How to Solve Physics Problems

Or

Translating from English to MathBy Matthew Allen

Mandeville High School Physics

Physics applies the logic of mathematics to the worldin an effort to understand why things do what they do and predict what will happen in the future.

Real world problems are stated in English as word problems.These word problems are are then “translated” into mathematics, applying the rules which have been experimentally proventhrough through the centuries.

Some of the problems may be simple and you can solve them in your head.

Ex. A train travels 50 miles in 2 hours. What was his average speed?

Answer 25mi/hr

I will not give credit for this answer if there is no other work shown because it is not in standard units

And many problems will be multi-step and multi-variable problems that cannot be solved in your head. You need to practice the method on simple problems to succeed on the hard ones.

Ex. A bullet is fired at target 255m away. If the bullet has a velocity of 447 m/s, at what angle should the bullet be fired?

225m

?

]cos)447[(

225

00

tsmv

md

d

x

x

x

?

]sin)447[(

]sin)447[(

0

0

0

0

tsmv

smv

d

d

y

y

y

y

And then apply the equations we will learn.

DO NOT COPY: Visual Example only

While every problem is different the following steps will help to keep you organized and help to solve the problems. REQUIRED FOR FULL CREDIT!

1. Read the problem thoroughly. Understand what the problem is asking.

2. Reread the problem and write down important known quantities and unknown quantities in a list. We will discuss which variables to use.

3. Draw a picture or diagram that represents the problem. Stick figures are fine. Label important elements as listed in step #2. 4. Convert units to SI units if needed. Show work.

6. Fill in the values for the variables in equation from the list. If the number is not in the list or from a conversion, you may not use it in the equation.

5. Decide which equation to use from the information in the list and drawing. You may have to use one equation to find an unknown to solve another equation for the answer.

Write down the equation in variable form.

7. Solve for unknown variable(s) to answer he problem. Show steps used to solve. Each step below the one from which it was derived. 8. BOX / CIRCLE THE ANSWER. I do not play seek and find for answers. All answers must be in SI units unless otherwise noted in problem

Helpful HintsWrite the units for each number every time including in the equation. Math operates on units like variables and is a good way to check answer. If you end up with the wrong units in the answer, you did something wrong.

I will deduct 0.1 points for incorrect significant digits.

Do not give up. If at first you don’t succeed, try again a different way.

Answer must have correct units. I will deduct 0.5 points for missing or incorrect units. All answers in SI units unless otherwise specified.

GOOD LUCK

Common Variables For Unit Motion 1-D

timeonaccelerati

speed finalspeed/ speedinitial

distancefinal distance/ distanceinitial

0

0

VARIABLES SCALAR

tassdd

timeonaccelerati

velocityfinal velocity/

velocityinitial

position finalposition/

position initial

VARIABLES VECTOR

ta

v

v

x

x

o

o

Common Variables For Unit Motion 1-D

EXAMPLE PROBLEM

tds Using the equation

The Earth has a circumference of 38,076 km at the equator. This is the distance a you would travel in one day standing on the equator at sea level as the Earth rotates. If there are exactly 24 hours in one day, how fast does the Earth rotate in meters per second on the equator?

Warning: I may give you a radius and you would have to remember the equation to find circumference.

Step 1and 2: Read the Problem and list known

and unknown quantities

?24

076,38

shrt

kmd

Step 3: Draw and label diagram

Earth

hrtkmd

24076,38

Step 4: Convert to SI units

sshr

hrt

mmkmmkmd

400,86)min60)(

1min60)(

124(

108076.3000,076,38)1

1000)(1

076,38( 7

Steps 5-8: Choose, fill in, and solve equation. BOX ANSWER

sm440.69444s

s108.64m103.8076s

tds

4

7

sms 2104.4

What final answer should look like

?24

076,38

shrt

kmd

sshr

hrt

mmkmmkmd

400,86)min60)(

1min60)(

124(

108076.3000,076,38)1

1000)(1

076,38( 7

sm440.69444s

s108.64m103.8076s

tds

4

7

Earthhrtkmd

24076,38

sms 2104.4