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Check Your Understanding Given that 1 lb (pound) is 4.45 N, were the numbers being output by SM_FORCES too big or too small?

The numbers were too small, by a factor of 4.45.

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Summary

  • To convert a quantity from one unit to another, multiply by conversions factors in such a way that you cancel the units you want to get rid of and introduce the units you want to end up with.
  • Be careful with areas and volumes. Units obey the rules of algebra so, for example, if a unit is squared we need two factors to cancel it.

Problems

The volume of Earth is on the order of 10 21 m 3 . (a) What is this in cubic kilometers (km 3 )? (b) What is it in cubic miles (mi 3 )? (c) What is it in cubic centimeters (cm 3 )?

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The speed limit on some interstate highways is roughly 100 km/h. (a) What is this in meters per second? (b) How many miles per hour is this?

a. 27.8 m/s; b. 62 mi/h

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A car is traveling at a speed of 33 m/s. (a) What is its speed in kilometers per hour? (b) Is it exceeding the 90 km/h speed limit?

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In SI units, speeds are measured in meters per second (m/s). But, depending on where you live, you’re probably more comfortable of thinking of speeds in terms of either kilometers per hour (km/h) or miles per hour (mi/h). In this problem, you will see that 1 m/s is roughly 4 km/h or 2 mi/h, which is handy to use when developing your physical intuition. More precisely, show that (a) 1.0 m/s = 3.6 km/h and (b) 1.0 m/s = 2.2 mi/h .

a. 3.6 km/h; b. 2.2 mi/h

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American football is played on a 100-yd-long field, excluding the end zones. How long is the field in meters? (Assume that 1 m = 3.281 ft.)

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Soccer fields vary in size. A large soccer field is 115 m long and 85.0 m wide. What is its area in square feet? (Assume that 1 m = 3.281 ft.)

1.05 × 10 5 ft 2

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What is the height in meters of a person who is 6 ft 1.0 in. tall?

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Mount Everest, at 29,028 ft, is the tallest mountain on Earth. What is its height in kilometers? (Assume that 1 m = 3.281 ft.)

8.847 km

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The speed of sound is measured to be 342 m/s on a certain day. What is this measurement in kilometers per hour?

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Tectonic plates are large segments of Earth’s crust that move slowly. Suppose one such plate has an average speed of 4.0 cm/yr. (a) What distance does it move in 1.0 s at this speed? (b) What is its speed in kilometers per million years?

a. 1.3 × 10 −9 m ; b. 40 km/My

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The average distance between Earth and the Sun is 1.5 × 10 11 m. (a) Calculate the average speed of Earth in its orbit (assumed to be circular) in meters per second. (b) What is this speed in miles per hour?

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The density of nuclear matter is about 10 18 kg/m 3 . Given that 1 mL is equal in volume to cm 3 , what is the density of nuclear matter in megagrams per microliter (that is, Mg/ μ L )?

10 6 Mg/ μ L

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The density of aluminum is 2.7 g/cm 3 . What is the density in kilograms per cubic meter?

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A commonly used unit of mass in the English system is the pound-mass, abbreviated lbm, where 1 lbm = 0.454 kg. What is the density of water in pound-mass per cubic foot?

62.4 lbm/ft 3

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A furlong is 220 yd. A fortnight is 2 weeks. Convert a speed of one furlong per fortnight to millimeters per second.

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It takes 2 π radians (rad) to get around a circle, which is the same as 360°. How many radians are in 1°?

0.017 rad

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Light travels a distance of about 3 × 10 8 m/s. A light-minute is the distance light travels in 1 min. If the Sun is 1.5 × 10 11 m from Earth, how far away is it in light-minutes?

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A light-nanosecond is the distance light travels in 1 ns. Convert 1 ft to light-nanoseconds.

1 light-nanosecond

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An electron has a mass of 9.11 × 10 −31 kg. A proton has a mass of 1.67 × 10 −27 kg . What is the mass of a proton in electron-masses?

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A fluid ounce is about 30 mL. What is the volume of a 12 fl-oz can of soda pop in cubic meters?

3.6 × 10 −4 m 3

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Practice Key Terms 1

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Source:  OpenStax, University physics volume 1. OpenStax CNX. Sep 19, 2016 Download for free at http://cnx.org/content/col12031/1.5
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