Measuring Motion: Position, Time, and Units
Describing Where and When
Position and coordinates
To study motion, we need a coordinate system. In one dimension, we can use a line and choose a positive direction. Position is then written as , measured from an origin.
Time as a variable
Time is the quantity we use to compare events. In mechanics, we usually write it as . Motion is described by how position changes as time changes.
A useful habit
Always define your origin and positive direction before solving a problem. Many mistakes happen when the coordinate system is unclear.
Common units
| Quantity | SI unit |
|---|---|
| Position | meter () |
| Time | second () |
| Velocity | meter per second () |
| Acceleration | meter per second squared () |
| Force | newton () |
Why units matter
Units keep calculations meaningful. If you compute velocity, the units should be distance divided by time, such as . If the units do not match, something is wrong.
What is the SI unit of force?
Force is measured in newtons, symbol .
Correct answer: Newton
What two things must you define before describing 1D motion clearly?
These define the coordinate system.
Correct answer: An origin and a positive direction.
Problem setup checklist
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Step 1: Choose a coordinate system.
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Step 2: Identify the object of interest.
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Step 3: List the known quantities with units.
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Step 4: Decide which motion quantities you need to find.