Describing Motion — Practice · Grade 12 Science

Chapter: Motion & Kinematics

What this practice covers

By the end of Describing Motion you should be able to:

10 questions, numeric, multiple choice and written answer, each with a worked explanation.

Vocabulary used: position, distance, displacement, scalar, vector, frame of reference.

Problem 1

A jogger runs 8 km east, then turns around and runs 5 km west along the same road. What total distance did she cover?
Answer and explanation

13 km

Distance is total path length: 8 + 5 = 13 km.

Problem 2

A jogger runs 8 km east then 5 km west along one road. What is the magnitude of the displacement?
Answer and explanation

3 km

Displacement combines opposite directions: 8 − 5 = 3 km east.

Problem 3

Which of the following is a vector quantity?
  1. displacement
  2. speed
  3. distance
  4. mass
Answer and explanation

displacement

Displacement has both magnitude and direction, making it a vector. Distance, speed, and mass are scalars.

Problem 4

A race car drives exactly one full lap and returns to the starting line. Which statement is true?
  1. Its distance and displacement are both zero
  2. Its distance is zero but its displacement is large
  3. Its displacement is zero but its distance equals the track's length
  4. Its displacement equals the track's length
Answer and explanation

Its displacement is zero but its distance equals the track's length

Returning to the start makes the displacement zero, while the distance traveled equals the full length of the lap.

Problem 5

A drone flies 12 m north, then 5 m east. What is the magnitude of its displacement?
Answer and explanation

13 m

The legs are perpendicular, so displacement = √(12² + 5²) = √169 = 13 m.

Problem 6

Explain how two observers in different frames of reference can honestly disagree about a passenger's velocity. Use a passenger sitting still on a moving train as your example.
Answer and explanation

Velocity is always measured relative to a frame of reference. In the train's frame, the seated passenger isn't moving, so her velocity is zero. In the platform's frame, she is carried along with the train at, say, 100 km/h. Both are correct within their own frame because there is no absolute resting point in the universe — motion is relative to whatever origin you measure from.

Problem 7

A runner completes a 400 m lap ending at the start. Find distance and displacement. Show the reasoning, not just the conclusion.
Hints

Final minus initial position should match the signed path sum.

Answer and explanation

Distance 400 m; displacement 0 m.

Problem 8

Why must a position be specified relative to an origin and axis?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

Coordinates depend on the chosen reference frame; changes can then be interpreted consistently.

Problem 9

Average speed and average velocity use different numeratorsAverage speed and average velocity use different numeratorsIntervalPath lengthSigned displacement0–4 s8 m+2 mAverage rate8/4=2 m/s2/4=+0.5 m/sAverage speed uses total path length.Average velocity uses endpoint change.
Illustrative aggregate record; no uniform speed between endpoints is assumed.
Interpret the supplied visual. Can you infer the maximum instantaneous speed from this table?
Answer and explanation

No. It supplies totals and duration, not the full motion history between events.

An average rate divides a defined change or amount by elapsed time. Average speed is total distance divided by duration; average velocity is displacement divided by duration. The same 4-second journey can therefore have 2 m/s average speed but only +0.5 m/s average velocity. Averaging two segment speeds by ordinary arithmetic works only for equal segment times. For unequal times, use total distance and total duration or the appropriate time-weighted calculation.

No. It supplies totals and duration, not the full motion history between events.

Response criteria

  • Interpret labels, structures, axes, units and directions correctly.
  • Explain the physical or environmental relationship or mechanism.
  • Respect stated assumptions and distinguish observation from inference.

Problem 10

A student walks 12 m north in 6 s, then 4 m south in 4 s. Find average speed and average velocity, taking north positive.
Answer and explanation

Distance=16 m and elapsed time=10 s, so average speed=1.6 m/s. Displacement=+8 m, so average velocity=+0.8 m/s. Do not average the segment speeds without time weighting.

Distance=16 m and elapsed time=10 s, so average speed=1.6 m/s.

Displacement=+8 m, so average velocity=+0.8 m/s.

Do not average the segment speeds without time weighting.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the physical or environmental mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

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