Lesson 2 of 6 · Speed and acceleration
Tell speed from a change in velocity
8 minute lesson · 3 checks along the way
Your goal for this lessonCalculate average speed and acceleration while preserving units, direction, and the limits of sampled measurements.
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Time changes a distance into a rate
Speed tells us how quickly an object covers distance. If a rover travels 18 m along its route in 6 s, its is 3 m/s. The word average matters: the rover could pause, move faster, and still achieve that same overall result. One total distance and one elapsed time do not reveal the speed at every moment of the journey.
includes direction. A rover moving east at 3 m/s has a different velocity from one moving west at 3 m/s, although their speeds match. uses displacement divided by elapsed time. On an out-and-back trip it can be zero while average speed is positive. Both calculations are useful; they answer different questions about the same recorded movement.
Measure how velocity changes
is the rate of change of velocity. Along one axis, average acceleration equals final velocity minus initial velocity, divided by elapsed time. Its unit, meters per second squared, means a change of so many meters per second during each second. Positive acceleration is not automatically speeding up: the object’s velocity direction must also be considered before drawing that conclusion.
For example, choose east as positive. A vehicle changing from −6 m/s to −2 m/s in 2 s has average acceleration +2 m/s², yet its speed decreases from 6 to 2 m/s. A vehicle turning at a steady speed also accelerates because its velocity direction changes. A circular orbit is therefore not an example of zero acceleration, even in an ideal model with constant orbital speed.
Check 1 of 3
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Work through a decision
Decide what a telemetry log supports
Consider an invented straight-line test with velocities +4 m/s at 0 s, +10 m/s at 3 s, and +10 m/s at 7 s. The first interval has average acceleration (10 − 4)/3 = +2 m/s². The second has zero average acceleration because its endpoint velocities match. These are calculations from the supplied log, not measurements extracted from the STS-114 launch photograph.
A report claims the vehicle’s velocity stayed constant throughout the second interval. The two endpoint samples alone cannot establish that: it could have slowed and accelerated again between them. More frequent measurements could test the claim. If continuous data instead establish constant +10 m/s throughout those four seconds, the during that interval is 10 × 4 = +40 m.
Check 2 of 3
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Use direction and evidence together
The STS-114 image records Discovery above the launch pad with a bright exhaust plume. It helps locate a real launch event, but plume length and the rocket’s height within the frame are not calibrated acceleration instruments. To reconstruct motion, engineers combine timing, or velocity measurements, and knowledge of the observing geometry. A dramatic still photograph cannot replace those inputs.
Before accepting a statement that something “accelerated,” identify the initial and final velocity, the interval, and the chosen frame. Ask separately whether speed increased, decreased, or remained constant while direction changed. Finally, distinguish an interval average from a claim about every instant. These habits let you compare a slow rover, a rising launcher, and an orbiting spacecraft using the same physical language.
Discovery launches on STS-114 from Kennedy Space Center on July 26, 2005, in NASA photograph STS114-S-024. This archived color still supplies the launch setting. The lesson’s velocity log is an invented exercise, not telemetry measured from this image.
Look closely
Can plume length establish that the vehicle gained the same amount of speed every second?
See what to notice
No. A quantitative acceleration claim needs timed velocity or suitably calibrated position measurements. Exhaust appearance, viewing geometry, and exposure are not a substitute for that evidence.
Check 3 of 3
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Put it together
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