Physics of Motion: What is What?

Intro to Chemistry and Physics Index

 

Physics Index

 

Physics of Motion: What is What?

The equations:

df = di + (vi)(t) + (½)(a)(t2)   This will answer almost all “how far” questions.

Use this if:
Only 1 velocity is given
You have a distance given (df is rarely 0, but sometimes it is the unknown)

∆d = v(ave)∆t

Use this if:
You only need to find how much something moved in a period of time, not how far it ends up from a point of reference.

vf = vi + (a)(t)    This will answer almost all “how fast” questions.

Use this if:
2 velocities are given
You don’t have distance

∆v = a∆t

Use this if:
You only need to find how much velocity changed over a period of time, not what its final or initial velocity was/is.

v(ave) = (vi + vf)/2 (See * below)

Use this if:
You need to find the average velocity
You know BOTH vi and vf (you might have to calculate vf!

vf = (vi + ∆v)

Use this if:
You need to find the final velocity AND
You know BOTH vi and how much v changes (e.g. speed increases by 10 m/s)

df = (di + ∆d)

Use this if:
You need to find the final distance, total distance, or final position AND
You know BOTH di and how much d changes (e.g. it moves 10 meters)

The hints and helps:

It is vi if…

“…traveling at a rate of…”

“…moving at…”
"…has a velocity of…"

If it is “at rest” then vi = 0 and di is probably 0

The words "begin" and "starts" generally go with the initial values.

If it is “moving at a constant” rate or if it “has a constant velocity” then a = 0

It is possible that something not given (but which is not the thing to be found) should have a value of 0...

*Another v(ave) approach:

v(ave) = ((vi + at) + vi)/2
Use this if: You need to find the average velocity You know vi and but you have to calculate vf