Instructive Classes (Force)
Instructive Classes (Force)
Instructive Classes (Force)
In previous chapter motion class 9 we already learned about motion and variables of motion. You
must have noticed that in that chapter we only analyzed motion using variables like speed, velocity,
acceleration etc. Here in this i.e., force and laws of motion we will get to know what causes motion.
In this page, find easy to learn and understand ,class 9 science, chapter 9 - Force and laws of
motion notes.
Introduction
○ While studying kinematics, we have already studied about the position, distance and
displacement, and acceleration of a moving particle.
○ Here in this chapter, we would take our understanding one step further to learn about origins
of acceleration or force.
○ Here we will specifically consider the cause behind the moving objects i.e. what causes the
objects to move.
○ Thus we will learn the theory of motion based on the ideas of mass and force and the laws
connecting these physical concepts to the kinematics quantities.
○ Concept of force is central to all of physics whether it is classical physics, nuclear physics,
quantum physics or any other form of physics
○ So what is force? When we push or pull anybody we are said to exert force on the body
○ Push or pull applied on a body does not exactly define the force in general. We can define
force as an influence causing a body at rest or moving with constant velocity to undergo
acceleration
○ There are many ways in which one body can exert force on another body. Few examples are
given below
(a) Stretched springs exerts force on the bodies attached to its ends
(b) Compressed air in a container exerts force on the walls of the container
(c) Force can be used to deform a flexible object
○ Force of gravitational attraction exerted by earth is a kind of force that acts on every physical
body on the earth and is called the weight of the body
○ There are two types of forces namely balanced forces and unbalanced forces
○ If the resultant of all forces acting on a body is zero then the forces are called balanced
forces.
○ To understand this concept consider whether an object rests on a surface such as a block on
the table as shown below in figure 1. Weight of the block is balanced by the reaction force
from the table. The table pushes up against the block.
So the weight of a block lying on a table is balanced by the reaction force from the table top
○ If the forces on an object are balanced (or if there are no forces acting on it) then the object
that is not moving stays still and the object that is moving continues to move at the same
speed and in the same direction.
○ Although balanced forces cannot produce motion in a stationary body or stop a moving body
they can however change the shape of the body.
○ If the resultant forces acting on a body is not zero the forces are called unbalanced forces.
○ To understand this consider the figure 2 which shows a block on a horizontal table and two
strings X and Y are tied to the two opposite faces of the block.
○ Let us now pull the block using this string in two different directions such that two opposite
forces of different magnitudes act on the block.
○ Since two forces acting on the block are of different magnitude the block would begin to
move in the direction of the greater force.
○ Thus, the two forces acting on the block are not balanced and the unbalanced force acts in
the direction the block moves.
○ So unbalanced forces can move a stationary body and they can stop a moving body.
○ The size of the overall force acting on an object is called the resultant force. If the forces are
balanced, this is zero. In the example above, the resultant force is the difference between the
two forces F1 and F2, which is 120 - 60 = 60 N.
○ If all the forces acting on a body result in an unbalanced force, then the unbalanced force
can accelerate the body. It means that a net force or resultant force acting on a body can
either change the magnitude of its velocity or change the direction of its velocity.
○ The force that opposes the relative motion between the surfaces of two objects in contact
and acts along the surfaces in contact is called the force of friction or simply friction.
Laws of motion
○ Newton gave three laws of motion that describe the motion of bodies. These laws are known
as Newton's Laws of motion.
○ They describe the relationship between the forces acting on a body and its motion due to
those forces.
○ The three laws of motion were first compiled by Sir Isaac Newton in his work Principia
Mathematica, first published in 1687. Newton used these laws to explain and investigate the
motion of many physical objects and systems.
○ We shall now learn about Newton's First law of motion
Momentum
○ Before discussing the second law of motion we shall first learn about momentum of a moving
object.
○ From our daily life experiences like during the game of table tennis if the ball hits a player it
does not hurt him. On the other hand, when a fast moving cricket ball hits a spectator, it may
hurt him.
○ This suggests that impact produced by moving objects depends on both their mass and
velocity.
○ So, there appears to exist some quantity of importance that combines the object's mass and
its velocity called momentum and was introduced by Newton.
○ Momentum can be defined as "mass in motion". All objects have mass; so if an object is
moving, then it has momentum - it has its mass in motion.
○ The momentum, p of an object is defined as the product of its mass, m
Momentum has both direction and magnitude so it is a vector quantity. Its direction is the same as
that of velocity, v
○ .
○ The SI unit of momentum is kilogram-meter per second (kg m s-1).
○ Since the application of an unbalanced force brings a change in the velocity of the object, it
is therefore clear that a force also produces a change of momentum.
○ We define the momentum at the start of the time interval is the initial momentum and at the
end of the time interval is the final momentum.
○ When the object moves then it gains momentum as the velocity increases. Hence greater the
velocity greater is the momentum.
○ Newton's second law of motion gives the relationship between the force and acceleration.
○ The second law of motion states that
the rate of change of momentum of an object is proportional to the applied unbalanced force
in the direction of force
○ The rate of change of momentum of an object is proportional to the applied force. So,
Newton's second law of motion can be expressed as
is moving along a straight line with an initial velocity, u. It is uniformly accelerated to velocity, v in
time, t by the application of a constant force, F throughout the time, t
○ .
○ The initial and final momentum of the object will be, p1 = mu and p2 = mv respectively.
○ Now change in momentum would be
(5)
Now, m1 u1+ represents the total momentum of particles A and B
m2u2
before collision and m1v1+m2v2 represents the total momentum of particles after collision. This
means that
○ Thus we conclude that when two particles are subjected only to their mutual interactions ,the
sum of the momentums of the bodies remains constant in time or we can say the total
momentum of the two particles does not change because of any mutual interactions between
them.
○ For any kind of force between two particles then the sum of the momentum ,both before and
after the action of force should be equal i.e total momentum remains constant.
○ Law of conservation of linear momentum is one of the most fundamental and important
principles of mechanics.
○ Once again ,the total momentum of two or any number of particles of interacting particles is
constant if they are isolated from outside influences (or no resultant external forces are
acting on the particles).