Find out your options for physics homework help.Physics help is provided to university and college students by expert physics tutors. Also get physics homework help, tutorials, solved problem sets.Find help for physics here by entering search words or phrases.
Find physics help or a physics tutor here.

Physics Help >> Index to Physics Homework Help » Converting Units


Using Conversion Factors

 

Converting units is like riding a bicycle: once you know how it’s easy. One method is to multiply the given unit by a conversion factor that cancels the unwanted unit and replaces it with the desired unit.

For example, to convert 2 hours to minutes you intuitively multiply by 60. But why does that work?

 

Using dimensional analysis, multiplying hours by minutes per hour converts hours to minutes because hours cancel.

 


You see that h is in both the numerator and the denominator, so the h cancels, leaving you with minutes. (Related term: dimensional analysis.)

 

A common problem is how to convert grams to kilograms. For example, how many kg is 250 g?

 

250 g x (1 kg)/(1000 g) = 0.25 kg

 

Converting km/h to m/s

 

If you have two units at a time, you use two conversion factors. For example, to convert 50 km/h to m/s you convert km to m and 1/ h to 1/ s:

 

To convert km per h to m per s, multiply be m per km and by h per s.

Converting three units at a time, requires you use three conversion factors. For example, to convert 50 mi/h to m/s you convert mi to km, km to m and 1/ h to 1/ s:

 

To convert mi/h to m/s requires converting mi to km, km to m, and h to s.

 

 

Metric Conversions

 

Another method works well for converting between metric units. For example, the axon of a neuron has a diameter of 0.20 microns. Express this diameter in terms of nanometers.

(1 micron = 1 micrometer).

 

Metric conversions may be done easily be replacing the prefix with power of 10, multiplying by one, then replacing the power of 10 with a prefix.

 

(Related search term: metric multipliers, SI multipliers, metric prefix.)


Problems

1.

a. The Washington National Monument is 555 ft high. Express this height in meters.
b. A basketball player is 7 ft tall. What is his height in meters?

2. The axon of a neuron has a diameter of 0.20 microns. Express this diameter in terms of a centimeter. (1 micron = 1 micrometer).

3. Change the following to the SI unit and in standard form:

a. 2.31 MJ

b. 1.5 gcm-3

4. 45,000,000 liters of crude oil are dropped into a body of water. 50% of this oil forms a slick with a thickness of 5.57*10^-7 cm. What area would the slick cover in square miles and in square kilometers?

5. An average active person consumes about 2500 Cal a day. a)What is this in joules? b)What is this in kilowatt-hours? c) Your power company charges about a dime per kilowatt-hour. How much would your energy cost per day if you bought from the power company?

6. Convert 10 km/hr/s to m/s2.

 

Answers

 

 

For solutions to all the problems on this page click here.

 

 

 

6.

(10 km/h/s)*(1000 m / km)*(3600 s / h) = 2.78 m/s/s = 2.78 m/s2