5 Pius Augustine Newton's Laws of Motion

Force - Push or Pull
Contact Force – eg. Friction, Spring
Non contact – electric, magnetc, gravity.
Non contact force – also known as
action at distance.
Non contact force – field force
Dr. Pius Augustine, S H College, Kochi

Effects of Force
1. Set a stationary body into motion
2. Stop moving body
3. Change speed or direction of motion
of a moving body
4. Bring about change in dimension
Give examples for each

Force
Qualitative definition
Physical cause which
changes or tends to change
the state of rest or uniform
motion of a body along a
straight line.

Force:
Quantitative definition
Force is an agent whose
action can produce
acceleration in a body. It is
represented as the product of
mass and acceleration.
F = ma

External and internal force
External – force is coming from
another frame.
Eg. Pushing auto from road.
Internal – from same frame or
within.
Eg. Pushing auto from inside the
auto rickshaw.

Balanced and unbalanced force
Balanced – resultant force is zero
,cannot change state of rest of
motion.
Eg. Book on a table – wt is
balanced by reaction.
Unbalanced –net force ≠ 0
Eg. Cart is pulled.Dr. Pius Augustine, S H College, Kochi

Newton’s first law. or Law of inertia
Every body continues in its
state of rest or uniform
motion along a straight line
until and unless compelled
by an external unbalanced
force to change its state .

Why does a person fall when he jumps out from
a moving train?
Feet suddenly comes to rest on
touching the ground , but upper
part of his body has a tendency
to continue motion due to
inertia of motion.

Coin falls into tumbler when card below is
suddenly flicked off
Due to inertia of rest coin
continue in its state of rest.
As the card is flicked off,
coin falls into the tumbler.

Ball thrown upward in a train moving with
constant velocity come back to thrower’s
hand
Due to inertia of motion .
Body is in motion along
with the train due to
inertia of motion .

Corridor train suddenly starts doors move?
Due to inertia of rest , door
continue in its state of rest
and hence moves
backward and getting
opened when train
suddenly starts.

A force of 8N givesa mass m1, an accelerationof 8m/s2
and a mass m2, an accelerationof 24 m/s2. What
accelerationwouldit give, if both the massesare tied
together?
M1 = F/a1 = 8/8 = 1kg
M2 = F/a2 = 8/24 = 4/3kg
Total mass = M1 + M2 =

Greater the mass greater is the inertia
Better to run in zig zag path to
escape from elephant. Comment.
Heavy truck and a car moving
with same velocity.Difficult to
stop truck compared to car due
to large mass and inertia.

More the mass more difficult it is to
move the body from rest.
Pushing a bike and a car .
Difficult to make the car
move.

Explain, why a cart pulled by a horse
moves, when action is always equal to
reaction.
It is because the component of
reaction which horse gets
from the ground which is an
external unbalanced force .

Explain why some of the leaves may fall
from a tree, if we vigorously shake its
branch.
When the branch is suddenly set in
motion, the leaves attached to it
tend to continue in their state of
rest, on account of inertia of rest.
Hence the leaves get detached
from the tree.

The weight of a man on the earth is 100
kg. Does this weight on the moon
increase or decrease?
As the acceleration due to gravity
on the moon is 1/6th that on the
earth, weight will decrease on
the surface of moon.

State Newton’s second law of motion?
The rate of change of momentum
is directly proportional to the
applied force and the change
takes place in the direction of
the applied force.

F = ma
It is a fundamental law in Physics
According to 2nd law,
F = K (P2-P1)/t
= K(mv – mu)/t
F = ma
K= 1

Define momentum. SI unit ??
Total quantity of motion
contained in a body is
called momentum.
SI unit is Ns or kgm/s

Name the two factors on
which the force needed to
stop a moving body in a
given time depends?
Mass of the body .
Velocity of the body.

An electron of mass 9 x 10-31 kg is
moving with a linear velocity of 6 x 107
m/s. Calculate linear momentum of
electron?
P = mv
= 9 x 10-31 x 6 x 107
= 54 x 10-24 Ns

A motor cycle of mass 100 kg is
running at 10 m/s. If its engine
develops an extra linear momentum of
2000 Ns, calculate the new velocity of
motorcycle.
Pinitial = mv
= 100 x 10 = 1000 m/s
Pfinal = 1000 + 2000 = 3000 m/s
New velocity = 3000/mass = 30 m/s

Is force a scalar or a vector?
Vector

State and define SI unit of force.
Newton
One N is that force which
when applied on a mass of
1kg produces in it an
acceleration of 1m/s2 in the
direction of the force.

State Newton’s second law of motion? Derive an
expression for force?
Rate of change of momentum of a
body is directly proportional to
the applied force and the change
takes place in the direction of
the applied force.
F = K (mv – mu)/t = ma
Constant K = 1(by suitably defining 1N force)

Give absolute and gravitational
units of force? Which is
universal?
N and dyne
They are absolute units which
are universal
Kgf and gf are gravitational
It vary from place to place.

Absolute unit of fore:
A force which produce unit
acceleration on unit mass.
SI system – Newton.
CGS system – Dyne

One newton (1N = 1 kgm/s2) is that
force which produce 1 m/s2
acceleration on a body of mass 1 kg.
One dyne (1dyne cm/s2) is that force
which produce an acceleration 1
cm/s2 on a body of mass 1 g.
1N = 1 kgm/s2 = 1000 g x 100 cm/s2
= 105 gm/s2 = 105 dyne

Gravitational unit of fore:
A force which produce an
acceleration equal to acceleration
due to gravity of on unit mass.
SI system – kgf.
CGS system – gf

One kgf is that force which produce
9.8 m/s2 acceleration on a body of
mass 1 kg.
1 kgf = 1 kg x 9.8 m/s2 = 9.8 kgm/s2
1 kgf = 9.8 N
One gf is that force which produce an
acceleration 980 cm/s2 on a body of
mass 1 g.
1 gf = 1 g x 980 cm/s2 = 980 gcm/s2
1 gf = 980 dyne Dr. Pius Augustine, S H College, Kochi

Impulse and impulsive force.
Large force acting for a short interval of
time is known as impulse.
Force is impulsive force and effect is
impulse.
It is measured as the product of F and
time.
Impulse = Ft = mv – mu = change in
momentum . This is known as impulse
momentum principle.Dr. Pius Augustine, S H College, Kochi

A car initially at rest, picks up a velocity of 72
km/h in 20 s. If the mass of the car is 1000 kg,
find (i) force developed by its engine (ii)
distance covered by the car.
u = 0 v = 72 x1000/3600 = 20 m/s
F = m(v-u)/t = 20000/20 = 1000N
a = F/m = 1000/1000 = 1m/s2.
S = ½ at2 = ½ x 1 x 400 = 200m.

A bullet leaves a fine hole on a glass of a
window, while smashed irregularly
otherwise.
Bullet exerts impulsive force.
Since time of interaction between
bullet and glass is very small, bullet
passes through the glass without
transferring the impact to other
parts of the window.

Obtain 1st law from 2nd law?
According to 2nd law,
F = ma =(mv – mu)/t
If F = 0 , 0 = m ( v – u)/ t
m ≠ 0 t ≠ 0 , ie. v-u = 0
Or v = u which is 1st law.
If body is at rest initially, it will
continue rest; or same velocity.

Mention the condition under
which F = ma hold good?
Mass of the body is constant
or velocity of the moving
body is much less than
velocity of light or less than
106 m/s.

How does Newton’s second law of
motion differ from first law?
First law – qualitative
definition for force.
Second law – quantitative
measurement of force.

Draw a graph showing variation of
acceleration with mass under a constant
force?
Hint – Inversely related.
Give few values (Say F = 10 N)
m and a can take values
1 and 10, 2 and 5, 4 and 2.5, 5 and 2, 10 and 1 etc.
Plot graph

How can a person standing in the middle of
a perfectly smooth island of ice, get to a
corner?
Throwing something
opposite to the
direction in which he
wants to move. (3 law)

Two similar vehicles are moving with
same velocity on the road, such that
one of them is loaded and the other one
is empty. Which of the two vehicles will
require larger force to stop it in a given
distance?
Large force for loaded vehicle.
Greater momentum should be
reduced to zero.

When a carpet is beaten with a stick, dust
comes out. Explain.
inertia

What happens when you shake a wet piece of
cloth? Explain.
Cloth is shaken, it is suddenly set
into motion, but loose water in
it, on account of inertia of rest
continues in its state of rest and
detached .

A javelinthrow is markedfoul if an athlete crossesover
the line marked for throw. Explainwhy oftenfails to
stop?
Inertia of motion.
He builds up momentum, which is
helpful in throwing javelin a longer
distance.
Momentum acquired by the body
prevents him from stopping
immediately.

How much momentum will a dumbbell
of mass 10kg transfer to the floor, if it
falls from a height of 0.8m? g = 10
N/Kg.
V2 – u2 = 2gS
V = 4m/s
Momentum of dumbbell = mv =
40 kgm/s

A force of 2oo N acts on a body for 5s. It
gives the body a velocity of 50 m/s. Find the
mass of the body.
Given F = 200N t = 5s u = 0 v = 50 m/s
v = u + at
50 = 0 +a 5
solving, a = 10 m/s 2.
F = ma , m = F/a = 20 kg.

What is weightlessness?
It is a state when objects do not
weigh anything.
This occurs when bodies are in a
state of free fall under the effect of
gravity.
Or when weight is used for
centripetal force.

If someone jumps to the shore from a boat, the
boat moves in the opposite direction. Explain.
3 law of motion .

State the purpose of Newton’s third law of
motion.
1st law – qualitative definition for force.
2nd law – quantitative measurement .
But both these are silent about how does
force act on a body.
It is third law tells that force always acts
in pairs. - action and reaction-equal
and opposite act on two different
bodies.

Identify action reaction pairs
Firing bullet from gun : gun spring
exerts action on bullet and bullet
exerts reaction on gun.
Hammering a nail – force exerted by
hammer on nail is action and nail on
hammer is reaction

Moving rocket – force exerted by rocket
on ejected gas is action and gas on
rocket is reaction .
Walking – force exerted by the person
against the ground backward is action
and ground exerts forward force on the
person ( reaction )

Book on table – wt of book on table
is action and table on book is
reaction .
Moving train colliding stationary
train – force exerted by moving
train on stationary train is action

When bullet is fired from a gun, the
gun recoils. Why?
On triggering , Gun spring exerts
force (action) on bullet and
bullet exerts reaction on gun due
to which gun recoils.Dr. Pius Augustine, S H College, Kochi

Discuss an experiment to show that action and
reaction are equal?
Take two spring balances.
Lock their hooks.
Care fully pull both the spring balances
Note readings indicated
Both of them register same reading – one is
action and other reaction .

To move a boat ahead in water, boatman
has to push the water backwards by his oar.
Explain
To make the boat move forward , an
external force is required, for which ,
he pushes the water backward (action)
which in turn provides forward
reaction. Dr. Pius Augustine, S H College, Kochi

Though action and reaction are equal
and opposite, they never cancell each
other. Why?
Because, they act on two
different bodies.

A person pushing a wall hard is liable to fall
back. Give reason.
As applied force (action) by the
person on the wall increases ,
reaction on him by the wall also
increases according to N 3rd LM
and may fall back .Dr. Pius Augustine, S H College, Kochi

How can a person standing in the
middle of a perfectly smooth island
of ice, get to a corner ?
Throwing something opposite
to the direction in which he
wants to move. (3 law)

Mass
Measure of quantity of
matter contained in
the body at rest.
Scalar
SI unit is Kg
Measured by physical
balance
Constant , doesnot vary
from place to place.
Weight
Force with which earth
attracts the body
Vector directed to
centre of earth.
SI unit is N
Spring balance
calibrated in N.
( physical balance give
wt in kgf)
Not constant , vary
according to the value
of g.

What do you understand by 1kgf = 1.6 N
It tells weight of a body of mass
1kg at a place where value of
acceleration due to gravity = 1.6
m/s2.( not on earth’s surface)

5 Pius Augustine Newton's Laws of Motion

5 Pius Augustine Newton's Laws of Motion

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5 Pius Augustine Newton's Laws of Motion