This MCQ module is based on: Newton’s First and Second Laws of Motion and Inertia
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Newton’s First and Second Laws of Motion and Inertia
🎓 Class 9
Science
CBSE
Theory
Ch 6 — How Forces Affect Motion
⏱ ~13 min
🌐 Language: [gtranslate]
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This assessment will be based on: Newton’s First and Second Laws of Motion and Inertia
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Recap: Linking Force to Motion
In Part 1 we saw that a force is a push or pull and that an unbalanced force changes the state of motion of an object. But how exactly is the change in motion related to the size of the force and to the body itself? The answers lie in three remarkable laws first stated by Sir Isaac Newton (1642–1727). We meet the first two in this part.
6.5 Galileo's Incline Experiments
Long before Newton, the Italian scientist Galileo Galilei performed clever experiments with smooth inclined planes. He let a ball roll down one incline and roll up another. He noticed that the ball climbed up the second incline almost as high as the height from which it started. The smoother the surfaces, the closer it came.
Galileo then asked: what if the second incline were stretched flat with no friction at all? The ball, never reaching its starting height, would simply keep rolling forever. This thought experiment told him that motion does not need a continuous force to maintain it — only friction (an external force) brings moving objects to rest.
6.6 Newton's First Law of Motion
Building on Galileo's idea, Newton stated his First Law:
Newton's First Law of Motion: A body at rest will remain at rest, and a body in uniform motion in a straight line will continue to move that way, unless acted upon by an external unbalanced force.
The First Law is often called the law of inertia, because it states that all bodies have a natural tendency to resist changes in their state of motion.
What is inertia?
Inertia is the property by which a body opposes any attempt to change its state of motion. The greater the mass of a body, the greater its inertia. That is why mass is called the measure of inertia.
Three faces of inertia
| Type | Meaning | Everyday example |
|---|---|---|
| Inertia of rest | A body at rest stays at rest until forced to move. | When a bus suddenly starts, passengers jerk backwards — their bodies want to stay at rest while the bus moves forward. |
| Inertia of motion | A body in motion continues to move uniformly until forced to stop. | When a moving bus brakes suddenly, passengers lurch forward — their bodies want to keep moving while the bus stops. |
| Inertia of direction | A body in motion keeps moving in the same straight line until forced to change direction. | When a car turns sharply, passengers lean outward — their bodies want to continue in a straight line. |
🚌 Three Faces of Inertia — Click each bus to reveal L1 Remember
Inertia hides in plain sight every time you ride a bus. Click each panel to reveal which type of inertia is at play and the everyday observation that proves it.
Click any of the three bus panels above to reveal the type of inertia and what you feel.
More everyday demonstrations of inertia
- Coin on a card: Place a stiff card over a glass and a coin on the card. Flick the card sharply sideways — the coin drops straight into the glass. The card moves but the coin's inertia of rest keeps it in place momentarily, and gravity then pulls it down.
- Dust from a carpet: When a carpet is beaten with a stick, the carpet jerks forward but the dust particles, due to inertia of rest, remain behind and so come out into the air.
- Athlete's run-up: A long jumper runs before the jump so that the body acquires inertia of motion, which carries them further across the pit.
Activity 6.2 — The Coin and the CardL3 Apply
Predict first: If you place a coin on a stiff card resting on a glass and you snap the card horizontally with a quick flick, where will the coin end up?
- Set an empty glass tumbler on a table.
- Place a smooth, stiff card (e.g. a postcard) on top of the glass.
- Place a one-rupee coin in the centre of the card, directly above the mouth of the glass.
- Give the card a sharp, horizontal flick with your finger so it shoots sideways.
- Observe what happens to the coin.
Observation: The card shoots out and the coin falls vertically into the glass.
Why? The flick is so quick that friction acts on the coin only for a tiny instant — too short to drag it along. The coin's inertia of rest keeps it almost in place. Once the card is gone, gravity pulls the coin straight down into the glass.
Why? The flick is so quick that friction acts on the coin only for a tiny instant — too short to drag it along. The coin's inertia of rest keeps it almost in place. Once the card is gone, gravity pulls the coin straight down into the glass.
6.7 Newton's Second Law of Motion
The First Law tells us that a force changes motion — but not by how much. The Second Law makes this quantitative.
Newton's Second Law of Motion: The acceleration produced in a body is directly proportional to the net force applied on it and inversely proportional to its mass. The acceleration is in the direction of the net force.
Mathematically, if a net force \(F\) acts on a body of mass \(m\) and produces an acceleration \(a\), then
\(F = m \, a\)
The SI unit of force is the newton (N). From the equation, \(1 \text{ N} = 1 \text{ kg} \cdot \text{m/s}^2\) — that is, 1 newton is the force needed to give a 1 kg mass an acceleration of 1 m/s².
Working tip: If a body of mass \(m\) changes its velocity from \(u\) to \(v\) in time \(t\), its acceleration is \(a = (v-u)/t\). Substituting in \(F = ma\) gives \(F = m(v-u)/t\), a useful form for many problems.
Worked Numericals
Example 1 — Direct application of F = ma
Q. A force acts on a 5 kg block at rest and produces an acceleration of 2 m/s². Find the force.
Solution. Given \(m = 5\) kg, \(a = 2\) m/s². Using \(F = ma\):
\(F = 5 \times 2 = 10\) N.
Answer: The applied force is 10 N in the direction of acceleration.
Example 2 — Finding mass
Q. A net force of 20 N produces an acceleration of 4 m/s² in a body. What is its mass?
Solution. \(m = F/a = 20/4 = 5\) kg.
Example 3 — Acceleration of a car
Q. A car of mass 1000 kg moving with a velocity of 36 km/h is brought to rest in 5 s by applying brakes. Find the braking force.
Solution. Convert: \(u = 36 \text{ km/h} = 36 \times \tfrac{5}{18} = 10\) m/s. Final velocity \(v = 0\), \(t = 5\) s.
Acceleration \(a = (v-u)/t = (0-10)/5 = -2\) m/s² (negative ⇒ deceleration).
\(F = ma = 1000 \times (-2) = -2000\) N.
Answer: Braking force = 2000 N opposite to the direction of motion.
Example 4 — Two forces on the same body
Q. A body of mass 10 kg is pushed forward by 50 N while friction of 30 N acts backwards. Find its acceleration.
Solution. Net force \(F_{net} = 50 - 30 = 20\) N. Acceleration \(a = F/m = 20/10 = 2\) m/s² in the forward direction.
Example 5 — Velocity change calculation
Q. A ball of mass 0.2 kg, initially at rest, is given a force of 4 N for 0.5 s. Find its final velocity.
Solution. \(a = F/m = 4/0.2 = 20\) m/s². Using \(v = u + at\): \(v = 0 + 20 \times 0.5 = 10\) m/s.
Example 6 — Same force, different masses
Q. A force of 12 N acts separately on a 2 kg and a 6 kg block. Compare the accelerations produced.
Solution. \(a_1 = 12/2 = 6\) m/s²; \(a_2 = 12/6 = 2\) m/s². Ratio \(a_1 : a_2 = 3 : 1\). The lighter body accelerates 3 times more — a clear demonstration that mass is a measure of inertia.
Quick Recap
| Concept | Statement / Formula |
|---|---|
| Newton's First Law | A body continues at rest or in uniform straight-line motion unless an external unbalanced force acts on it. |
| Inertia | Tendency to resist change in state of motion; measured by mass. |
| Newton's Second Law | \(F = ma\); 1 N = 1 kg·m/s². |
| Acceleration | \(a = (v-u)/t\) |
Competency-Based Questions
A school bus moving at 18 km/h applies brakes and stops in 3 s. The total mass of the bus with passengers is 2400 kg. A standing passenger is not holding any support.
Q1. Convert 18 km/h to m/s and find the deceleration of the bus. L3
\(u = 18 \times 5/18 = 5\) m/s, \(v = 0\), \(t = 3\) s. \(a = (0-5)/3 = -1.67\) m/s². Deceleration ≈ 1.67 m/s².
Q2. Calculate the braking force on the bus. L3
\(F = ma = 2400 \times 1.67 \approx 4000\) N opposite to motion.
Q3. Why does the standing passenger fall forward when the bus stops? L2
(b) Inertia of motion. The passenger's body wants to keep moving forward at 5 m/s when the bus suddenly slows down.
Q4. Fill in the blank: The SI unit of force is _______ and 1 of this unit equals _______ kg·m/s². L1
newton; 1.
Q5. The same 10 N force is applied to two trolleys of mass 2 kg and 5 kg. Which accelerates faster and in what ratio? L4
\(a_1 = 10/2 = 5\) m/s²; \(a_2 = 10/5 = 2\) m/s². The 2 kg trolley accelerates faster, in the ratio 5 : 2.
Assertion–Reason Questions
Options: (A) Both true, R explains A. (B) Both true, R doesn't explain A. (C) A true, R false. (D) A false, R true.
A: A heavier body is harder to set in motion than a lighter one with the same force.
R: Mass is the quantitative measure of inertia.
(A) Both correct and R correctly explains A. By \(a = F/m\), a larger mass gives a smaller acceleration for the same force.
A: A passenger jerks backward when a stationary bus suddenly starts moving.
R: The passenger's lower body moves with the bus while the upper body resists motion due to inertia of rest.
(A) Both true and R explains A correctly — a classic demonstration of inertia of rest.
A: If no force acts on a body, it must be at rest.
R: Newton's First Law says a body in motion needs no force to keep moving uniformly.
(D) Assertion is false — a body with zero net force could equally be moving at constant velocity. Reason is true.
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