Force
Click on the Main Topics to go to the topic
Rest and motion
Speed and Velocity
Uniform velocity
Non-uniform velocity
Acceleration
Equation of motion
Inertia
Relation between Mass and Inertia
Newton’s First Laws of Motion
Newton’s Second Laws of Motion
Newton’s Third Laws of Motion
Balanced and imbalanced force
Force:
When we are traveling from one place to another
place through bus, when the bus stops suddenly or it moves suddenly we feel its
motion. When a ball is thrown upward, it returns back to the earth. We experience
such motion and force regularly and repeatedly in our day to day life. Such
cases are related with speed, acceleration, force, etc. In this chapter we are
going to study about above mentioned topics. Force is simply a push or pull on
an object. It can be defied as an external agency
that changes or tends to change the state of a body.
Rest and Motion
Nobody in this universe stays completely at rest. It
means all the bodies in the universe are in motion. Flying birds, running
vehicles, walking men, etc. are in motion. Our earth is in the state of motion.
All the bodies present in this universe such as constellations, galaxies, sun,
stars, planets, satellites, etc. are always in motion.
When we see a building, it is at rest. Similarly, we
see the flying birds,
Running vehicles, walking people and flowing rivers.
These are in motion. Buildings around us are at rest, it means they don’t
change their position with respect to their surrounding objects. But the moving
objects change their position with respect to their surrounding objects. An
object may be in motion with respect to other objects. An object is in motion
or rest in different situation.
For example; while travelling in a bus, the
passengers in it are at rest with respect to the other passengers in the bus,
but at the same time they feel themselves in motion with respect to the trees,
electric poles, houses, etc. present along the road side.
So, rest and motion are the relative terms. Thus, we
can define motion and rest as follows.
Motion: When an object changes its position with respect to its surrounding objects,
the object is said to be in motion.
Rest: When an object does not change its position with respect to its surrounding objects, then the object is said to be at rest.
Speed and Velocity
The
speed of an object gives information on how fast or slow an object is moving. It
does not tell us about the direction of its motion. So, the total distance covered by an object per unit time in any
direction is called its speed. Its SI unit is metre per second
(m/s).
Speed
=Total distance covered in any direction (s)/Time taken (t)
Speed
is a scalar quantity as it has only magnitude but no direction.
The physical quantity which has magnitude only is called scalar
quantity.
Magnitude is simply the possession of any quantity like length,
mass, time, etc.
The distance covered by a body in a fixed direction
is called displacement.
It
has both magnitude and direction. If a body moves 6 meter towards South, its
displacement is 6 metre. Here, 6 meter is magnitude and South is direction.
The physical quantity which has both magnitude and direction is
called a vector quantity.
Velocity
=Total distance covered in a fixed direction or displacement (d)/Time taken (t)
V
= s/t
(Where,
v= velocity, s= displacement and t= time)
In
SI system, the unit of displacement is metre (m) and the unit of time is second.
So the unit of velocity is metre per second (m/s).
Uniform Velocity
If an object covers equal distance in equal
interval of time in a fixed direction in a straight line, the velocity of the
object is called uniform velocity.
To
become the uniform velocity, the direction of the speed must
have
a fixed direction. If a body is
continuously changing its position or moving in a circular path then it is not
said to be in uniform velocity.
Non-unification Velocity
If an object covers unequal distance in a
fixed direction in each second, it is said to be in non-uniform velocity. In the same way, if it covers an equal distance in
each second in different directions, it is also said to be
in
non-uniform velocity. In this condition, the speed of the body is equal but its
velocity is unequal.
Acceleration
The
velocity of a body may be decreased or increased with time.
If
an object shows unequal velocity in a given interval of time, then the change
in its velocity does not become zero. Such type of motion is called accelerated
motion.
For
example; If you apply more force on the peddle of your bicycle, the velocity of
the bicycle increases and if you apply the brake, its velocity decreases. Here,
the motion of the bicycle is called accelerated motion.
The rate of change in velocity of an
object is called acceleration.
Acceleration
(a) = Change in velocity/Time taken
Or,
Acceleration (a) =Final velocity(v) - Initial velocity(u)/Time taken
(Where,
v = final velocity, u = initial velocity, t = time taken)
The
SI unit of acceleration is m/s2.
· An object has zero acceleration if it moves with the
uniform velocity.
· The negative acceleration is called retardation. If
the velocity of an object goes on decreasing, it is said to have retardation.
Equation of motion
The
relation among displacement (s) initial velocity (u), final velocity (v), acceleration
(a) and time taken (t) is called equation of motion.
We
can derive equation of motion from the definition of average velocity and of
acceleration.
Suppose
a body is moving with an equal acceleration along a straight line, then
Displacement
= s Initial velocity = u
Final
velocity = v Acceleration = a
Time
taken = t
The
relation among a, v, u and t
From
the definition of acceleration,
Acceleration
(a) =Final velocity - Initial velocity/Time taken
Or,
a =(v – u)/t
Or,
at = v - u
Or
v = u + at ……………(i)
The relation among s, u, v and t
Average
velocity =
(Initial
velocity + Final velocity)/2
The
total distance covered by an object = Average velocity Time taken
Total
distance covered (s) = (Initial velocity + Final velocity)/2 x time
The relation among u, v, a and t
The
total distance covered by an object = Average velocity Time taken
Things to remember
1.
If a body is starting from rest, its initial velocity becomes zero. (u = 0)
2.
If a body finally comes at rest after covering certain distance, the final
velocity
becomes zero.(v = 0)
3.
If a body is retarding, the acceleration has negative value.
Inertia
All bodies in the universe tries to be at their
position. A body at rest tries to be at
rest and a body in motion tries to be in motion in the same direction with the
same velocity. Position of a body changes only when external force is applied
on the body. In this way, the tendency of a body to
maintain its state of rest or uniform motion in a straight line is called
inertia.
Types of Inertia
There
are two types of inertia. They are:
(A) Inertia of motion (B) Inertia of rest
(A) Inertia of motion
The inertia present in a moving object is
called inertia of motion. Due to the inertia
of motion, the moving object continues its motion with the same velocity
towards the same direction.
Examples
o Inertia of motion:
1.
A fan keeps on moving for some time even after the electricity is switched off.
Electric fan runs because of electric power, but it keeps on moving even after
the electric supply is cut off due to the inertia of motion. The fan should be
stopped as soon as the switch is off but it doesn’t stop because of inertia of
motion.
2.
The passengers in a moving bus are thrown ahead when
the bus stops suddenly. When the bus is in motion, the passengers are also in
motion along with the bus but when the bus stops suddenly, the lower part of
the passengers come to the rest but the upper part of the body tries to be in
motion due to inertia of motion. As a result, the upper part of the body is
thrown ahead due to the inertia of motion.
3.
An athlete runs some distance before taking a long
jump. By doing so, the velocity of run acquired by the athletic is added to the
velocity taken by him at the time of jump. Hence, the athlete is able to jump
longer distance.
4.
A person getting down from a moving bus falls in the
direction of motion of the bus. This is because the feet of the person come to
rest on stepping the ground but the upper part of the body continuous to be in
motion due to inertia of motion. Hence, the person falls in the direction of
the motion of the bus.
(B) Inertia of rest
The inertia present in a resting body is
called inertia of rest. Due to inertia of
rest, a resting body tries to be at rest and it does not change its position
unless any external force is applied on it.
1.
Due to inertia of rest, a mango fruit falls down
from the branch while shaking it forcefully. In the beginning, the fruit along
with the branch remains at rest. When we shake the branch, it comes into
motion, but due to inertia of rest, the mango fruit tries to be at rest. As a
result of inertia of rest of the fruit, it detaches from the branch and falls
down.
2.
Due to the inertia of rest, the dust particles of
the clothes fall down when we beat the clothes. When a dusty cloth is beaten
with a stick, dust particles are cleared. This is because on beating, the cloth
comes in motion but the dust particles tries to remain in rest due to inertia
of rest. Hence dust particles falls down under the effect of gravity.
3.
When a bus starts suddenly, the passengers inside
the bus fall backwards. This is because the lower part of the body of the
passengers comes in motion along with the motion of the bus but the upper part
of the body tries to remain at rest due to inertia of rest. Hence they fall
backwards.
Relation between Mass and Inertia
We
can push a bicycle by applying certain force but we can’t push a truck by
applying the same magnitude of force. This is because; the inertia of rest of a
truck is more than that of a bicycle. Form the given example; we can say that a
body having more mass has more inertia and a body having less mass has less
inertia. Look at the video for the relation between mass and inertia.
Momentum
It
is experienced that more force is required for a body having more mass to bring
it into rest. Similarly, more force is required to bring a body moving with
high velocity.
From
the statement, it is clear that the magnitude of motion of a body depends on
its mass and velocity. So, the magnitude of motion of a body is called its
momentum.
The product of mass of a body and its
velocity is called momentum.
Mathematically
Momentum
= mass × velocity
In
the above formula, unit of mass is kg and unit of velocity is m/s. So, SI unit
of momentum is kgm/s. The momentum of a body at rest is zero.
Momentum
of a body depends on the mass and its velocity. A body having more mass and
velocity; has more momentum than the body having less mass and less velocity.
This
is the reason why a cricket ball hurts more than a shuttle cock.
Newton’s Laws of Motion
You
might have heard the name of the famous scientist, Newton, of Britain.
The
laws of motion formulated by him are famous till today. He formulated
three
laws of motion in 1687. Now, let’s discuss these three laws.
Newton’s first law of motion
Newton's
first law of motion states that “everybody
continues to be in its state of rest or uniform motion in a straight line
unless an external force is applied on it.”
This
law is fully related to the law of inertia. It means that, force is required to
change the inertia of rest of a body into inertia of motion and inertia of motion
of a body into inertia of rest. Hence, Newton’s first law of motion is also
called law of inertia. This law also gives the definition of force.
Newton's second law of motion
Newton's
second law of motion states that “acceleration
produced on a body is directly proportional to the force applied on it and
inversely proportional to its mass”
Mathematically,
Acceleration
(a) Force (F) a?F………….(i) (If mass of the body is kept constant)
Acceleration
(a)a?1/m …………….(ii) (If applied force is kept constant)
Combining
both equations,
a?F/m
Or,
F ? ma ……………….(iii)
F
= kma (Here, k is a constant)
In
equation (iii) if a = 1 m/s2, m = 1 kg, the value of F becomes 1N.
So, in this condition the value of k also becomes 1. Therefore;
F = ma
The force which produces 1m/s2 acceleration in a
body of mass 1 kg is called 1 Newton.
Newton's third law of motion
Newton's
third law of motion states that “every
action has equal but opposite reaction”
The third law of motion describes the features of force. It also proves that forces act in pair. According to this law, every action has equal but opposite reaction.
Some
Examples:
· While launching a rocket into the space, the hot
gases produced by the rapid burning of fuel rush downward but the rocket goes
upward.
· While firing a gun, bullet moves in the forward
direction but the gun recoils in the backward direction.
· While rowing a boat, water is pulled in backward
direction but the boat moves forward.
· While opening an air filled balloon turning its
mouth downward, the air comes down but the balloon moves upward.
· Birds while flying push the air downwards with their
wings. Due to this action air exert equal and opposite force to the wings and
hence lift them upwards.
· We are able to walk on the ground due to the
opposite reaction of the ground on our feet.
Balanced and Imbalanced Forces
If resultant of the applied forces on an object is
zero, such forces are called balanced forces. Balanced forces can’t bring a
body into motion.
For example; in the game tug of war, the rope
remains steady if both the team apply equal force to the opposite direction.
If resultant of the applied forces on an object does
not become zero, such forces are called imbalanced forces. Imbalanced forces
bring an object into motion. While pushing a bicycle, frictional force creates
obstruction to bring it into motion but our force is more than the frictional
force and hence motion is produced in the bicycle. At this time, the resultant
of the applied forces in the bicycle does not become zero.
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