Speed, Velocity & Acceleration — Drills · Grade 8 Science

Chapter: Motion

What these drills covers

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

8 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 400 m in 50 s. What is the average speed in m/s?
Answer and explanation

8 m/s

speed = distance / time = 400 / 50 = 8 m/s.

Problem 2

A car travels at 15 m/s for 12 s. How far does it go, in metres?
Answer and explanation

180 m

distance = speed × time = 15 × 12 = 180 m.

Problem 3

A cyclist covers 90 m at 6 m/s. How long does it take, in seconds?
Answer and explanation

15 s

time = distance / speed = 90 / 6 = 15 s.

Problem 4

Velocity differs from speed because velocity also states
  1. the mass
  2. the direction
  3. the time
  4. the distance
Answer and explanation

the direction

Velocity is speed with a direction, so 5 m/s north and 5 m/s south are different velocities.

Problem 5

A car speeds up from 4 m/s to 20 m/s in 8 s. What is the acceleration in m/s²?
Answer and explanation

2 m/s²

(20 − 4) / 8 = 2 m/s².

Problem 6

A train slows from 30 m/s to 10 m/s in 10 s. What is the magnitude of its acceleration in m/s²?
Answer and explanation

2 m/s²

(30 − 10) / 10 = 2 m/s², directed opposite to the motion.

Problem 7

A car goes round a roundabout at a steady 20 km/h. Is it accelerating? Explain.
Answer and explanation

Yes. Acceleration is a change in velocity, and velocity includes direction. The car's direction is changing continuously even though its speed is constant, so its velocity is changing and it is accelerating. The force producing that acceleration comes from friction between the tyres and the road.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

Problem 8

Explain the difference between average speed and instantaneous speed using a car journey.
Answer and explanation

Average speed is the total distance divided by the total time, so a 60 km journey in 1 hour gives 60 km/h regardless of what happened along the way. Instantaneous speed is the speed at one moment, which the speedometer shows — it may read 0 at a traffic light and 90 on a clear stretch. The average hides the variation that the instantaneous readings reveal.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

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