Unit 3

Force

A force is a push or a pull that starts, stops or changes the magnitude and direction of velocity of a body.

Contact force

A contact force is a force that is exerted by one object on the other at the point of contact.

Friction

Friction: It is the force that resists motion when the surface of one object comes in contact with the surface of another.

Drag

The drag force is the resistant force caused by the motion of a body through a fluid. It acts opposite to the relative motion of any object moving with respect to surrounding fluid.

Thrust

It is an upward force exerted by a liquid on an object immersed in it.

Normal force

It is the force of reaction exerted by the surface on an object lying on it. This force acts outward and perpendicular to the surface.

Air resistance

It is the resistance (opposition) offered by air when an object falls through it.

Tension force

It is the force experienced by a rope when a person or load pulls it.

Elastic force

It is a force that brings certain materials back to their original shape after being deformed.

Non-contact force

A non-contact force is defined as the force between two objects which are not in physical contact.

Gravitational force

The gravitational force is an attractive force that exists among all bodies which have mass.

Gravitational force formula

[ F = G \frac{m_1 m_2}{r^2} ] where ( m_1 ) and ( m_2 ) are two masses distant ( r ) apart and ( G ) is constant of gravitation. Its value is ( 6.67 \times 10^{-11} ) N m² kg⁻².

Electrostatic force

An electrostatic force is a force that acts between two charged objects. The opposite charges attract each other and similar charges repel each other.

Magnetic force

It is a force which a magnet exerts on other magnets and magnetic materials like iron, nickel and cobalt.

Newton's first law of motion

"A body continues its state of rest or of uniform motion in a straight line unless acted upon by some external force."

Force (Newton's first law)

Force is an agency which changes or tends to change the state of rest or of uniform motion of a body.

Inertia

The property of a body to maintain its state of rest or of uniform motion in a straight line is called inertia.

Newton's second law of motion

"If a net external force acts upon a body, it accelerates the body in the direction of force. The magnitude of acceleration is directly proportional to the magnitude of force and is inversely proportional to the mass of the body."

Newton's second law (mathematical form)

[ a = \frac{F}{m} ] or [ F = ma ]

1 Newton

One newton is the force which produces an acceleration of 1 ms⁻² in a body of mass 1 kg. [ 1 \text{ N} = 1 \text{ kg ms}^{-2} ]

Newton's third law of motion

For every action, there is always an equal and opposite reaction.

Mass and weight (difference)

Mass: The amount of matter in a substance is called the mass. It is a scalar quantity. Unit: kg. Measured by physical balance. Weight: The weight of a body on the earth is the force with which the earth pulls the body towards itself. It is a vector quantity. Unit: N. Measured by spring balance.

Gravitational field

The gravitational field is a space around a mass in which another mass experiences a force due to gravitational attraction.

Gravitational field strength

The gravitational field strength is defined as the gravitational force acting on unit mass. Its value is 10 N kg⁻¹.

Friction (definition)

Friction is the force that tends to prevent the bodies from sliding over each other.

Dissipation of energy during friction

Friction is a dissipative force due to which the energy is wasted in doing work to overcome against friction. The lost energy appears in the form of heat.

Sliding friction

The friction between two solid surfaces is called sliding friction.

Static friction

Static friction is the force of friction arising due to applied external force before motion of one body over the other.

Kinetic friction

Kinetic friction is the force of friction arising due to applied external force after motion of one body over the other.

Terminal velocity

When upward air resistance balances the downward force of gravity on a falling object, it falls down with constant (safe) velocity, it is called terminal velocity.

Rolling friction

When an object rolls over a surface, the friction produced is called rolling friction.

Momentum

The product of mass and velocity of a moving body is called momentum. [ p = m \times v ] Unit: kg ms⁻¹ or Ns.

Impulse

It is the product of force acting on a body for a very short interval of time t. [ \text{Impulse} = F \times \Delta t ] Unit: Ns.

Principle of conservation of momentum

If no external force acts on an isolated system, the final total momentum of the system is equal to the initial total momentum of the system. [ m_1 v_1 + m_2 v_2 = m_1 v_1' + m_2 v_2' ]