1. Unit Conversions
And the SI system

2. Unit Conversion Conversions are part of daily life
Hours worked → $ on paycheck
Travel distance → Minutes of driving
Kind of cell phone → Cost of calling plan

3. Unit Conversion
Both things are equivalent, but you change them in order to make better sense of the information.
Def: Conversion- changing a number from one kind of unit to another in order to use it in an equation.

4. Why convert?
In physics we take basic measurements of time, mass, and length all the time.
Before we can do ANY math, these measurements HAVE to be in these SI units:
Time- seconds (s)
Mass- kilogram (kg)
Length- meter (m)

5. Common Conversions Time: 1 min = 60 s 1 hr = 3600 s 1 day = 86,400 s
Mass:
2.2 lb = 1kg
Length:
1 in = 2.54 cm
1 mi = 5280 ft

6. How do we convert?
Another way to write that two things are equivalent is to write them as a fraction.
Example:
1 in = 2.54 cm is the same as
and also

7. Conversion Factors
Writing an equivalency like this is called a Conversion Factor.
This is what we use to go from one unit to another.

8. Conversion Factors 1 min = 60 s or 1 hr = 3600 s or
Any of these can be written as conversion factors. (It doesn’t matter which number is on the top and which is on the bottom)
1 min = 60 s or
1 hr = 3600 s or
1 day = 86,400 s or
2.2 lb = 1kg or
1 in = 2.54 cm or
1 mi = 5280 ft or

9. Using Conversion Factors
Example: Convert 5 in to centimeters.
Step 1: Write the number down WITH UNITS.
5 in

10. Using Conversion Factors
Example: Convert 5 in to centimeters.
Step 2: Set up a “T chart”.
5 in

11. Using Conversion Factors
Example: Convert 5 in to centimeters.
Step 3: Find a conversion factor with the same units IN THE DENOMINATOR.
5 in cm
1 in

12. Using Conversion Factors
Example: Convert 5 in to centimeters.
Step 4: Cancel units (NOT numbers)
5 in cm
1 in =

13. Using Conversion Factors
Example: Convert 5 in to centimeters.
Step 5: Simplify and keep the units that AREN’T cancelled out
5 in cm * 2.54 cm cm
1 in = =

14. Practice
On a scratch piece of paper, perform the following conversions:
Convert 3 days into seconds.
Convert 600 feet into miles.
Convert 195lbs into kilograms.

15. Practice
On a scratch piece of paper, perform the following conversions:
Convert 3 days into seconds.
259,200s
Convert 600 feet into miles.
Convert 195lbs into kilograms.

16. Practice
On a scratch piece of paper, perform the following conversions:
Convert 3 days into seconds.
259,200s
Convert 600 feet into miles.
0.11 mi
Convert 195lbs into kilograms.

17. Practice
On a scratch piece of paper, perform the following conversions:
Convert 3 days into seconds.
259,200s
Convert 600 feet into miles.
0.11 mi
Convert 195lbs into kilograms.
88.63 kg

18. Two Step Conversions
Some conversions may require two conversion factors to complete.
Example: Convert 1 mi into inches.
1mi

19. Two Step Conversions
Some conversions may require two conversion factors to complete.
Example: Convert 1 mi into inches.
1mi 5280ft
1mi

20. Two Step Conversions
Some conversions may require two conversion factors to complete.
Example: Convert 1 mi into inches.
1mi 5280ft 12in
1mi 1ft

21. Two Step Conversions
Some conversions may require two conversion factors to complete.
Example: Convert 1 mi into inches.
1mi 5280ft 12in *5280*12 in
1mi 1ft *1 =

22. Two Step Conversions
Some conversions may require two conversion factors to complete.
Example: Convert 1 mi into inches.
1mi 5280ft 12in *5280*12 in ,360 in
1mi 1ft *1 = =

23. Practice Perform the following conversion:
Convert .5 years to seconds.
Convert 1 ton to kilograms (1 ton is 2000 lbs).

24. Practice Perform the following conversion:
Convert .5 years to seconds.
15,768,000 s
Convert 1 ton to kilograms (1 ton is 2000 lbs).

25. Practice Perform the following conversion:
Convert .5 years to seconds.
15,768,000 s
Convert 1 ton to kilograms (1 ton is 2000 lbs).
909.1 kg

26. SI Prefixes
These are some common prefixes to the SI units that can help you come up with more conversion factors:
Centi: 1/100th
Ex: 100 cm = 1 m
Milli: 1/1000th
Ex: 1000 ms = 1 s
Kilo: 1000
Ex: 1 kg = 1000 g

27. Converting Multi Units
Sometimes we have to convert two thing in one problem before doing the math.
Example: Convert 10 mph into m/s.

28. Converting Multi Units
Sometimes we have to convert two thing in one problem before doing the math.
Example: Convert 10 mph into m/s.
10 mi 5280ft
h mi

29. Converting Multi Units
Sometimes we have to convert two thing in one problem before doing the math.
Example: Convert 10 mph into m/s.
10 mi 5280ft 12in
h mi 1ft

30. Converting Multi Units
Sometimes we have to convert two thing in one problem before doing the math.
Example: Convert 10 mph into m/s.
10 mi 5280ft 12in 2.54cm
h mi 1ft in

31. Converting Multi Units
Sometimes we have to convert two thing in one problem before doing the math.
Example: Convert 10 mph into m/s.
10 mi 5280ft 12in 2.54cm 1m
h mi 1ft in cm

32. Converting Multi Units
Sometimes we have to convert two thing in one problem before doing the math.
Example: Convert 10 mph into m/s.
10 mi 5280ft 12in 2.54cm 1m h
h mi 1ft in cm 3600s

33. Converting Multi Units
Sometimes we have to convert two thing in one problem before doing the math.
Example: Convert 10 mph into m/s.
10 mi 5280ft 12in 2.54cm 1m h
h mi 1ft in cm 3600s
10*5280*12*2.54*1*1 m 1,609,344 m
1*1*1*100*3600 s ,000 s = =

34. Converting Multi Units
Sometimes we have to convert two thing in one problem before doing the math.
Example: Convert 10 mph into m/s.
10 mi 5280ft 12in 2.54cm 1m h
h mi 1ft in cm 3600s
10*5280*12*2.54*1*1 m 1,609,344 m m/s
1*1*1*100*3600 s ,000 s = = =

35. Practice
Using your notes as a guide, complete the classwork assignment on unit conversion.