Forest PHYS100 Chapt2

Chapt 2 Motion

Newton's Principia published in 1687 ( in latin and then 1726 in english).

Average speed

Average speed [math](s)[/math] is the ratio of the DISTANCE [math](d)[/math] you have traveled divided by the time [math](t)[/math] it took you to travel that distance.

[math]\mbox{Average Speed} = \frac{\mbox{Distance traveled}}{\mbox{Time traveled}}[/math]

We can write a formula to express the definition of speed using the above abreviations for speed, distance, and time

[math]\bar s = \frac{d}{t}[/math]

The bar over the [math]s[/math] just used to indicate that it is an average.

An example of calculating your average speed

Suppose you drive from Pocatello to a friends house in Salt Lake. You know that the distance is 150 miles.

If it takes you 3 hours to drive there then your average speed is

[math]\bar s = \frac{150 \mbox{miles}}{3 \mbox{hours}}= 50 \frac{ \mbox{miles}}{ \mbox{hours}} = 50 \mbox{ MPH}[/math]

International System's (SI) unit for speed is

[math]\frac{ \mbox{meters}}{ \mbox{sec}} =\frac{ \mbox{m}}{ \mbox{s}} [/math]

Unit conversion

Converting the units from MPH to m/s is a matter of multiplying by 1.

Here are some useful value for 1

[math]1 = \frac{ 1 \mbox{m}}{ 621.4 \mbox{miles}}=\frac{ 1 \mbox{hour}}{ 60 \mbox{minutes}}=\frac{ 1 \mbox{minute}}{ 60 \mbox{seconds}}=\frac{ 1 \mbox{hours}}{ 3600 \mbox{seconds}}[/math]

We can now convert MPH to m/s by multiplying by 1

[math]\bar s = 50 \frac{ \mbox{ \sout miles}}{ \mbox{hours}} \left ( \frac{ 1 \mbox{m}}{ 621.4 \mbox{miles}}\right )\left (\frac{ 1 \mbox{hours}}{ 3600 \mbox{seconds}}\right ) [/math]

Instantaneous speed

Speed at a given instant in time.

Graphing Motion

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