FORCE AND LAWS OF MOTION
Class 9, NCERT Science Physics - CBSE Guess and Guide
Solutions of CBSE Guess Sample Questions
Question.1: Define ‘Balanced’ and
‘Unbalanced’ forces. Illustrate with examples.
Solution:
Balanced
Force - If the resultant of several forces acting on a body is zero, the
forces are said to be ‘Balance Forces’. Balanced forces do not change the
speed. They usually change the shape of an object e.g. in a tug-of-war the rope
does not move in either direction if the two teams pull the rope with equal
efforts. Here the forces exerted by the two teams are equal and opposite and
so, get balanced.
Unbalanced
Force
- If the resultant of several forces acting on a body is not zero, the forces
are said to be ‘Unbalance Forces’. Unbalance Forces produce a change in the
state of rest or uniform motion of a body. In other words objects continue to
move, with the same velocity unless acted upon by an unbalanced force. For
example, a force larger than the force of gravity has to be applied on an
object in order to raise it to a certain height from the ground. Here the two
forces are unbalanced and result in the upward motion of the object.
Question.2: What is inertia? How many types
of inertia are there?
Solution:
Inertia
- It is the inability of a body to change by itself its state of rest or
uniform motion in a straight line. Generally there are three types of inertia:
Inertia of
Rest
- This can be defined as the tendency of a body to remain in its state of rest.
Inertia of
Motion
- It is the tendency of a body to remain in its state of uniform motion along a
straight line.
Inertia of
Direction
- It is the inability of a body to change by itself its direction of motion.
Question.3: Define: (a) Momentum, (b)
Newton
Solution:
Momentum
- It is the quantity of motion possessed by a body and is equal to the product
of the mass and velocity of the body. Momentum is a vector quantity and is
expressed by the symbol p.
Momentum =
Mass x Velocity
or, p =
mv
The SI unit
of momentum is kg.m.s–1
Newton - Newton is
the SI unit of force and is expressed by the symbol N. One Newton is that quantity of force which can produce an
acceleration of 1 ms–2 in a body of mass 1 kg.
1 N = 1
kg.m.s–2
Question.4: State the various effects
of force.
Solution: The
various effects produced by a force are as follows:
=> It can
change the speed of an object making it move faster or slower.
=> It can
change the direction of motion of an object.
=> It can
change the shape of an object.
Question.5: State Newton’s first law of
motion. Why the Newton’s first law of motion is also called ‘Law of Inertia’?
Solution: Newton’s
First Law of Motion - According to this law a body at rest or in uniform
motion will remain at rest or in uniform motion along a straight line unless an
unbalanced force acts upon it.
According to
Newton’s first law of motion, a body by itself is not able to change its state
of rest or of uniform motion along the same direction. This property of the
body is called ‘inertia’ which in other words, can be defined as the tendency
of undisturbed objects to stay at rest or keep moving with the same velocity.
That is why the Newton’s first law of motion is called ‘law of inertia’.
Question.6: What is the relation
between mass and inertia?
Solution: The mass
of a body is measure of its inertia. The larger the mass of a body, larger is
the inertia or opposition offered by a body to change its state of motion. For
example if we kick a football, it flies a long way. If we kick a stone of the
same size it hardly moves. Rather we may get injured in our leg while hitting
the stone. The reason is that the stone resists a change in its motion better
than the football because of its greater mass. Thus the stone has more inertia
than the football.
Question.7: State and explain Newton’s
second law of motion.
Solution: Newton’s
Second Law of Motion - This law states that the rate of change of momentum
of a body is directly proportional to the applied unbalanced force and the
change takes place in the direction of the force. This law can be divided into
two parts:
(i) The rate of change of momentum of a body is
directly proportional to the applied force. The larger the force acting on
a body, the greater is the change in its momentum. Since the change in momentum
is equal to the product of mass and the change in velocity, and the mass remaining
constant, the rate of change of momentum is directly proportional to the rate
of change in velocity i.e. acceleration. Hence, force (F) is directly
proportional to the mass (m) and acceleration (a).
F α ma.
(ii) The change of momentum occurs in the
direction of the force. If a body is at rest, a force can set it in motion.
If a body is moving with a certain velocity, a force will increase or decrease
this velocity accordingly as the force acts in the same or opposite direction.
Question.8: Give two examples to
illustrate Newton’s second law of motion.
Solution: Examples
to illustrate Newton’s second law of motion -
(i) A cricket player lowers his hands while
catching a ball. The reason is that by lowering his hands, he increases the
tome of catching the ball. As a result, the rate of change of momentum
decreases and by Newton’s second law, the force exerted on his hands is less.
So he is less likely to get hurt.
(ii) We are hurt less when we jump on a muddy
floor than on a hard floor. When we jump on a muddy floor, the floor is
carried in the direction of the jump and the time interval for which force acts
is increased. This decreases the rate of change of momentum and hence the force
of reaction. Thus we get less hurt.
Question.9: Explain how Newton’s second
law of motion can be used to derive a quantitative definition of Force.
Solution:
Measurement
of force from Newton’s second law of motion -
Suppose a
force F, acts on a body of mass m, and change its velocity from u to v
in t seconds. Then,
Initial
momentum of the body, p1 = mu
Final
momentum of the body, p2 = mv
Change in
momentum = p2 – p1 = m(v - u)
Time taken =
t
Rate of
change in momentum = m(v - u) ÷ t
Rate of
change in momentum = ma, where, a is
the acceleration of the body. [a = (v -
u) ÷ t]
According to
Newton’s second law of motion, the rate of change of momentum of a body is
directly proportional to the applied force, so
F α ma.
or, F = kma
[k = constant]
The unit of
force is so chosen that k is equal to
one i.e. if m = 1, a = 1 and F = 1, then k = 1
Therefore, F
= ma.
So a unit of
force is that force which produces unit acceleration in a body of unit mass.
The SI unit of force is Newton. From the above formula we can say that one
Newton is that force which produces an acceleration of 1 m/s2 in a
body of mass 1 kg.
Thus, the
Newton’s second law of motion gives us a method to measure force.
Question.10: State Law of Conservation
of Momentum. Deduce this from Newton’s second law of motion.
Solution:
Law of
conservation of momentum - This law states that if a number of bodies are
interacting with each other, their total momentum remains conserved before and
after the interaction, provided there is no external force acting on them.
Derivation
of Law of Conservation of Momentum from Newton’s Second Law -
Suppose p1 and p2 represent the sum of momentum of a group of objects
before and after the collision respectively. Let t is time elapsed during collision. Then according to Newton’s
Second Law,
External
force = Rate of change of momentum
or, F = (p2
- p1) ÷ t
If there is
no external force, F = 0 and
(p2
- p1) ÷ t = 0
or, p1
= p2
Therefore in
the absence of an external force, the total momentum of a group of objects
remains unchanged or conserved during collision. This is the law of
conservation of momentum.
CBSE Board - Class 9, Force and Laws of Motion - further study
- Force and Laws of Motion - Class 9 Ncert Cbse Science Physics -MCQs
- Class IX, Solutions of Ncert Cbse Science Physics - Lesson 9,Force and Laws of Motion - Chapter In-text Questions
- Class 9 Ncert Cbse Science Solutions - Lesson 9 Physics, Forceand Laws of Motion
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