. The importantprinciples in step 2 will help recognize this.

4. Solve• Concepts are used to select the method.

• Equations• Equations are only useful in two ways:

• They organize the concepts – a good summary. This oftenshows up as ratio problems.

• You need them when you need a numerical answer.

• Be careful – make sure your units are compatible andwatch the signs of things.

• Be quick – most of the time you can round to 1 figure anddo a quick calculation.


17/46


problem

5. Think

• Reasonable in magnitude?

• Units match?

6. After about 1 minute• Eliminate the unlikely answers

• Guess

• Mark the problem if there is hope.


18/46

Sample MCAT physics problems

• These sample questions are from an old-style MCAT,

but they illustrate many of the principles above.• sampleitems.pdf


19/46

Paradigms

• A paradigm is a model or typical pattern

that can be followed, particularly to solveproblems.

• I will talk about several paradigms that can

be used to solve various classes of

problems in physics.


20/46

Notes on the Web

A printout of these notes can be found at the

following url:

http://www.physics.byu.edu/faculty/hart/MCAT/


21/46

Paradigms we will use• Block on Inclined Plane

(Energy Conservation)• Porsche (Power)

• Braking Car (Kinematics)

• Lifting a box (Equilibrium)

• Circuit (Resistance,

Current, Capacitance and

Voltage)

• Charge in Capacitor(Electric Forces)

• Water Tank (Fluids)

• Wave (Waves andSound)

• Ball hitting wall (Optics –

reflection)

• Cart going into sand

(Optics – refraction)

• 14C (Radioactivity and

Half-life)


22/46






Current and Voltage)

• Charge in Capacitor

(Electric Forces)




reflection)




Half-life)


23/46

Block on Inclined Plane Paradigm

• This is a paradigm for conservation of energy.

This is the easiest way to work a problem – if itworks.

• Energy and work:

2

2

1mvKE

mghPE

PE KE E

E E finalinitial


24/46


• There is no friction.

h


25/46


ghv

mghmv

mv E

mgh E

f

f

f final

initial

2

2

1

02

1

0

2

2

h

initial

final


26/46


• As long as there is no friction, the path

between start and finish doesn’t matter. – Free fall is the same as sliding down

something without friction in terms of what the

final velocity will be.

• For springs the PE is

. You can use

this in place of, or in addition to thegravitational PE.


27/46


• Use this technique whenever possible.

Key things to look for: – Only conservative forces involved (usually

gravity, electric forces, and springs.)

– Time is not involved in the problem, you have

just an initial state and a final state.

– Usually just one object is moving.


28/46

Possible biologically related

systems• A spring-loaded lancet is used to pierce a

fingertip. How fast is it going when it hitsthe end of the finger?

• A person is injured by falling off of a wall.

How fast where they going when they hit?


29/46







Voltage)





reflection)




Half-life)


30/46

Porsche Paradigm• Power:

t W

t E P

(Porsche speeding up)


31/46

Porsche Paradigm• It can go from 0 to 60 in 3 seconds, what is the power?

• ~ 1500 kg, ~ 25 m/s, so

~150,000

• ~ 210 HP

• Divide whatever change in energy you have by the timeinterval. That is the power, the rate at which energy changes. You don’t use this for electrical power in circuits,although it works at the microscopic level.

t

mv

t

K K

t

E P

i f

2

21


32/46

Work done• Important ideas:

– Work-Kinetic Energy relation:• Δ

Σ

– Definition of Work (units: Joules):

• cos

– If you have a conservative force, then• Δ


33/46

Power expended• Important ideas:

– Power is the rate of doing work (units:Joules/sec or Watts)

• Since

, then the work done is Δ.

– If an object is moving at speed , then the

power acting on it instantaneously is•


34/46

Possible biologically related

systems• How many Calories do you burn climbing

to the top of a tall tower?• How many horsepower can a person exert

if they run up a short flight of stairs?

• How deeply will a biopsy needle penetrate,

given the compression of the spring

shooting it?


35/46


(Energy Conservation)

• Porsche (Power)





Voltage)





reflection)




Half-life)


36/46

Braking Car Paradigm• This paradigm is for kinematics – description of

motion.

• This is used when the following quantities are

involved:

- Position - Time

- Velocity - Acceleration- Force


37/46

Braking Car Paradigm• Basic Equations:

N F

mgW

maF m

x xavv

t at v x x

t avv

f

2

2

1

020

2

2

00

0

aF


38/46

Braking Car Paradigm• Typical Problem:

v 0

x

d

v = 0

t = 0

t = tf


39/46

Possible questions:• What is the acceleration?

• What is the coefficient of friction?

d

v

a

ad v x xavv

2

2

2

2

0

2

0

0

2

0

2

g

a

mamg

F maF f


40/46

Possible questions:• How big is the frictional force?

• Given μ and d, what was v 0 ?

maF

t v

t t vva

f f

0

0

0

d gv

ad v

agmamg

2

2

0

2

0


41/46

Braking Car Paradigm• Remember – this is for anything speeding

up or slowing down, whether horizontallyor vertically.

• Use whichever equations have the right

variables in them.

• Make sure that conservation of energy isnot the easier way to do it.


42/46

Biologically related problems• How long does it take a nerve impulse to

travel the length of a neuron?

• A person blacks out at an acceleration higherthan 7 g. How long would it take a car to gofrom 0-60 mph with that acceleration? How

far would it travel? – (0.4 sec, 5 m)

• A person lands after a fall high. Their legs

bend when they land. How much force dotheir legs have to exert when they stop? Willthey break their legs?


43/46

Aside – Newton’s Laws• A number of conceptual questions address

Newton’s Laws directly, not in the context ofkinematics.

• In many ways Newton’s first law is conceptuallythe hardest.

– When an object has no net force acting on it, then it

moves at a constant speed in a straight line. – It does not take a force to keep something moving!


44/46

An Example:• A skydiver jumps out of a plane. His

speed increases until he reaches terminalvelocity. How big is the force of air

resistance on him at first?

– Greater than mg.

– Equal to mg .

– Less than mg .


45/46

Another Example:• A skydiver jumps out of a plane. His

speed increases until he reaches terminalvelocity. How big is the force of air

resistance on him after he reaches

terminal velocity? – Greater than mg.

– Equal to mg .

– Less than mg .


46/46

Still Another Example:• A monkey slides down a vine. At the time

he reaches velocity v , he starts to tightenhis grip on the vine. The frictional force

increases with time. At the time that the

force of friction equals his weight, – He moves with constant speed down the vine.

– He stops.

– He starts to move upward.

MCAT Physics Review 1

Documents

Transcript of MCAT Physics Review 1