Force and Laws
of Motion
Topics to be Covered
Force
Galileo’s Analysis about Motion
Newton’s Laws of motion
Rest and Motion:
An object is at rest if its position relative to its surroundings doesn't
change over time.
An object is in motion if its position does change with time.
FORCE
“Force is defined as a push or pull on the body.”
Force is an external effort that can change an object's state of motion or rest, or
change its shape .
Force is a vector quantity.
Its SI Unit is Newton (N).
Its CGS unit is dyne.
1 N = 100000 dyne
Effects of Forces:
→ Force can make a stationary body move.
e.g.: a football can be set to move by kicking it i.e. by applying a
force.
→ Force can stop a moving body.
e.g., by applying brakes, a running cycle or a running vehicle can be
stopped.
→ Force can change the direction of a moving object.
e.g: by applying force i.e. by moving steering, the direction of a
running vehicle is changed.
→ Force can change the speed of a moving body.
When you push a swing, you apply force to make it go
higher and faster.
→ Force can change the shape and size of an object.
e.g.: by hammering, a block of metal can be turned
into a thin sheet. By hammering, a stone can be
broken into pieces.
Balanced Force
Unbalanced Force
1. When two or more forces act on a body and
produce a net force equal to zero, the forces
are called balanced forces.
1. When two or more forces act on a body and
produce a net force not equal to zero, the
forces are called unbalanced forces.
2. A balanced force does not produce any
change in the state of uniform motion or rest
of the body.
2. An unbalanced force can produce a change
in the state of uniform motion or rest of the
body.
3. A balanced force does not cause a body to
accelerate.
3. An unbalanced force can accelerate a body.
4. In balanced forces, the forces are equal in
magnitude and opposite in direction,
canceling each other out.
4. In unbalanced forces, the forces are
unequal in magnitude, causing the object to
move in the direction of the greater force.
RESULTANT FORCE
The resultant force is the total force acting on an object when multiple forces are combined.
It's also known as the net force. The resultant force has the same effect on an object as the
original system of forces.
950 N
730 N
760 N
500 N
1050 N
Galileo’s Analysis on Motion:
Aristotle's Belief: Natural state of bodies is rest.
Galileo's Opposition: Challenged Aristotle's view.
Galileo's Observation:
Ball rolling down an inclined plane: Speed increases.
Ball rolling up an inclined plane: Speed decreases.
Experiment on Horizontal Plane:
On smooth surface, ball continues to move.
Speed remains constant with no external force or friction.
Conclusion: Bodies naturally oppose changes in their state of rest or motion.
Inertia:
Inertia is defined as a property of matter by which it tries to maintain its
state of rest or of uniform motion along a straight line
Inertia of an object is measured by its mass. Inertia is directly proportional
to the mass. This means inertia increases with increase in mass and
decreases with decrease in mass.
Inertia of Rest:
An object stays at rest, and it remains at rest until an
external force affects it. Example: When a car accelerates,
passengers may feel as though their bodies are moving
backwards. In reality, inertia is making their bodies stay in
place as the car moves forward.
Inertia of Motion:
An object will continue to be in motion until a
force acts on it. Example: A hockey puck will
continue to slide across the ice until acted upon
by an outside force.
Q. Why does a person standing in a bus fall backward when the
bus suddenly starts moving?
Initially, both the bus and the person are at rest. When the bus
suddenly starts moving, the legs of the person move with the
bus, but the upper part of their body tends to remain at rest
due to inertia, causing them to fall backward if they are not
alert.
Q. Why does a person standing in a moving bus fall forward when
the driver suddenly applies the brakes?
When the bus is moving, the person is also in motion along
with it. When the brakes are suddenly applied, the bus slows
down or stops abruptly, but the upper part of the person’s
body tends to remain in motion due to inertia, causing them to
fall forward.
Q. Why are seat belts used in cars and other vehicles?
Seat belts are used to prevent passengers from being
thrown forward in case of sudden braking or an accident.
When the vehicle stops suddenly, passengers tend to
continue moving forward due to inertia, and the seat belt
restrains them and prevents injuries.
Q. Why do we give many jerks to wet clothes before hanging
them to dry?
Jerks cause the water droplets in the clothes to separate
from the fabric. When the clothes are suddenly moved, the
water droplets tend to remain at rest due to inertia and fall
out, reducing the water content in the clothes and helping
them dry faster.
Q. Why does the coin fall into the glass tumbler when the card is
flicked away quickly?
The coin remains in its state of rest due to inertia. When the card
is flicked away quickly, the coin's inertia causes it to stay in place
while the card moves, so the coin falls straight down into the
tumbler.
Q. Why does water spill when you turn around quickly holding a tray
with a water-filled tumbler?
The water tends to stay in its state of rest due to inertia.
When the tray is turned quickly, the water does not follow
the motion of the tray immediately, causing it to spill out.
Q. Why is the head of a hammer tightened by banging the handle
against a hard surface?
When the handle is struck against a surface, the handle comes to
rest, but the head of the hammer, due to inertia, continues
moving downward and tightens onto the handle after repeated
strikes.
Q. Why does only the bottom coin move when a pile of coins on a
carom board is hit with a striker?
When the striker hits the pile of coins, only the bottom coin
moves due to the force applied, while the rest of the coins in the
pile tend to remain at rest due to inertia, causing them to drop
vertically and stay in place.
Q. Between a football and rock of same size which one will have more inertia?
CBSE 2016,2018
Newton’s First Law of Motion
Newton’s first law of motion states that:
A body remains in the state of rest or uniform motion in a straight line unless
and until an external force acts on it.
There are two conditions on which the 1st law of motion is dependent:
Objects at rest: When an object is at rest, velocity (v = 0) and acceleration
(a = 0) are zero. Therefore, the object continues to be at rest.
Objects in motion: When an object is in motion, velocity is not equal to
zero (v ≠ 0), while acceleration (a = 0) is equal to zero. Therefore, the object
will continue to be in motion with constant velocity and in the same
direction.
Newton’s First Law of Motion: Examples
A person standing in a bus falls backward when bus starts moving suddenly.
A person standing in a moving bus falls forward if driver applies brakes suddenly.
Before hanging the wet clothes over laundry line, usually many jerks are
given to the clothes to get them dried quickly.
MOMENTUM
Momentum is the quantity of motion an object has
The product of velocity and mass is called the momentum. Momentum is
denoted by ‘p’.
p=mxv
where, p = momentum, m = mass of the object and v = velocity of the object.
Impulse = (Force × time)
Momentum: Examples
Even a small bullet is able to kill a
person when it is fired from a gun
because of its momentum due to
great velocity.
A person get injured in the case of
hitting by a moving object, such as
stone, pebbles or anything because
of momentum of the object.
A person get injured severely when hit by a moving
vehicle because of momentum of vehicle due to
mass and velocity.
20 m/s
10 m/s
v
v
-1
Q. What is the momentum of a body of mass 5 kg moving with a velocity of 0.20 ms.
CBSE 2016,2019
Q. The mass of a goods lorry is 4000 kg and the mass of goods loaded on it is 20000 kg.
If the lorry is moving with a velocity of 2 m/s what will be its momentum ?
Newton’s Second Law of Motion
The rate of change of momentum of an object is directly proportional
to the applied unbalanced force in the direction of the force.
Newton's second law of motion states that the acceleration of an object is directly
proportional to the net force acting on it and inversely proportional to its mass.
If any object is in the state of rest, then it will remain in rest until a external force is
applied to change its state. Similarly, an object will remain in motion until any
external force is applied over it to change its state.
This means all objects resist to in changing their state.
The state of any object can be changed by applying external forces only.
Q: A person is prone to more serious injuries when falling from a certain height on a
hard concrete floor than on a sandy surface. Explain why.
[CBSE 2011, 2012, 2015]
Ans: When a person falls from a height on a hard concrete floor, he immediately
comes in rest position. It means change in momentum is taking place in an extremely
short time and consequently, force exerted by the floor on the person to destroy its
momentum is extremely large. Hence, chances of more injuries.
When a person falls on a sandy surface, the surface gets compressed downward and
it increases the time of fall. As a result for same change in momentum force exerted
by sandy surface on the person is less and chances of his being hurt are less.
Q. Why does a continuous push over time help to
accelerate a car with a dead battery more effectively
than a sudden push?
Continuous pushing applies a steady force over
time, gradually changing the car's momentum,
which is more effective than a sudden push that
does not provide enough time for momentum
change.
Q. Why does a fast-moving cricket ball hurt a spectator
more than a table tennis ball hitting a player?
A fast-moving cricket ball has greater momentum
due to its higher mass and velocity, which results in
a more forceful impact compared to a table tennis
ball.
Mathematical Expression:
Newton’s second law of motion states that the force exerted by a body is directly proportional to
the rate of change of its momentum.
mass = ‘m’
velocity changes from u to v
time = ‘t’,
force = ‘F’
Newton’s Second Law of Motion
F = ma
S.I. Unit of Force is kg m/² or Newton.
1 Newton: When an acceleration of 1 m/s² is seen in a body of mass 1 kg,
then the force applied on the body is said to be 1 Newton.
Force
Momentum
Force = mass * acceleration (F = ma)
Momentum = mass * velocity (p = mv)
Forces can exist even when the object is
stationary.
Momentum for stationary objects is always
zero.
The direction of the force depends on the
acceleration.
The direction of momentum depends on the
direction of velocity.
Force is inversely proportional to time (F ∝
1/t).
Momentum is directly proportional to time
(p ∝ t).
Force remains constant for constant
acceleration.
Momentum changes with constant
acceleration, but remains constant if
velocity is constant.
Q. What will be the effect on force if:
a) mass of the body is doubled
b) acceleration of the body is halved
Q. Why does a fielder pull his hands backwards while
catching a fast-moving cricket ball?
Answer: The fielder pulls his hands backward to
increase the time over which the ball's momentum
decreases to zero. This reduces the acceleration of
the ball and thus the force of impact, which helps in
reducing the chance of injury.
Q. Why are athletes in high jump events made to fall on
cushioned or sand beds?
Answer: Athletes fall on cushioned or sand beds to
increase the time over which their momentum
decreases to zero. This reduces the rate of change
of momentum and thus the force of impact,
minimizing the risk of injury.
Q. Why does a karate player use a quick, powerful strike to break a slab of ice?
Answer: A karate player uses a quick, powerful strike to achieve a high acceleration
of the fist, which generates a large force over a very short period. This force is
sufficient to break the slab of ice despite its resistance.
Q. The velocity-time graph of a car of 1000 kg mass is given alongside. From the graph
answer the following: [CBSE 2010, 2016, 2022]
(a) When is the maximum force acting on the car? Why?
(b) What is the retarding force?
(c) For how long is there no force acting?
Q. If the time taken to bring a ball to rest from a certain velocity v is reduced to half,
what will be the changes in values of:
(a) initial and final momentum
(b) change of momentum
(c) rate of change of momentum.
[CBSE 2012, 2013, 2018]
Q: Define the SI unit of force. A force of 2N acting on a body changes its velocity
uniformly from 2 m/s to 5 m/s in 10 s. Calculate the mass of the body.
(CBSE 2018, 2022)
Q: A 8000 kg engine pulls a train of 5 wagons, each of 2000 kg, along a horizontal track.
If the engine exerts a force of 40000 N and the track offers a friction force of 5000 N,
then calculate the net accelerating force.
Newton’s Third Law of Motion
Newton’s 3rd law states that every action has an equal and opposite reaction.
Action and reaction forces are equal, opposite and acting on different bodies.
“When one body exerts a force on the other body, the first body
experiences a force which is equal in magnitude in the opposite direction of
the force which is exerted”.
The mathematical representation of Newton’s third law of motion is let A be
the body exerting force F₁ on the body B, then body B too exerts a force
F₂ on body A, which is given as:
F₁ = -F₂
Q. Why do both players get hurt when they collide
while playing football?
Answer: Both players get hurt because, according
to Newton's third law, each player exerts an equal
and opposite force on the other. These action and
reaction forces are equal in magnitude but opposite
in direction, resulting in both players feeling the
impact.
Q. Why do you push the ground backward when trying
to walk forward?
Answer: When you try to walk forward, you push the
ground backward. According to Newton's third law,
the ground exerts an equal and opposite reaction
force on your feet, which propels you forward.
Q. Why does a gun recoil when a bullet is fired?
Answer: When a gun is fired, it exerts a forward
force on the bullet, and the bullet exerts an equal
and opposite reaction force on the gun. This
reaction force causes the gun to recoil backward.
The difference in mass between the gun and the
bullet results in different accelerations.
Q. Why does a rowing boat move backward when a
sailor jumps out of it?
Answer: When a sailor jumps forward from a rowing
boat, the force exerted by the sailor on the boat has
an equal and opposite reaction force on the boat,
causing it to move backward. This demonstrates
Newton's third law of motion.
Q. Why does a swimmer push water backward to move forward in the water?
Answer: A swimmer pushes water backward using their arms and legs. According to
Newton's third law, the water exerts an equal and opposite force on the swimmer,
propelling them forward. The reaction force from the water helps the swimmer move
forward in the water.
Q. Why does a rocket move upwards when it is launched?
Answer: A rocket moves upwards because it expels
gases downward. According to Newton's third law, the
gases push back on the rocket with an equal and
opposite force, causing the rocket to accelerate
upward.
Q. Why is it difficult to walk on ice compared to a rough
surface?
Answer: Walking on ice is difficult because there is
less friction between the ice and your shoes.
According to Newton's third law, the reaction force
you exert on the ice is the same in magnitude but
opposite in direction to the friction force, which is
much lower on ice than on a rough surface.
MORE QUESTIONS
Q. Give a reason of the following:
(a) A footballer kicks a ball, which rolls on the ground and after covering some distance
comes to rest.
(b) Only the carrom coin at the bottom of a pile is removed when a fast moving striker hits
it.
[CBSE 2015, 2018, 2022]
Ans:(a) When a football is rolling on the ground, a force of friction acts on it due to ground
in a direction opposite to its motion. As a result, the motion of football gets slowed down
and after covering some distance it comes to rest.
(b) The carrom coin at the bottom of a pile comes in a state of motion due to force exerted
by the striker on it. However, other coins of pile remain intact due to their inertia of rest.
Q. A bullet with a mass of 10 g is fired from a rifle. The bullet takes 0.004 seconds to
move through the barrel and leaves it with a velocity of 400 m/s. Calculate the force
exerted on the bullet by the rifle.
(CBSE 2018, 2022)
Q. If the moon attracts the earth, why does the earth not move towards the moon?
1. (a) State the reason why a bullet of small mass fired from a gun can be fatal.
(b) A bullet with a mass of 4 g is fired with a velocity of 50 m/s and penetrates a wall to a
depth of 10 cm. Calculate the average resistance offered by the wall.
(c) How will the depth of penetration into the wall change if a bullet of mass 5 g strikes it
with a velocity of 40 m/s? Provide a reason to justify your answer.
(2016, 2018, 2022)
2. Name the physical quantities corresponding to the following units:
(CBSE 2015, 2016, 2022)
3. An object of mass 50 kg is accelerated uniformly from a velocity of 4 m/s to 8 m/s in 8
seconds. Calculate the initial and final momentum of the object. Also, find the
magnitude of the force exerted on the object.
(CBSE 2015, 2018, 2023)
4. A cricket player catches a ball of mass 0.1 kg moving with a speed of 10 m/s in 0.1
second. What is the force exerted by him?
a) 4 N
b) 2 N
c) 1 N
d) 10 N
5. When an athlete comes running from a distance, he is able to jump longer. Why?
(CBSE 2015, 2017, 2022)
"Shake off the inertia of
distractions—don’t lose focus until
your goals are in motion!" ”