Study guide H C Verma JEE Physics Ch.4 THE FORCES

The study guides provides only the main issues covered in the chapter to provide some introduction to the chapter. Some past JEE questions are provided to link the issues in the chapter to questions.

One has to read the chapter fully. Study all solved examples and solve exercise problems.

The points may be used as revision points later.
------------------------------


No topic in JEE syllabus. But study of the chapter is necessary as there are full chapters on each of these topics later on and this introduction is essential to understand the in depth chapters.

-----------------

Sections

4.1 Introduction
4.2 Gravitational forces
4.3 Electromagnetic (EM) forces
4.4 Nuclear Forces
4.5 Weak forces
4.6 Scope of Classical physics

---------

Study Plan

Day 1

4.1 Introduction
4.2 Gravitational forces

Session important points

http://iit-jee-physics.blogspot.com/2009/05/physics-study-diary-ch-4-forces-day-1.html


Day 2
4.3 Electromagnetic (EM) forces
Ex. 4.1

Session - important points
http://iit-jee-physics.blogspot.com/2009/05/iit-jee-physics-study-diary-ch3-forces.html

Day 3

4.4 Nuclear Forces
4.5 Weak forces
4.6 Scope of Classical physics


Session important points
http://iit-jee-physics.blogspot.com/2009/05/iit-jee-2011-physics-study-diary-ch4.html

Day 4
Chapter 4 Forces
Worked out examples 1 to 3
Exercises 1 to 7

Day 5
Chapter 4 Forces
Exercises 8 top 12
Objective I and II

Day 6
Chapter 4 Forces
Questions for short answer 1 to 10
Concept Review
Formula Review

Day 7 to 10
Chapter 4 Forces Revision

Day 11 to  20
Revision of all the four chapters completed so far.

Don't waste the time available. Always use the time to revision the concepts, formulas, difficult problems and to do test paper questions. You have to sqeeze the time every day and use it in a focused way.

Master each chapter as early as possible. That will give the energy to study one more chapter. Always feel happy after you do the planned work for a day. That will relax you. The feeling of happiness after you complete the day's task will relax you and make you feel energetic to eat and sleep and if some time is there to see the TV or read a paper or talk to your parents. It is important to focus, put in the required effort and then feel happy and relax every day.









Concepts covered

Force is an interaction between two objects. Force is exerted by an object A on another object B
Force is a vector quantity.

If more than one force act on a particle, we can find the resultant force using the laws of vector addition.

Gravitational forces

Any two bodies attract each other by virtue of their masses.

The force of attraction between two point masses is

F = G m1m2/r^2

where m1 and m2 are the masses of the particles and r is the distance between them.

G is a universal constant having the value 6.67 x 10^-11 N-m²/kg²

Gravitational force on small bodies by the earth

For earth, the value of radius R and mass M are 6400 km and 6 x 10^24 kg respectively.

The quantity GM/r² is a constant and has the dimensions of acceleration. It is called acceleration due to gravity and is denoted by letter g.

g and G are different.


Electro magnetic force:

Apart from gravitational force between any two bodies, the particles may exert upon each other electromagnetic forces.

If two particles hving charges q1 and q2 are at rest with respect to the observer, the force between them has a magnitude

F = (1/4πε0)(q1q2/r^2)

Where ε0 = permittivity of air or vacuum = 8.8549 x 10^-12 C² /N-m²
The quantity (1/4πε0) = 9.0 x 10^9 N-m² /C²

q1, q2 = charges
r distance between q1 and q2

This is called coulomb force and it acts along the line joining the particles.

Force due to a spring

When a metallic wire is coiled it becomes a spring.

when a spring is stretched, it pulls the bodies attached to its ends and when compressed, it pushes the bodies attached to its ends. the force exerted by thes spring is proportional to the change in its length.

If the spring has natural length of x0 and if it is changed to x, the magnitude of force exerted by the spring will be

f = k|x-x0| = k|Δx|

If the spring is extended, the force will be directed towards its centre and it compressed, it will be directed away from its centre.

Nuclear forces

The alpha particles is a bare nucleus of Helium. It contains two protons and two neutrons. It is a stable object and once created it can remain intact until it is not made to interact with other objects.

The protons in the nucleus will repel each other due to coulomb force and try to break the nucleus. Why does the Coulomb force fail to break the nucleus.

There are forces called nucluear forces and they are exerted only if the interacting particles are protons or neutrons or both. They are largely atractive, but with a short range. They are weaker than the Coulomb force if the separation between particles is more than 10^-14 m. For separation smaller than this the nuclear force is stronger than the Coulomb force and it holds the nucleus stable.

Radioactivity, nuclear energy (fission, fusion) etc. result from nuclear force.

Weak Forces

A neutron can change into proton and simulataneously emit an electron and a particle called antinutrino.

a proton can also change into neutron and simulataneously emit a positron (and a neutrino). The forces responsible for these changes are called weak forces. The effect of this force is experienced inside protons and neutrons only.


Scope of classical physics

Physics based on Newton's Laws of motion, Newton's law of gravitation, Maxwell's electromagetism, laws of thermodynamics and the Lorentz force is called classical physics. The behaviour of all the bodies of linear sizes greater than 10^-6 m are adequately described by classical physics. Grains of sands and rain drops fall into this range as well as heavenly bodies.

But sub atomic particles like atoms, nuclei, and electrons have sizes smaller than 10^-6 m and they are explained by quantum physics.

The mechanics of particles moving at velocity equal to light are explained by relativistic mechanics formulated by Einstein in 1905.



http://plasma4.sr.unh.edu/ng/phys407/phys407-6-22-05.pdf

Updated on 30 April 2017, 23 October 2007

4. The forces - Revision Facilitator

4. The forces

Sections

4.1 Introduction
4.2 Gravitational forces
4.3 Electromagnetic (EM) forces
4.4 Nuclear Forces
4.5 Weak forces
4.6 Scope of Classical physics


Mind Map

Forces - Gravitational forces - Electromagnetic (EM) forces - Nuclear Forces - Weak forces - Scope of Classical physics

Gravitational forces - G (universal constant 6.67 *106-11 N-m^2/kg^2) – Acceleration due to gravity g = GM/R^2) - Spherical body treated as a point mass at their centres

Electromagnetic (EM) forces – Coulomb forces – Forces between two surfaces in contact – Tension in a string or rope – Force due to a spring

Nuclear Forces – Exerted when interacting particles are protons or neutrons

Weak forces – Forces responsible for beta decay – antinutrino - positron

Scope of Classical physics – Applicable to bodies of linear sizes greater than 10^-6 m – Subatomic bodies – Quantum physics applicable – If the velocity of bodies are comparable to 3*10^* m/s relativistic mechanics is applicable.

Concept Review - Chapter 4 The Forces

Force

Force is an interaction between two objects.
Force is exerted by an object A on another object B.
Force is a vector quantity. Hence if two or more forces act on a particle, we can find the resultant force using laws of vector addition.

The SI unit for measuring the force is called a newton.

Newton's third law of motion

If a body A exerts a force F on another body B, then B exerts a force -F on A,the two forces acting along the same line.


Gravitational force

Any two bodies attract each other by virtue of their masses.

The force of attraction between two point masses is

F = Gm1m2/r²
where m1 and m2 are the masses of the particles and r is the distance between them.

G is a universal constant having the value 6.67 x 10^-11 N-m²/kg²

The above rule was given for point masses. But it is analytically found that the gravitational force exerted by a spherically symmetric body of mass m1 on another such body of mass m2 kept outside the first body is Gm1m2/r² where r is the distance between teh centres of such bodies.

Thus, for the calculation of gravitational force between two spherically symmetric bodies, they can be treated as point masses placed at their centres.

Gravitational force on small bodies by the earth

For earth, the value of radius R and mass M are 6400 km and 6 x 10^24 kg respectively. Hence, the force exerted by earth on a particle of mass m kept at its surface is, F = GMm/R². The direction of this force is towards the centre of the earth.

The quantity GM/R² is a constant and has the dimensions of acceleration.
It is called acceleration due to gravity and is denoted by letter g.
Hence, g and G are different.
Value of g is approximately 9.8 m/s².
In calculations, we often use 10 m/s².

Now we know that force exerted on a small body of mass m, kept near the earth's surface is mg in the vertically downward direction.

Gravitational constant is so small that the gravitational force becomes appreciable only when one of the masses has a very large mass.

HC Verma gives the example of Force exerted by a body of 10 kg on another body of 10 kg when they are separted by a distance of 0.5 m. The force comes out to be 2.7*10^-8 N which can hold only 3 microgram. Such forces can be neglected in practice.
Hence we consider only gravitational force exerted by earth



Electromagnetic force

Apart from gravitational force between any two bodies, the particles may exert upon each other electromagnetic forces.

If two particles having charges q1 and q2 are at rest with respect to the observer, the force between them has a magnitude

F = (1/4πε0)(q1q2/r^2)

Where ε0 = permittivity of air or vacuum = 8.8549 x 10^-12 C² /N-m²
The quantity (1/4πε0) = 9.0 x 10^9 N-m² /C²

q1, q2 = charges
r distance between q1 and q2

This is called coulomb force and it acts along the line joining the particles.

Atoms are composed of electrons, protons and neutrons.

Each electron has 1.6*10^-19 coulomb of negative charge. Each proton has an equal amount of positive charge.

In atoms, the electrons are bound by the electromagnetic force exerted on them by charge on protons. Even the combination of atoms in molecules are brought about by electromagnetic forces only. A lot of atomic and molecular phenomena result from electromagnetic forces between subatomic particles (for example, theory is put forward that charged mesons are responsible for the stability of nucleus).

Examples of electromagnetic force:

1. Bodies in contact: The contact force between bodies in contact arises out of electromagnetic forces acting between the atoms and molecules of the surfaces of the two bodies. The contact force may have a components parallel to the contact surface. This component is known as friction.

2. Tension in a string: Tension in the string is due to electromagnetic forces between atoms or electrons and protons (free electrons and nucleus in metals).

3. Force due to spring: If a spring has natural length x0 and if it is extended to x, it will exert a force

F = k|x-x0| = k|∆x|

k, the proportionality constant is called the spring constant. This force comes into picture due to the electromagnetic forces between the atoms of the material.



Nuclear forces

The alpha particles is a bare nucleus of Helium. It contains two protons and two neutrons. It is a stable object and once created it can remain intact until it is not made to interact with other objects.

The protons in the nucleus will repel each other due to coulomb force and try to break the nucleus. Why does the Coulomb force fail to break the nucleus.

There are forces called nucluear forces and they are exerted only if the interacting particles are protons or neutrons or both. They are largely atractive, but with a short range. They are weaker than the Coulomb force if the separation between particles is more than 10^-14 m. For separation smaller than this the nuclear force is stronger than the Coulomb force and it holds the nucleus stable.

Radioactivity, nuclear energy (fission, fusion) etc. result from nuclear force.



Weak forces

A neutron can change into proton and simulataneously emit an electron and a particle called antinutrino.

a proton can also change into neutron and simulataneously emit a positron (and a neutrino). The forces responsible for these changes are called weak forces. The effect of this force is experienced inside protons and neutrons only.


Scope of classical physics

Physics based on Newton's Laws of motion, Newton's law of gravitation, Maxwell's electromagetism, laws of thermodynamics and the Lorentz force is called classical physics. The behaviour of all the bodies of linear sizes greater than 10^-6 m are adequately described by classical physics. Grains of sands and rain drops fall into this range as well as heavenly bodies.

But sub atomic particles like atoms, nuclei, and electrons have sizes smaller than 10^-6 m and they are explained by quantum physics.

The mechanics of particles moving at velocity equal to light are explained by relativistic mechanics formulated by Einstein in 1905.

X-revision Force

What is a force?

A force is that physical cause which changes or tends to change the state of rest or the state of motion of a body.

Newton’s laws of motion

First law – a body continues to be in its state of rest or of uniform motion in a straight line, unless an external force is applied on it.

Second law - the rate of change of momentum of a body is directly proportional to the force applied on it and this change takes place in the direction of force.

Third law – to every action, there is always an equal and opposite reaction.

What is linear momentum?
The linear momentum of a body is the product of the mass of the body and its linear velocity. It is a vector quantity

One dyne is that force which when acting on a body of mass 1 gramme, produces an acceleration of 1 cm s^-2(to the power of -2) in it.

One Newton is that force which when acting on a body of mass 1 kg, produces an acceleration of 1 m s^-2 in it.

The Newton and dyne are called the absolute units of force.

What is mass?
The mass of a body is the quantity of matter it contains.

The S.I. Unit of mass is kilogramme (kg).
The C.G.S. unit of mass is gramme (g).

What is weight of a body?
The force with which the earth attracts a body, is called the weight of the body on earth.