Force and Laws of Motion Class 9 Flashcards

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Questions and Answers

What is force?

A force is a push or pull acting on a body which changes or tends to change the state of rest or of uniform motion. It's a vector quantity.

What is the SI/CGS unit of force?

Newton/dyne

What is one newton?

One newton is a force which, when acted upon a body of mass 1 kg, produces in it an acceleration of 1 m/s^2.

1 N is equal to how many dynes?

10^5 dynes Signup and view all the answers

What is a balanced force?

When a number of forces acting simultaneously on a body do not bring about any change in its state of rest or of uniform motion in a straight line. Signup and view all the answers

What is an unbalanced force?

When a number of forces acting simultaneously on a body bring about a change in its state of rest or of uniform motion. Signup and view all the answers

What is Newton's First Law of Motion?

Every body continues to be in its state of rest or of uniform motion in a straight line, unless compelled by some external applied force to change its state. Signup and view all the answers

What is another name for Newton's First Law of Motion?

Law of inertia Signup and view all the answers

What is inertia?

The property of a body by virtue of which it resists any change in its state of rest or of uniform motion in a straight line. Signup and view all the answers

What is the measure of inertia of a body?

Mass Signup and view all the answers

What is inertia of rest?

A body at rest remains at rest and cannot start on its own due to inertia of rest. Signup and view all the answers

What is inertia of motion?

A body in uniform motion can neither get accelerated or decelerated nor can come to rest on its own due to inertia of motion. Signup and view all the answers

What is inertia of direction?

The resistance offered by a body to change its direction of motion. Signup and view all the answers

What is Newton's Second Law of Motion?

The rate of change of momentum of a body is directly proportional to the applied unbalanced force in the direction of force. Signup and view all the answers

What is momentum?

The linear momentum of a body of mass 'm' traveling with a velocity 'v' is defined as the product of its mass and velocity; P = mv. Signup and view all the answers

What is the SI unit of momentum?

kg m/s Signup and view all the answers

What does the equation v^2 - u^2 = 2as represent?

It represents the relationship of velocity with acceleration and displacement. Signup and view all the answers

Frictional force is always?

Negative Signup and view all the answers

Give an application of Newton's Second Law.

An athlete taking a high jump falls onto a cushion bed rather than onto a rough surface. Signup and view all the answers

What are impulsive forces?

Forces which act on the bodies for a short time. Signup and view all the answers

What is impulse?

Impulse = force x time (SI unit = Ns) Signup and view all the answers

What is Newton's III Law?

To every action, there is an equal and opposite reaction where action and reaction forces act on different bodies. Signup and view all the answers

Give an example of Newton's Third Law of Motion.

A gun recoils when a bullet is fired from it. Signup and view all the answers

What is the Law of Conservation of Momentum?

According to this law, in the absence of an external force, the total momentum of a system remains constant. Signup and view all the answers

What is the formula for recoil velocity?

V = -mv/M Signup and view all the answers

When is Newton's III Law applicable?

When action and reaction forces are acting on two different bodies. Signup and view all the answers

Study Notes

Force

Defined as a push or pull acting on a body, it can change the state of rest or uniform motion.
Considered a vector quantity, it has both magnitude and direction.

Units of Force

SI unit is the Newton (N), while the CGS unit is the dyne.
One Newton is the force needed to accelerate a 1 kg mass at 1 m/s².

Force Conversions

1 N = 10^5 dynes.

Balanced and Unbalanced Forces

Balanced forces do not change an object's state of rest or motion.
Unbalanced forces result in a change in the state of an object.

Newton's First Law of Motion

States that an object remains at rest or in uniform motion unless acted upon by an external force.
Also known as the law of inertia.

Inertia

Refers to an object's resistance to change in its state of rest or motion.
Measured by mass, which quantifies the amount of inertia.

Types of Inertia

Inertia of rest: A stationary object stays at rest; e.g., a passenger falling backward when a bus starts.
Inertia of motion: A moving object stays in motion; e.g., a passenger leaning forward when the bus stops.
Inertia of direction: An object's resistance to changing direction; e.g., falling to the left when a bus turns right.

Newton's Second Law of Motion

The rate of change of momentum is directly proportional to the applied unbalanced force.
Momentum (P) is the product of mass (m) and velocity (v); expressed as P = m*v, a vector quantity.

Momentum

SI unit of momentum is kg·m/s.

Friction

Frictional force is always negative, opposing the direction of motion.

Applications of Newton's Second Law

High jump athletes land on cushioning to reduce impact.
Karate experts use focused force to break materials.
Seat belts are advised for car safety to restrain passengers.

Impulsive Forces and Impulse

Impulsive forces act for a short time.
Impulse equals force multiplied by time; SI unit is Newton-seconds (Ns).

Newton's Third Law of Motion

States that for every action, there is an equal and opposite reaction, influencing different bodies.

Examples of Third Law of Motion

A gun recoils upon firing a bullet.
Swimming, walking, and rowing illustrate this law.
Hitting a nail results in discomfort from reaction force.

Law of Conservation of Momentum

In an isolated system, total momentum remains constant without external forces.
Expressed as total initial momentum = total final momentum.

Recoil Velocity

Calculated as V = -mv/M, where V is recoil velocity, M is the mass of the gun, and m is the mass of the bullet.

Applicability of Newton's Third Law

Only valid when action and reaction forces act on different bodies.

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This quiz contains flashcards focusing on key concepts from the 'Force and Laws of Motion' chapter relevant to Class 9. Each card presents a term along with its definition, helping students to grasp the fundamental principles of force, motion, and their measurements. Ideal for quick reviews and test preparations.