Question Think It Over 1
How does the safe distance from a truck ahead depend on our speed?
Steps only
Higher speed needs a larger safe gap.
Answer
Keep a larger gap at a higher speed.
CLASS 9 · SCIENCE · CHAPTER 4
Describing Motion Around Us
Simple answers to motion activities, all sixteen review questions, and the four end projects.
26 worked answersComplete chapter answers
26 worked answers · short steps first, explanations when you need them
Chapter Activities and Review Questions
Textbook pages 48–70
Question Activity 4.1
How can we describe where a moving object is at a particular time?
Steps only
Use distance and direction from a fixed reference point.
Answer
Distance and direction from a reference point.
Question Activity 4.2
How do we find average acceleration from a velocity change?
Steps only
aₐᵥ = (v − u) / t
Answer
Average acceleration is change in velocity divided by time.
Question Activity 4.3
How do we plot a position-time graph from observations?
Steps only
Time on x-axis; position on y-axis; plot and join points.
Answer
Plot position against time.
Question Activity 4.4
How do we plot a velocity-time graph from observations?
Steps only
Time on x-axis; velocity on y-axis.
Answer
Plot velocity against time.
Question Activity 4.5
What happens when the ring around a moving marble is lifted, and why?
Steps only
It leaves along the tangent when the ring is lifted.
Answer
The marble travels approximately straight along the tangent.
Question 1
A shop is 250 m from home on a straight road. A person goes home to shop, returns home for a bag, goes to the shop again, and then comes home. Find the total distance and final displacement.
Steps only
Distance = 4 × 250 m = 1000 m
Answer
Distance: 1000 m; displacement: 0 m.
Question 2
A student runs from the ground floor to the fourth floor, then comes down to the second floor. Each floor is 3 m high. Find the total vertical distance and displacement.
Steps only
Distance = 12 + 6 = 18 m; displacement = 6 m upward.
Answer
Distance: 18 m; displacement: 6 m upward.
Question 3
Can a scooter accelerate when its speedometer reading is constant? Explain.
Steps only
Turning changes velocity direction, so acceleration is present.
Answer
Yes, while turning.
Question 4
A car starts from rest and reaches 24 m s⁻¹ in 6 s. Find its average acceleration and distance travelled, assuming uniform acceleration.
Steps only
a = (24 − 0) / 6 = 4 m s⁻²; s = [(0 + 24) / 2] × 6 = 72 m
Answer
4 m s⁻²; 72 m.
Question 5
A motorbike moving at 28 m s⁻¹ stops after travelling 98 m. Find its acceleration and stopping time, assuming constant acceleration.
Steps only
0 = 28² + 2a(98), so a = −4 m s⁻²; 0 = 28 − 4t, so t = 7 s.
Answer
−4 m s⁻²; 7 s.
Question 6
Figure 4.27 shows position-time graphs of objects A and B. Do they ever have equal velocity?
Steps only
The lines have different slopes, so the velocities differ.
Answer
No. They meet at 5 s, but their velocities are not equal.
Question 7
For objects A and B in Fig. 4.28, which statements about average velocity and average speed over 10 s are correct?
Steps only
Same displacement gives equal average velocity; B travels farther.
Answer
(i) and (iii).
Question 8
A truck slows uniformly from 54 km h⁻¹ to 36 km h⁻¹ in 36 s. Find the distance travelled.
Steps only
[(15 + 10) / 2] × 36 = 450 m
Answer
450 m.
Question 9
A car accelerates from rest to 20 m s⁻¹ in 5 s, travels at 20 m s⁻¹ for 10 s, then brakes to rest in 6 s. Find the total distance.
Steps only
50 + 200 + 60 = 310 m
Answer
310 m.
Question 10
A bus travels at 36 km h⁻¹. An obstacle is 30 m ahead. The driver reacts for 0.5 s, then brakes with acceleration −2.5 m s⁻². Will the bus stop in time?
Steps only
Reaction: 10 × 0.5 = 5 m; braking: 10² / (2 × 2.5) = 20 m; total = 25 m.
Answer
Yes, it stops 5 m before the obstacle.
Question 11
If Earth moves around the Sun, can an object on Earth be considered at rest?
Steps only
It is at rest relative to Earth, but moving relative to the Sun.
Answer
Rest depends on the reference point.
Question 12
For the cyclist graph in Fig. 4.30, identify the displacement during constant and decreasing velocity, then find total displacement and average acceleration over 120 s.
Steps only
Area under the graph: 30 + 240 + 50 = 320 m. Acceleration = (2 − 0)/120.
Answer
Constant part: 240 m; slowing part: 50 m; total: 320 m; average acceleration: 0.0167 m/s².
Question 13
Estimate the distance from the runner's velocity-time graph in Fig. 4.31.
Steps only
Area under the speed-time graph ≈ 47 km.
Answer
About 47 km (estimate).
Question 14
A car moves at 6 m s⁻¹ for 2 minutes, then accelerates at 1 m s⁻² for 6 s. Find its displacement in the full 2 min 6 s using a velocity-time graph.
Steps only
6 × 120 + [(6 + 12) / 2] × 6 = 774 m
Answer
774 m.
Question 15
Car A reaches 5 m s⁻¹ in 5 s and car B reaches 3 m s⁻¹ in 10 s, both from rest with constant acceleration. Describe their velocity-time graphs and find their displacements over those intervals.
Steps only
A: ½ × 5 × 5 = 12.5 m; B: ½ × 10 × 3 = 15 m.
Answer
A: 12.5 m; B: 15 m.
Question 16
From 6 PM to 7:30 PM, find the distance, displacement, speed, and velocity of the tip of a 7 cm minute hand.
Steps only
Distance = 1.5 × 2π × 7 = 21π cm; displacement = 14 cm; divide each by 90 min for averages.
Answer
66 cm; 14 cm; 0.733 cm min⁻¹; 0.156 cm min⁻¹ toward the opposite point.
Question Project 1
On a spinning disc, why may numbers near the edge disappear while letters closer to the centre remain visible? Are their speeds equal?
Steps only
Outer marks have greater linear speed and blur more.
Answer
Same angular speed; outer numbers have greater linear speed.
Question Project 2
What may an accelerometer show when a phone is held in an outstretched hand compared with when it rests on the floor?
Steps only
A held phone usually moves slightly more than one resting on the floor.
Answer
Compare repeated accelerometer readings in both positions.
Question Project 3
Derive s = vt − ½at² and s = ½(u + v)t for constant acceleration.
Steps only
u = v − at ⇒ s = (v − at)t + ½at² = vt − ½at²; graph area = ½(u + v)t.
Answer
s = vt − ½at² and s = ½(u + v)t.
Question Project 4
How do road and driving conditions affect stopping distance, and what should a school safety message say?
Steps only
Poor grip, high speed, low visibility, and delayed reactions increase stopping distance.
Answer
Slow down and keep extra space, especially in poor conditions.