Unit notes and practice questions

Grade 9 Physics unit 3: Motion in a Straight Line

Everything here is for unit 3 of the Grade 9 Physics textbook: what the chapter covers, the short notes, practice questions with their answers, and the lesson video where one exists.

5
Cards: 10
Practice questions on unit 3: 36–59
The textbook pages for unit 3

What unit 3 covers

1.Position, Distance and Displacement
2.Average Speed and Instantaneous Speed
3.Average Velocity and Instantaneous Velocity
4.Acceleration
5.Uniform Motion
6.Graphical Representation of Motion

Short notes for unit 3

The cards filed under this chapter on the Grade 9 Physics page. Open one at a time.

Average AccelerationFormula

\vec{a}_{av} = \frac{\vec{v}_f - \vec{v}_i}{t_f - t_i} = \frac{\Delta \vec{v}}{\Delta t}

\vec{a}_{av} = \frac{\vec{v}_f - \vec{v}_i}{t_f - t_i} = \frac{\Delta \vec{v}}{\Delta t}

$\vec{a}_{av}$: average acceleration $m\,s^{-2}$
$\vec{v}_f$: final velocity $m\,s^{-1}$
$\vec{v}_i$: initial velocity $m\,s^{-1}$
$\Delta \vec{v}$: change in velocity $m\,s^{-1}$
$\Delta t$: time interval $s$

When you use it

Use to calculate the rate of change of velocity over a given time interval.

Watch out

When an object slows down, final velocity is less than initial velocity, resulting in negative acceleration (deceleration).

Drafted from Grade 9 Physics, pages 36-59, then checked twice before it went up

Average SpeedFormula

v_{av} = \frac{s_{tot}}{t_{tot}}

v_{av} = \frac{s_{tot}}{t_{tot}}

$v_{av}$: average speed $m\,s^{-1}$
$s_{tot}$: total distance $m$
$t_{tot}$: total time $s$

When you use it

Use to calculate the rate at which total distance is covered over the total time taken.

Watch out

Always convert non SI units before calculating. Multiply km/h by 10/36 to convert to m/s.

Drafted from Grade 9 Physics, pages 36-59, then checked twice before it went up

Average VelocityFormula

\vec{v}_{av} = \frac{\Delta \vec{S}}{\Delta t} = \frac{\vec{S}_f - \vec{S}_i}{t_f - t_i}

\vec{v}_{av} = \frac{\Delta \vec{S}}{\Delta t} = \frac{\vec{S}_f - \vec{S}_i}{t_f - t_i}

$\vec{v}_{av}$: average velocity $m\,s^{-1}$
$\Delta \vec{S}$: displacement $m$
$\vec{S}_f$: final position $m$
$\vec{S}_i$: initial position $m$
$\Delta t$: time interval $s$
$t_f$: final time $s$
$t_i$: initial time $s$

When you use it

Use to calculate the rate of change of position in a specific direction over a time interval.

Watch out

Displacement depends on initial and final positions only. It is not the total distance travelled along the path.

Drafted from Grade 9 Physics, pages 36-59, then checked twice before it went up

Motion Graphs InterpretationReference tableS-t · v-t

Graph	Slope	Area Under Curve
$S-t$	velocity	No physical meaning
$v-t$	acceleration	distance or displacement

When you use it

Use when finding velocity, acceleration, or distance from position-time and velocity-time graphs.

Watch out

The slope of a v-t graph gives acceleration, while the area under it gives distance or displacement. Do not mix slope and area.

Drafted from Grade 9 Physics, pages 36-59, then checked twice before it went up

Round Trip DisplacementCommon mistake

When you use it

Reach for this when an object completes a trip and returns to its starting point.

Watch out

When an object returns to its starting point, net displacement is zero and average velocity is zero, but total distance and average speed are greater than zero.

Drafted from Grade 9 Physics, pages 36-59, then checked twice before it went up

Practice questions on unit 3

10 multiple-choice questions written against this chapter, each with its answer and the reason. Try one before you open the answer.

Question 1

Which of the following statements correctly differentiates between distance and displacement?

A.Distance is a scalar quantity, while displacement is a vector quantity.
B.Distance is a vector quantity, while displacement is a scalar quantity.
C.Distance depends only on the initial and final positions.
D.Displacement is always greater than distance.

Show the answer and why

Answer: A Distance is a scalar quantity, while displacement is a vector quantity.

Why

Distance is the total path length traveled and has no direction, so it is a scalar quantity. Displacement is the change in position in a specific direction, which makes it a vector quantity.

Question 2

Which instrument in an automobile is used to measure its instantaneous speed?

A.Odometer
B.Speedometer
C.Thermometer
D.Barometer

Show the answer and why

Answer: B Speedometer

Why

A speedometer shows the speed of a vehicle at a specific instant in time, which is its instantaneous speed.

Question 3

What physical quantity is represented by the slope of a position-versus-time graph?

A.Acceleration
B.Total distance
C.Velocity
D.Displacement

Show the answer and why

Answer: C Velocity

Why

The slope of a position-versus-time graph is the rise (change in position) over run (change in time), which gives velocity (v = ΔS / Δt).

Question 4

A student walks 500 m from home to school in Bahir Dar and then returns home along the exact same path. What is the student's total displacement for the entire trip?

A.0 m
B.500 m
C.1000 m
D.250 m

Show the answer and why

Answer: A 0 m

Why

Displacement depends only on the initial and final positions. Because the student returns to the starting point (home), the final position equals the initial position, so the displacement is 0 m.

Question 5

A train travels on a straight track from Addis Ababa towards Mojo at a constant speed. If it covers a distance of 2400 meters in 80 seconds, what is its speed?

A.30 m/s
B.40 m/s
C.60 m/s
D.100 m/s

Show the answer and why

Answer: A 30 m/s

Why

Speed is calculated as distance divided by time: v = s / t = 2400 m / 80 s = 30 m/s.

Question 6

A car accelerates uniformly from rest to a speed of 20 m/s in 10 seconds. What is the acceleration of the car?

A.0.5 m/s2
B.1 m/s2
C.2 m/s2
D.5 m/s2

Show the answer and why

Answer: C 2 m/s2

Why

Starting from rest means initial velocity v_i = 0. Acceleration is a = (v_f - v_i) / Δt = (20 m/s - 0 m/s) / 10 s = 2 m/s².

Question 7

A bus travels along a straight highway from Hawassa at a constant speed of 90 km/h. How far does the bus travel in 15 minutes?

A.12.5 km
B.22.5 km
C.30.0 km
D.50.0 km

Show the answer and why

Answer: B 22.5 km

Why

Convert time to hours: 15 minutes = 15 / 60 hours = 0.25 hours. Then distance is s = v × t = 90 km/h × 0.25 h = 22.5 km.

Question 8

A cyclist rides 3 km west and then turns around to ride 2 km east. What is the net displacement of the cyclist?

A.1 km west
B.1 km east
C.5 km west
D.5 km east

Show the answer and why

Answer: A 1 km west

Why

Taking west as negative and east as positive, the displacement is -3 km + 2 km = -1 km, which means 1 km west.

Question 9

An airplane lands on a runway with an initial velocity of 70.0 m/s and decelerates at 1.50 m/s² for 40.0 s. What is its final velocity?

A.0.0 m/s
B.10.0 m/s
C.60.0 m/s
D.130.0 m/s

Show the answer and why

Answer: B 10.0 m/s

Why

Deceleration is negative acceleration (a = -1.50 m/s²). Using v_f = v_i + aΔt: v_f = 70.0 m/s + (-1.50 m/s² × 40.0 s) = 70.0 m/s - 60.0 m/s = 10.0 m/s.

Question 10

Which of the following is correct for an object undergoing uniform rectilinear motion?

A.Its acceleration is zero, and the area under its velocity-time graph gives the distance covered.
B.Its acceleration is constant and positive, and the slope of its position-time graph is zero.
C.Its instantaneous velocity is always different from its average velocity.
D.The velocity-time graph is a vertical line.

Show the answer and why

Answer: A Its acceleration is zero, and the area under its velocity-time graph gives the distance covered.

Why

In uniform rectilinear motion, velocity is constant, which means acceleration is zero (a = 0). The area under a horizontal velocity-time graph represents the distance traveled (s = v × t).

The app holds 12 questions on this unit, marked as you go. These are the first 10.

The textbook pages for unit 3

Unit 3 runs from page 36 to page 59 of the Ministry of Education edition.

Ministry of Education edition, English, 179 pages

Download the PDF

All Grade 9 Physics short notes

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