9th Class Physics Chapter 3 Dynamics Long Questions

Question 1

Explain the concept of force with the help of practical examples.

Accepted Answer

Definition Force: A push or a pull that changes, or tends to change, the state of rest or of motion of a body. It is a vector; SI unit newton (N). Effects of a force, with examples Start a body moving, Pushing a box makes it slide; kicking a ball at rest; Stop a moving body, A goalkeeper catches the ball; brakes stop a car; Change its speed, Pressing the accelerator or the brake; Change its direction, A batsman hits the ball back; a car turns a corner; Change its shape or size, Squeezing a sponge; stretching a spring Forces in daily life Opening a door: a push or pull at the handle turns the door.; Turning a corner in a car: the seat pushes sideways on you, which is why you feel pressed against it.; Pushing a wheelbarrow: you push forward to move it; to turn it, you push harder on one handle than the other. Kinds of force Friction, normal reaction, tension, air resistance, thrust, Gravitational, magnetic, electrostatic A force can change speed or direction (or both); since velocity is a vector, either is a change in motion.

Question 2

State and explain Newton's laws of motion.

Accepted Answer

Newton's first law (law of inertia) Statement: A body at rest stays at rest, and a body in motion keeps moving with uniform velocity in a straight line, unless a resultant external force acts on it. Explanation: every body resists a change in its state of motion; this is inertia. A force is needed to start, stop, speed up, slow down or turn a body. Passengers lurch forward when a bus stops suddenly.; Dust falls out of a carpet when it is beaten. Newton's second law Statement: When a resultant force acts on a body, it produces an acceleration in its own direction that is directly proportional to the force and inversely proportional to the mass. F = ma a ∝ F (twice the force, twice the acceleration) and a ∝ 1/m (twice the mass, half the acceleration). Combining them gives F = ma. Newton: The force that gives a 1 kg mass an acceleration of 1 m s⁻². 1 N = 1 kg m s⁻². Newton's third law Statement: To every action there is an equal and opposite reaction. The two forces act on different bodies. A book presses down on a table; the table pushes up on the book (normal reaction).; A gun pushes a bullet forward; the bullet pushes the gun back (recoil).; Your feet push the ground backward; the ground pushes you forward, so you walk. Action and reaction act on different bodies, so they never cancel each other.

Question 3

Define momentum and express Newton's second law in terms of change in momentum.

Accepted Answer

Momentum Momentum: The product of the mass and velocity of a body: p = mv. It is a vector in the direction of the velocity; SI unit kg m s⁻¹ (= N s). A loaded truck is harder to stop than a bicycle at the same speed because it has more momentum; a bullet is dangerous despite its tiny mass because of its huge velocity. Second law in terms of momentum Start from F = ma. Acceleration a = (vf − vᵢ) ÷ t, so F = m(vf − vᵢ) ÷ t. Multiply out: F = (mvf − mvᵢ) ÷ t. mvf is the final momentum and mvᵢ the initial momentum, so F = (change in momentum) ÷ time. F = Δp ÷ t Statement The resultant force on a body equals its rate of change of momentum, and the change in momentum is in the direction of the force. Impulse Rearranging: F × t = Δp. The product F × t is called impulse. Δp = 0.16 × (52 − (−28)) = 12.8 kg m s⁻¹. F = 12.8 ÷ 0.004. 3200 N

Question 4

State and explain the law of conservation of momentum.

Accepted Answer

Statement Law of conservation of momentum: The total momentum of an isolated system (one with no external force) remains constant. m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂ Explanation During a collision ball 1 pushes ball 2, and ball 2 pushes ball 1 back with an equal and opposite force (third law).; Both forces act for the same time, so the impulses are equal and opposite.; Ball 2 gains exactly the momentum that ball 1 loses, so the total does not change. Example Momentum before = 3 × 5 + 2 × 0 = 15 kg m s⁻¹. Momentum after = (3 + 2) v = 5v. v = 3 m s⁻¹ Applications Recoil of a gun: the gun moves back as the bullet goes forward.; Rockets: hot gases thrown backward push the rocket forward.; Newton's cradle: equal balls in an elastic collision swap their motion; the moving ball stops and the other moves off with the same speed.

Question 5

Describe the motion of a block on a table, taking friction into account. Explain static and kinetic friction.

Accepted Answer

The experiment A wooden block rests on a table. A string tied to it passes over a pulley at the edge and holds a light pan. Weights are added to the pan one by one, so the pull on the block grows slowly. What happens With small weights the block does not move. Friction from the table pulls back on the block exactly as hard as the string pulls it forward: this is static friction.; As weights are added, static friction grows to match the pull.; Static friction can only grow up to a maximum value, called limiting friction.; When the pull becomes just greater than limiting friction, the block starts to slide.; While it slides, a slightly smaller kinetic friction acts, so a slightly smaller pull keeps it moving at steady speed. Definitions Static friction: The friction on a body at rest that stops it sliding; it can have any value up to limiting friction. Limiting friction: The maximum value of static friction, just before sliding begins. Kinetic (sliding) friction: The friction on a body while it is sliding; slightly less than limiting friction. It is harder to start pushing a heavy box than to keep it moving, because limiting friction is greater than kinetic friction.

Question 6

Explain the effect of friction on the motion of a vehicle, with reference to the tyre surface and the braking force.

Accepted Answer

Grip: friction that makes a car move When the engine turns the wheels, each tyre pushes backward on the road. By the third law, friction from the road pushes the tyre, and so the car, forward. Without friction the wheels would just spin, as on ice. Braking The brakes slow the turning of the wheels. Friction between the tyres and the road then provides the backward force that slows and stops the car. The greater this friction, the shorter the stopping distance. Tyre surface (treads) Treads (grooves) on a tyre push water out from under it on a wet road, so the rubber keeps touching the road and gripping it.; Worn, smooth tyres cannot clear water, so they lose grip and can skid.; On wet or oily roads friction is smaller, so braking takes longer; drivers must slow down and keep a bigger gap. Unwanted effects Friction wears tyres down over time.; It heats the tyres and brakes.; Friction in the engine and air resistance waste fuel energy. In a vehicle, friction is both useful (grip and braking) and a nuisance (wear, heat, wasted fuel).

Question 7

What is inertia? Explain it with examples. How is it related to mass?

Accepted Answer

Definition Inertia: The tendency of a body to keep its state of rest or of uniform motion in a straight line, and to resist any change in it. Newton's first law is called the law of inertia because it describes this property. Inertia of rest When a bus starts suddenly, passengers fall backward: their bodies tend to stay at rest.; A coin on a card over a glass drops into the glass when the card is flicked away quickly.; Dust falls out when a carpet is beaten: the carpet moves, the dust stays. Inertia of motion When a bus stops suddenly, passengers lurch forward: their bodies tend to keep moving.; An athlete runs on past the finishing line before stopping.; Seat belts are needed because the body keeps moving forward in a sudden stop. Inertia and mass The more mass a body has, the harder it is to start, stop or turn: mass is the measure of inertia. An empty trolley is easy to push and stop; a loaded one is not. Inertia depends only on mass, not on speed, shape or colour.

Question 8

Differentiate between mass and weight.

Accepted Answer

Mass Mass: The quantity of matter in a body, and the measure of its inertia. Scalar; SI unit kilogram (kg). Weight Weight: The force with which the Earth (or another body) pulls an object towards its centre. Vector; SI unit newton (N). W = mg Differences Quantity of matter, Force of gravity on the body; Scalar, Vector, towards the centre of the Earth; Unit kg, Unit N; Same everywhere, Changes with g: less on the Moon, zero far out in space; Measured with a beam (physical) balance, Measured with a spring balance Weight on the Earth = 80 × 10 = 800 N. Weight on the Moon = 80 × 1.6 = 128 N. mass 80 kg in both places

Question 9

What is friction? Explain its advantages and disadvantages and the methods of reducing it.

Accepted Answer

Definition Friction: The force that opposes the motion, or attempted motion, of one surface over another. Cause Even smooth-looking surfaces have tiny bumps. When two surfaces touch, the bumps catch on each other and resist sliding. Advantages (friction is useful) We can walk without slipping.; Vehicles can start, turn and brake.; We can hold a pen and write, and grip things.; Nails and screws stay in wood; knots hold. Disadvantages (friction is a nuisance) It wears out tyres, shoe soles and machine parts.; It produces unwanted heat in engines and machines.; It wastes energy and fuel, making machines less efficient. Methods of reducing friction Lubricants (oil, grease), A thin layer keeps the surfaces apart; Ball bearings and wheels, Change sliding into rolling, which has far less friction; Smooth, polished surfaces, Fewer bumps to catch on each other; Streamlining, Reduces air and water resistance on cars, aircraft, boats We reduce friction where it wastes energy and increase it where we need grip, for example with treaded tyres and studded shoes.

Question 10

What is rolling friction? Why is it less than sliding friction? Explain the use of ball bearings.

Accepted Answer

Rolling friction Rolling friction: The friction that acts on a body that rolls over a surface, such as a wheel or a ball. Why it is less than sliding friction In sliding, the whole contact area rubs along the surface and the bumps keep catching. A rolling wheel touches the ground at only a small area, and each part of it is lifted off the ground rather than dragged along. So rolling friction is very much smaller, in many cases about a hundred times smaller, than sliding friction. Pull a heavy box across the floor with a spring balance and note the reading. Put the box on round pencils or a trolley and pull again. the reading is much smaller: rolling friction < sliding friction Ball bearings Ball bearings are small hard steel balls placed between the moving parts of a machine, for example between a wheel and its axle. The parts then roll on the balls instead of sliding against each other, so friction, heat and wear are greatly reduced. They are used in bicycles, cars, fans and motors. Wheels and ball bearings work by changing sliding friction into much smaller rolling friction.

Question 11

Define impulse. Explain, with daily-life examples, how a longer time of impact reduces the force.

Accepted Answer

Definition Impulse: The product of a force and the time for which it acts. It equals the change in momentum; SI unit N s. Impulse = F × t = Δp Why a longer time means a smaller force When a moving body is stopped, its momentum must change from mv to zero; the change Δp is fixed. Since F = Δp ÷ t, making the stopping time t longer makes the force F smaller. Examples Catching a fast cricket ball, The hands are drawn back with the ball, Smaller force, hands not hurt; Car crash, Air bags, seat belts and crumple zones stop the passenger gradually, Smaller force on the body; Jumping down, Bending the knees on landing, Smaller force on the legs; Packing glassware, Wrapping it in foam or paper, Smaller force if the box is dropped; High jump and pole vault, Landing on a thick soft mattress, Smaller force on the athlete Δp = 0.4 × 20 = 8 N s. Stopped in 0.01 s: F = 8 ÷ 0.01 = 800 N. Stopped in 0.1 s (hands drawn back): F = 8 ÷ 0.1 = 80 N. ten times the time, one-tenth of the force

Question 12

Explain terminal velocity with the example of a skydiver.

Accepted Answer

Forces on a falling body Two forces act on a skydiver: weight, pulling down, and air resistance, pushing up. Air resistance is small at low speed and grows as the speed grows. Stages of the fall Just after jumping: speed is low, air resistance is small, so the net force is downward and the skydiver accelerates.; Speeding up: as the speed rises, air resistance grows, so the net force and the acceleration get smaller.; Terminal velocity: air resistance grows until it equals the weight. The net force is zero, so the acceleration is zero and the skydiver falls at a steady speed.; Parachute opens: air resistance suddenly becomes much larger than the weight, so the skydiver slows down until a new, much lower terminal velocity is reached for a safe landing. Terminal velocity: The constant velocity reached by a falling body when the air resistance on it equals its weight. At terminal velocity the body is still moving fast; only its acceleration has become zero.

A force can…	Example
Start a body moving	Pushing a box makes it slide; kicking a ball at rest
Stop a moving body	A goalkeeper catches the ball; brakes stop a car
Change its speed	Pressing the accelerator or the brake
Change its direction	A batsman hits the ball back; a car turns a corner
Change its shape or size	Squeezing a sponge; stretching a spring

Mass	Weight
Quantity of matter	Force of gravity on the body
Scalar	Vector, towards the centre of the Earth
Unit kg	Unit N
Same everywhere	Changes with g: less on the Moon, zero far out in space
Measured with a beam (physical) balance	Measured with a spring balance

Method	How it helps
Lubricants (oil, grease)	A thin layer keeps the surfaces apart
Ball bearings and wheels	Change sliding into rolling, which has far less friction
Smooth, polished surfaces	Fewer bumps to catch on each other
Streamlining	Reduces air and water resistance on cars, aircraft, boats

Situation	What is done	Result
Catching a fast cricket ball	The hands are drawn back with the ball	Smaller force, hands not hurt
Car crash	Air bags, seat belts and crumple zones stop the passenger gradually	Smaller force on the body
Jumping down	Bending the knees on landing	Smaller force on the legs
Packing glassware	Wrapping it in foam or paper	Smaller force if the box is dropped
High jump and pole vault	Landing on a thick soft mattress	Smaller force on the athlete

Long Questions with Answers

Set A: Textbook comprehensive questions 6 questions

Explain the concept of force with the help of practical examples.

Definition

Effects of a force, with examples

Forces in daily life

Kinds of force

State and explain Newton's laws of motion.

Newton's first law (law of inertia)

Newton's second law

Newton's third law

Define momentum and express Newton's second law in terms of change in momentum.

Momentum

Second law in terms of momentum

Statement

Impulse

State and explain the law of conservation of momentum.

Statement

Explanation

Example

Applications

Describe the motion of a block on a table, taking friction into account. Explain static and kinetic friction.

The experiment

What happens

Definitions

Explain the effect of friction on the motion of a vehicle, with reference to the tyre surface and the braking force.

Grip: friction that makes a car move

Braking

Tyre surface (treads)

Unwanted effects

Set B: More important long questions 6 questions

What is inertia? Explain it with examples. How is it related to mass?

Definition

Inertia of rest

Inertia of motion

Inertia and mass

Differentiate between mass and weight.

Mass

Weight

Differences

What is friction? Explain its advantages and disadvantages and the methods of reducing it.

Definition

Cause

Advantages (friction is useful)

Disadvantages (friction is a nuisance)

Methods of reducing friction

What is rolling friction? Why is it less than sliding friction? Explain the use of ball bearings.

Rolling friction

Why it is less than sliding friction

Ball bearings

Define impulse. Explain, with daily-life examples, how a longer time of impact reduces the force.

Definition

Why a longer time means a smaller force

Examples

Explain terminal velocity with the example of a skydiver.

Forces on a falling body

Stages of the fall

The rest of Chapter 3

Long Questions from other Physics chapters