Speed, Velocity & Acceleration — Practice · Grade 8 Science

Chapter: Motion

What this practice covers

By the end of Speed, Velocity & Acceleration you should be able to:

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

Vocabulary used: speed, velocity, acceleration, model, evidence, system boundary, assumption.

Problem 1

A runner covers 100 meters in 20 seconds. What is the average speed?
Answer and explanation

5 m/s

Speed = distance ÷ time = 100 ÷ 20 = 5 m/s.

Problem 2

A train travels 300 km in 4 hours. What is its average speed?
Answer and explanation

75 km/h

Speed = distance ÷ time = 300 ÷ 4 = 75 km/h.

Problem 3

A skateboarder speeds up from 2 m/s to 8 m/s in 3 seconds. Find the acceleration.
Answer and explanation

2 m/s²

Acceleration = (8 − 2) ÷ 3 = 6 ÷ 3 = 2 m/s².

Problem 4

Which quantity includes BOTH speed and direction?
  1. distance
  2. velocity
  3. time
  4. mass
Answer and explanation

velocity

Velocity is speed given with a direction.

Problem 5

Explain the difference between speed and velocity using an example.
Answer and explanation

Speed only tells you how fast something is moving, like 50 km/h. Velocity tells you how fast AND in what direction, like 50 km/h north. Two cars traveling the same speed in opposite directions have the same speed but different velocities.

Problem 6

A car slows from 30 m/s to 10 m/s in 4 seconds. What is its acceleration? (Include the correct sign.)
Answer and explanation

-5 m/s²

Acceleration = (10 − 30) ÷ 4 = −20 ÷ 4 = −5 m/s². The negative sign shows it's slowing down.

Problem 7

A student walks 30 m east then 30 m west in 40 s. Find average speed and average velocity. Explain the reasoning in your own words.
Answer and explanation

Average speed 1.5 m/s; average velocity 0 m/s.

Total distance=60 m; speed=60/40=1.5 m/s.

Displacement=0 m; average velocity=0/40=0 m/s.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 8

A cyclist covers 180 m in 30 s. Calculate average speed. Show the reasoning, not just the conclusion.
Hints

Distance counts the whole path; displacement compares endpoints.

Answer and explanation

180/30=6 m/s.

Problem 9

Velocity changes from 2 m/s east to 10 m/s east in 4 s. Calculate average acceleration.
Hints

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

Answer and explanation

(10−2)/4=2 m/s² east, assuming east is positive.

Problem 10

Right is positive. A cart changes velocity from −2 to −8 m/s in 3 s. Calculate average acceleration and explain whether it speeds up or slows down.
Answer and explanation

(−8−(−2))/3 = −2 m/s². It speeds up toward the left because speed rises from 2 to 8 m/s.

Subtract signed velocities before dividing.

Compare velocity direction with acceleration direction.

Response criteria

  • Calculate −2 m/s².
  • Explain leftward motion.
  • Reject negative-always-slows.

Problem 11

Distance and displacement differDistance and displacement differ0 m: start20 m: finish30 m: turn30 m east10 m westDisplacement: 20 m eastIllustrative model • use the labels and stated assumptions
The path covers 30 m east and 10 m west, for 40 m distance and 20 m east displacement. Arrows separate the journey from the start-to-finish comparison. The schematic horizontal positions are not a precision scale and do not establish a changing speed without elapsed-time data.
For this unit’s illustrated case, how can a trip have zero displacement without zero distance?
Answer and explanation

The final position can equal the initial position even after a nonzero path. Displacement uses endpoints; distance adds the traveled path.

A student walks 6 m east and 6 m west back to the start. Distance is 12 m; displacement is zero. Mark the route and endpoints separately so the calculation answers the intended question.

The final position can equal the initial position even after a nonzero path. Displacement uses endpoints; distance adds the traveled path.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 12

Right is positive. A cart’s velocity changes from −3 to −9 m/s in 2 s. Find average acceleration and explain its speed change.
Answer and explanation

Average acceleration = (−9−(−3))/2 = −3 m/s²; speed increases leftward from 3 to 9 m/s. Negative acceleration does not always mean slowing.

Subtract signed velocities.

Compare the magnitudes and directions instead of using a sign slogan.

Average acceleration = (−9−(−3))/2 = −3 m/s²; speed increases leftward from 3 to 9 m/s. Negative acceleration does not always mean slowing.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

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