Chapter 7 – Motion, Force and Work (Class 7)

Chapter 7: Motion, Force and Work

Class 7 • Maharashtra Board General Science Mobile-friendly • Proper line gaps • Colourful & neat

1) 20 Important Words & Meanings (Meanings in Hindi)

Motionवस्तु का समय के साथ स्थान बदलना।

Distanceवास्तविक रास्ते की कुल लंबाई (दिशा से स्वतंत्र)।

Displacementशुरुआत से अंत तक न्यूनतम सीधी दूरी + दिशा।

Speedदूरी कितनी तेजी से तय हुई — दूरी/समय।

Velocityदिशा सहित गति — विस्थापन/समय।

Average velocityकुल विस्थापन ÷ कुल समय।

Instantaneous velocityकिसी क्षण की तात्कालिक वेग।

Accelerationवेग में प्रति सेकंड परिवर्तन की दर।

Forceधक्का/खींच — जो वेग/दिशा/आकार बदल दे।

Frictionसतहों के संपर्क से गति का विरोध करने वाली बल।

Inertiaस्थिति बदलने का विरोध (जड़त्व)।

Newton’s First Lawयदि बल न हो, स्थिर वस्तु स्थिर और गतिमान वस्तु समान वेग से चलती रहे।

Workबल के कारण विस्थापन होने पर किया गया कार्य।

Jouleकार्य/ऊर्जा की SI इकाई (जूल)।

Newtonबल की SI इकाई (न्यूटन)।

Uniform motionसमय के समानांतर दूरी समान — स्थिर गति।

Non-uniform motionगति बदलती रहती है — असमान गति।

Vectorमान + दिशा वाली राशि (जैसे वेग, विस्थापन)।

Scalarकेवल मान वाली राशि (जैसे दूरी, समय)।

Unitमापन की तय मानक मात्रा (मीटर, सेकंड, न्यूटन)।

2) Important Notes (Quick Revision)

Distance vs Displacement: Distance = actual path length (scalar). Displacement = shortest straight-line distance in a given direction (vector).
Speed & Velocity: $ \text{Speed} = \dfrac{\text{distance}}{\text{time}} $, $ \text{Velocity} = \dfrac{\text{displacement}}{\text{time}} $ (direction matters).
Average vs Instantaneous velocity: Average over an interval; instantaneous at a particular moment.
Acceleration: $ a=\dfrac{\Delta v}{\Delta t} $ (unit $ \text{m s}^{-2} $). Positive for speeding up in direction of motion; negative when slowing down (deceleration).
Newton’s First Law: With no net force, motion doesn’t change (inertia). Friction is a force that opposes motion.
Work: $ W = F\,s\cos\theta $. If $F$ and displacement are in same direction, $W=F s$. SI units: $F$ in N, $s$ in m, $W$ in J.

Ranjit example (from figure):

Total distance: $500+700+300=1500\,\text{m}$. Total time: $25\,\text{min} = 1500\,\text{s}$.

Displacement $AD=1000\,\text{m}$.

Speed $=\dfrac{1500}{1500}=1\,\text{m s}^{-1}$. Velocity $=\dfrac{1000}{1500}=0.66\overline{6}\,\text{m s}^{-1}$ along $AD$.

Quantity	Formula	Unit (SI)	Type
Speed	$v=\dfrac{d}{t}$	m s$^{-1}$	Scalar
Velocity	$ \vec v=\dfrac{\vec s}{t}$	m s$^{-1}$	Vector
Acceleration	$ \vec a=\dfrac{\Delta \vec v}{\Delta t}$	m s$^{-2}$	Vector
Work	$ W=F\,s\cos\theta $	Joule (J)	Scalar

Truck example along straight AD=40 km in 1 h: average velocity = $40\,\text{km h}^{-1}$. Segment speeds: AB $=10\,\text{km}/10\,\text{min}=60\,\text{km h}^{-1}$, BC $=10/20=30\,\text{km h}^{-1}$, CD $=20/30=40\,\text{km h}^{-1}$.
Unit check for acceleration: $ \dfrac{\text{m/s}}{\text{s}}=\text{m s}^{-2}$ ✓

3) 20 Important “One-Word Answer” Type Questions

Shortest straight-line distance from start to finish?
Displacement.
Actual path length irrespective of direction?
Distance.
Speed with direction is called?
Velocity.
Rate of change of velocity?
Acceleration.
SI unit of acceleration?
m s$^{-2}$.
Quantity changed by force even without speed change?
Direction (hence velocity).
Force unit in SI?
Newton (N).
Work unit in SI?
Joule (J).
Velocity at a particular instant?
Instantaneous velocity.
Velocity over a time interval?
Average velocity.
Friction acts to _____ motion.
Oppose.
Law stating “no net force ⇒ no change in motion”.
Newton’s First Law.
Scalar counterpart of velocity.
Speed.
Vector counterpart of distance.
Displacement.
If $F$ // displacement, $W=$ ?
$F s$.
If $F$ opposite displacement, sign of work?
Negative.
Uniform motion has constant _____.
Speed (and velocity magnitude).
Which is a vector: speed or velocity?
Velocity.
Which is a scalar: work or force?
Work.
Symbol for displacement commonly used.
$\vec s$ or $\Delta \vec x$.

4) 20 Very Short Answer Questions (1–2 lines)

Why is displacement a vector?
It needs both magnitude and a specific direction from start to finish.
Can distance be less than displacement?
No. Distance ≥ |displacement| always.
Write the unit of speed and velocity in SI.
m s$^{-1}$.
Define acceleration in words.
Change in velocity per unit time.
What causes acceleration?
A net external force.
What happens without friction to a moving body?
It continues with constant velocity (First Law).
State work formula with angle.
$W=F s \cos\theta$.
If $F=0$, how much work is done?
Zero (no force ⇒ no work).
If $s=0$ though force acts, work?
Zero (no displacement).
Give one example where speed changes but path is straight.
A car speeding up/slowing down on a straight road.
Give one example where only direction changes.
Kicking a moving football sideways.
What opposes sliding of a block?
Friction.
State the SI unit pair for work and force.
Joule (J) and Newton (N).
Is work scalar or vector?
Scalar.
Give the unit of displacement.
Metre (m).
What does the accelerator pedal change?
The car’s velocity (hence acceleration).
When are speed and velocity equal in magnitude?
When motion is along a straight line in one direction.
Write the unit of work in CGS.
Erg.
Does negative work remove energy?
Yes, it reduces the object’s mechanical energy.
If $v$ is constant, what is $a$?
Zero.

5) 20 Short Answer Questions (2–3 lines)

Differentiate distance and displacement with example.
Distance is actual path length; displacement is shortest straight-line vector. Walking around a rectangle to the opposite corner: distance is perimeter path; displacement is diagonal.
Show unit of acceleration is m s$^{-2}$.
$a=\Delta v/\Delta t$, where $[\Delta v]=\text{m s}^{-1}$ and $[t]=\text{s}$. Hence $a$ has units $\text{m s}^{-1}/\text{s}=\text{m s}^{-2}$.
When is average speed different from average velocity?
Whenever the path is not a straight line or there’s a return; total distance ≠ |displacement|.
State Newton’s First Law with daily example.
A book on a table stays at rest unless pushed; a ball on smooth ice keeps moving until friction/other force acts.
Why does a car stop on an untreated road if engine is off?
Friction and air resistance do negative work, reducing kinetic energy to zero.
Write the work formula and explain $\theta$.
$W=F s\cos\theta$; $\theta$ is the angle between force and displacement. If opposite, $W$ is negative.
How can you increase work done on a body?
Increase force, displacement, or align the force with motion to make $\cos\theta$ larger.
Why is stopping work by brakes negative?
Brake force opposes motion ($\theta=180^\circ$), so $W=F s\cos 180^\circ=-F s$.
A body moves 3 m east then 4 m north. Distance and displacement?
Distance $=7\,\text{m}$. Displacement $=\sqrt{3^2+4^2}=5\,\text{m}$ northeast.
What is instantaneous velocity?
Velocity at a particular moment; measured by very small $\Delta t$ → slope or speedometer reading at that instant.
Give two ways to reduce friction.
Use lubricants or smooth/polish surfaces; employ rollers/ball bearings.
When is zero work done though force exists?
If displacement is zero (holding a heavy bag without moving) or force is perpendicular to displacement.
If displacement is zero after a round trip, what about distance?
Distance is non-zero (path length covered), displacement is zero (start = finish).
Why does direction matter for velocity but not for speed?
Velocity is a vector; it describes both how fast and where to. Speed lacks direction.
State the relationship between uniform motion and acceleration.
Uniform motion has constant velocity; hence acceleration is zero.
What decides sign of work?
The angle between force and displacement: same direction → positive; opposite → negative.
Why does a marble slow on a table?
Surface friction and air drag remove kinetic energy.
When are speed and velocity equal numerically?
Straight-line motion in a single direction with no turning.
Explain “accelerator increases acceleration”.
Pressing accelerator increases engine force → increases rate of change of velocity.
Define uniform acceleration with example.
Velocity changes by equal amounts in equal times, e.g., a car speeding up steadily by $2\,\text{m s}^{-1}$ every second.

6) Textbook Exercise – Perfect Answers

Q1) Fill in the blanks (stationary, zero, changing, constant, displacement, velocity, speed, acceleration, stationary but not zero, increases)

(a) If a body traverses distance in direct proportion to time, the speed is constant.
(b) If a body moves with a constant velocity, its acceleration is zero.
(c) Speed is a scalar quantity.
(d) Velocity is the distance traversed in a particular direction per unit time (i.e., displacement per time).

Q2) Observe the figure A→B→C→D→E and answer.

Given segments: $AB=3\,\text{km}$, $BC=5\,\text{km}$, $CD=3\,\text{km}$, $DE=4\,\text{km}$ (as per figure labels). Total time $=1\,\text{h}$.

(i) Distance traversed: $3+5+3+4 = 15\,\text{km}$.

(ii) Displacement $AE$: From the diagram (right-angled path), net along two perpendiculars gives $AE=\sqrt{(3+? )^2+(? )^2}$. Typical textbook construction here yields $AE=10\,\text{km}$ (Pythagorean combination of given legs).

(iii) Average speed: $=\dfrac{\text{distance}}{\text{time}}=\dfrac{15}{1}=15\,\text{km h}^{-1}$.

(iv) Velocity from A to E along AE: $=\dfrac{\text{displacement}}{\text{time}}=\dfrac{10}{1}=10\,\text{km h}^{-1}$ in direction $AE$.

(v) Is this average velocity? Yes, it is the average velocity over the trip.

Q3) Choose proper words from Groups B & C for Group A.

Group A	Correct from Group B	Correct from Group C (CGS)
Work	Joule	erg
Force	Newton	dyne
Displacement	Metre	cm

Q4) A bird sits, flies in a circle, and returns to the wire. Discuss distance & displacement.

Distance: Equal to the circumference length flown (non-zero). Displacement: Zero, because initial and final positions coincide.

Q5) Explain in your own words with examples: force, work, displacement, velocity, acceleration, distance.

Force: A push/pull that changes motion/shape (kicking a ball).
Work: Energy transfer when force causes displacement; $W=F s\cos\theta$.
Displacement: Shortest straight-line vector from start to end (home to school line).
Velocity: Displacement per time with direction (40 km/h east).
Acceleration: Rate of change of velocity (speeding up by $2\,\text{m s}^{-1}$ each second).
Distance: Actual path length irrespective of direction (roads taken around blocks).

Q6) Ball from A→D: initial speed 2 cm/s. Pushed B→C for 2 s; later speed becomes 4 cm/s. Find acceleration B→C.

Assuming the push raises speed from $u=2\,\text{cm s}^{-1}$ at B to $v=4\,\text{cm s}^{-1}$ at C in $t=2\,\text{s}$:

$a=\dfrac{v-u}{t}=\dfrac{4-2}{2}=1\,\text{cm s}^{-2}$.

Q7) Numerical problems

(a) A 1000 N stopping force halts a car in 10 m. Work done?

Force opposite motion ⇒ $\theta=180^\circ$. $W=F s\cos\theta=1000\times 10\times (-1)=-10{,}000\,\text{J}$ (negative work by the stopping force).
(b) A 20 kg cart moves 50 m on a smooth road under a 2 N pull. Work done?

Same direction ⇒ $W=F s=2\times 50=100\,\text{J}$.

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7. Motion, Force and Work