Physics formula for class 9 chapter-Force and Laws of Motion
Mar 23, 2023, 16:45 IST
The cause of motion is the
force
. No one has seen force. Hence, concept of force is explained by describing what happens when some force is applied on an object. Force is an external effort in the form of pushing, pulling, stretching, compressing etc.,
The forces are of two types:
-
Balanced forces and
-
Unbalanced forces.
Balanced and Unbalanced Forces
If the resultant of all the forces acting on a body is zero, the forces are called
balanced
forces
. The balanced forces cannot change the state of rest or state of uniform motion or the direction of motion of the body. They can however change the shape and size of the body.
If the resultant of all the forces acting on a body is not zero, the forces are called unbalanced
forces
.
Law of Motion
Newton first law of motion
According to newton first law of motion a body continue to be in a state of rest or in a state of uniform motion along a straight line, unless an external force is applied on the body to change the state.
Inertia of a body is of three types:
-
Inertia of rest
-
Inertia of motion and
-
Inertia of direction.
Inertia of Rest
-
When a bus suddenly starts moving forward, the passengers in the bus fall backward.
-
The carpet is beaten with a stick to remove the dust particles
-
Place a fifty paisa coin on a piece of a card-board that cover the glass. Coin falls into the glass when the card board is hit
-
When a tree is shaken, the tree moves to and fro while the fruits and leaves remain in their state of rest because of their inertia, and thus fruits and leaves separate from the tree and fall from the tree.
Inertia of motion
-
The passengers fall forward when a fast-moving bus stops suddenly
-
A person falls forward while getting down from a moving bus or train
-
A luggage is usually tied with a rope on the roof of buses
Inertia of Direction
-
When a fast-moving bus negotiates a curve on the road, passengers fall towards the centre of the curved road.
-
The sparks produced during sharpening of a knife against a grinding wheel leave the rim of the wheel tangentially. This is because of the inertia of direction
-
A stone tied to a string is whirling in a horizontal circle. If the string breaks, the stone flies away tangentially.
-
An umbrella protects us from rain.
Newton second law of motion
Linear momentum (p) of a body is the product of mass of the body (m) and velocity of the body (v), i.e.
p = mv
According to Newton’s second law of motion,
the rate of change of linear momentum of a body is directly proportional to the external force applied on the body, and this change takes place always in the direction of the applied force.
The SI unit of force is newton (N). One newton is that much force which produces an acceleration of 1 m/s2 in a body of mass one kilogram.
i.e., 1 N = 1 kg × 1 m/s
2
= 1 kg m/s
2
Newton’s Third Law of Motion
According to Newton’s third law of motion, “To every action, there is always an equal and opposite reaction”.
Law of Conservation of Momentum
According to the law of conservation of momentum, when two or more bodies interact with one another, the vector sum of their linear momenta remains constant (i.e., conserved), and is not affected due to their mutual action and reaction. The only condition is that no external unbalanced forces should be acting on the system of bodies. This law is deduced from Newton’s third law of motion.
To become expert in Science do read NCERT text book and solve questions given in the exercise with the help of
NCERT solutions for class 9 Science.
All
NCERT Solutions
are prepared by experts of Physics Wallah. If any students need to take the online test to check their concepts or undertstanding then they can visit
Quiz for Force Law of Motion
.
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