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1. What is motion?
Ans.:
The position of a body changes with time relative to any fixed point in its
surroundings, then the body is said to be in motion.
E.g. Cars and buses running on the road, Birds and aeroplanes flying in the air etc.
2. What is meant by a body at rest?
Ans.:
A body is said to be at rest, if it does not change its position with respect
to a fixed point in its
surroundings. E.g., a book lying
on a table.
3. Why are the states of motion and rest considered relative?
Ans.:
the given object/ body will be at rest or in motion relative to a fixed
reference point. Thus, the
states of rest and motion are
relative. E.g. a platform, as the
reference point, then the compartment, as well as all the passengers in
it are in motion. This example shows that the same object may be at rest with respect to one
reference point, and in motion with respect to another at the same time. So, we can say that the motion is
actually a relative motion.
4. How is the position of an object described?
Ans.:
The position of an object can be described by specifying the following two
parameters:
(i) The distance of the object from a fixed reference
point and
(ii)
the direction of the object.
5. What is a scalar quantity? Write its characters.
Ans.:
A physical quantity which is described completely by its magnitude is called a
scalar quantity. Thus, a scalar quantity has only
magnitude, and no direction. E.g., length, distance, area, volume, mass, time, power, energy, speed and
temperature etc.
Scalar quantities show the
following characteristics:
(i) A scalar quantity can be represented by a number with proper units. For
example, an object has a
mass of 2kg. The number 2
represents the magnitude of mass, and kg is the unit in which mass is
expressed.
(ii) The scalar quantities having similar units get added or subtracted by the
usual rules of arithmetic. For example, if 2kg of sand is mixed
with 1kg of cement, then the total mass of the mixture will be, 2kg
+ 1kg = 3kg.
6. What is a vector quantity? Write its characters.
Ans.:
A physical quantity which requires both magnitude and direction for its
complete descriptions
called a vector quantity. Thus,
a vector quantity has both magnitude and direction.
E.g., Displacement, velocity,
acceleration, force, weight and momentum etc.
Vector quantities show the
following characteristics:
(i) A vector quantity can be represented by a number with proper units and a
direction. For example, the velocity of a car is 45 – number
magnitude-km-unit-/hr-unit-
(ii) The vector quantities having similar units but different directions are
not added/ subtracted by
ordinary arithmetical rules.
However, if two or more vector quantities have the same units and the
same direction, then their
magnitudes get added subtracted by ordinary arithmetical rules.
7. How is a vector quantity shown graphically?
Ans.:
A vector quantity can be represented graphically by an arrow where,
(i) the length of the arrow represents the magnitude of
the vector quantity.
(ii) the arrowhead represents the direction of the
vector quantity.
To show a vector quantity graphically, we should have the following information:
(i)
The magnitude of the vector quantity
(ii)
The direction of the vector quantity.
8. What is meant by the term distance?
Ans.:
The actual length of the path travelled by a moving body irrespective of the
direction is called the distance travelled by the body.
The distance travelled by a body is denoted by s or S.
Note: During the calculation of
the distance travelled, the direction is which the body moves in not
considered. So, the distance
travelled by a body is a scalar quantity.
9. What is meant by the displacement?
Ans.:
The change in the position of a moving body in a particular direction is called
its displacement.
Displacement is generally
denoted by d or x.
Character of the displacement:
(i)
Displacement is a vector quantity. So,
displacement has magnitude as well as direction.
(ii)
The displacement of a body is for a
certain interval of time.
(iii)
The displacement of a body may have a
positive, zero or a negative value.
10. What is meant by positive, negative or zero displacement?
Ans:.
The displacement of a body in any direction is given by,
Displacement, d = Final position
– initial position = (x2 –
x1)
Depending upon the locations of
the initial and the final positions, the displacement may be
negative, zero or positive.
(i) When
is greater than
, i.e., x2
> x1then
d = (x2 –
x1) = positive. i.e., the displacement of the
body is positive. Thus, The
displacement of a body is positive if its final position lies on the right of
its initial position or above it.
(ii) When
is less than
, i.e., x2 < x1, then d = (x2 – x1) = negative
i.e., the displacement of the
body is negative. Thus, The displacement of a body is negative if its final
position is on the left of its
initial position or below it.
(iii) When x2 is equal to x1 , i.e., x2
= x1, then d =(x2 –
x1) =0
i.e., the displacement of the
body is zero. Thus, the initial and the final positions of the body are the
same, i.e., the body returns to
its initial position.
11. Differentiate between distance and displacement.
Ans.:
Distance
|
Displacement
|
(i) the distance travelled by a moving body is the
actual length of the path covered by the body,
irrespective of the direction
|
(i) the displacement of a body is the shortest
distance between the initial and the final positions of the body.
|
(ii) distance travelled is a scalar quantity.
|
(ii) displacement
is a vector quantity.
|
(iii) distance travelled is always positive.
|
(iii) displacement may be positive, negative or zero.
|
(iv) distance travelled is always greater than or
equal to the displacement.
|
(iv) Displacement is always less than or equal to
the distance travelled.
|
Ans.:
If a body moves along a straight line in its positive direction, then its
displacement and the
distance travelled by it are
equal and have the same sign.
13. "Displacement of a particle is independent of the path followed by
it.” Justify the statement.
Ans.:
The displacement of a moving particle is the shortest distance between its
initial and the final
positions. Thus, it does not
depend on the path followed by it.
14. What is uniform motion?
Ans.:
A body is said to have uniform motion when it travels equal distance in equal
intervals of time.
E.g.
A body moving with a constant speed is
said to be in uniform motion.
15. What is non-uniform motion?
Ans.:
A body is said to have non-uniform motion when it travels unequal distances in
equal intervals of time.
16. What is speed?
Ans.:
The distance travelled by a body per unit time over a short interval of time is
called its speed.
Thus, the speed of a moving body
may be expressed by the relationship,
Speed = Distance travelled in a
short interval of time/Time interval.
Speed is a scalar quantity.
Speed has only magnitude. If a body covers a distance s in time t, then its
speed is given by, v = s/t. The SI units
of distance and time are metre and second respectively. Therefore, SI unit of
speed = metre/second = m/s or ms-1.
17. Define average speed.
Ans.:
The average speed of a moving body is equal to the ratio of the total distance
travelled by the
body to the total time taken by
it to cover that distance.
Thus, Average speed = Total
distance travelled by the moving body/Total time taken to cover that
distance.
18. What is meant by uniform speed?
Ans.:
When a moving body travels equal distances in equal intervals of time
irrespective of direction, then it is said to have uniform
speed. For example, if a body covers a distance of 10m in every one second, then it is said to have
uniform speed of 10ms-1.
19. What is meant by non-uniform speed?
Ans.:
When a moving body travels unequal distances in equal intervals of time
irrespective of direction, then it is said to have
non-uniform speed. A car travelling unequal distances in equal intervals of
time is a typical case of the
non-uniform speed.
20. What is velocity?
Ans.:
The displacement of a body per unit time over a short interval is called
velocity.
Thus, Velocity, v =
Displacement/Time or, v = x/t.
The unit of velocity depends
upon the units of distance and time. The SI unit of distance is metre and that of time is second. Velocity
is a vector quantity.
21. What is meant by uniform velocity?
Ans.:
When a body travels in a straight line and covers equal distances in equal
intervals of time, it is
said to have uniform velocity.
Thus, a body is said to have uniform velocity if it covers equal distances in equal intervals of time in a
particular direction.
22. What is meant by non-uniform velocity?
Ans.:
A body is said to have non-uniform velocity when it covers unequal distances in
equal time
intervals in a particular
direction, or when it covers equal distances in equal intervals of time, but
changes its direction during the
motion. A car turning a corner at a constant speed has non-uniform
velocity.
23. Differentiate between speed and velocity.
Ans.:
Speed
|
Velocity
|
(i) the
distance travelled by a moving body per unit time is called its speed.
|
(i) The distance travelled by a moving body in a
particular direction per unit time is called its velocity.
|
(ii) Speed is a scalar quantity.
|
(ii) Velocity is a vector quantity.
|
Ans.:
The change in the velocity of a body per unit time is called acceleration.
That is, Acceleration = Change
in velocity/Time interval.
In mathematical term:
Let, Velocity of a body at t1 = v1
And, Velocity of a body at t2 = v2
Then, Change in velocity = v2 -v1
And,
Time interval = t2 – t1
Therefore, a = v2 -v1/ t2 – t1
Note: Acceleration is taken to be positive if it acts in the direction of
velocity, and negative when in the direction opposite to the
direction of velocity.
25. What is positive acceleration?
Ans.:
When the velocity of a body increases with time, its acceleration is positive.
Positive acceleration is simply called acceleration.
Note: The acceleration of a moving body is positive if its final velocity
is greater than the initial velocity.
Example: A body dropped from a
certain height gains velocity as it falls down towards the earth.
26. What is negative acceleration?
Ans.:
When the velocity of a body decreases with time, its acceleration is negative.
It is also called
deceleration or retardation.
Note: The acceleration of a moving body is negative when its final velocity
is less than the initial velocity.
Example: When a ball is thrown
vertically upwards, its velocity decreases with time. So, the
acceleration of a ball thrown vertically
upwards is negative.
27. What is retardation?
Ans.:
When the velocity of a body decreases with time, it is said to be under
retardation. It is actually acceleration with a negative
sign. For example, if a body has an acceleration of -5 m/s2, then
the retardation of the body is +5 m/s2.
28. What is meant by uniform acceleration?
Ans.:
When a body travels in a straight line and its velocity changes by equal
amounts in equal intervals of time, it is said to have
uniform acceleration.
Example: A body falling freely
under gravity has uniform acceleration.
29. What is meant by non-uniform acceleration?
Ans.:
when the velocity of a body changes by unequal amounts in equal intervals of
time, it is said to
have a non-uniform acceleration.
Example: An auto/car on a
crowded city-road with frequent application of brakes has a non-uniform
acceleration.
30. What is a distance-time graph? Write the conditions of distance-time
graph.
Ans.:
The geometrical relationship between the distance travelled by a boy and the
time taken is called distance-time graph. It provides
information about the position of the moving body at any time during its motion.
The distance-time graphs of a body under the following three conditions are
described below:
(i)
When the body is at rest
(ii)
When the body is moving with a
uniform speed
(iii)
When the body is moving with a
non-uniform speed
(a) Distance-time graph for a body at rest: When a body does not change its position relative to a fixed reference point, it is said to
be in a state of rest.
(b) Distance-time graph for a body moving with a uniform speed: When a body covers equal distances in equal intervals of time, it is
said to have uniform speed. The distance-time graph of a body which covers
a distance of 1 metre every 10 seconds is shown in Fig.
(c) Distance-time graph for a body moving with a
non-uniform speed: A body
moving with a non-uniform speed covers unequal distances in equal intervals of time.
Therefore, the distance-time
graph of a body moving with a non-uniform speed is a curve.
31. What are the uses of speed-time or velocity-time graphs for a straight
line motion?
Ans.:
Speed-time or velocity-time graphs can be used for:
(i)
determining the speed or velocity of
the body at any particular point of time.
(ii)
determining the acceleration of the
body. The slope of a speed-time graph equal the acceleration of the body.
(iii) determining the total distance travelled by the
body in a given time interval.
32. What is circular motion?
Ans.:
The motion of an object along a circular
path is called circular motion. When a body moves along a circular path with a constant
speed, its motion is called uniform circular motion.
The following are some examples
of circular motion:
(i)
the earth and the other planets move around
the sun in nearly circular paths.
(ii) the moon moves around the earth in a circular
path.
(iii)
a stone tied to a strong string when
rotated gives rise to circular motion of the stone.
33. Why is uniform circular motion called acceleration motion?
Ans.:
When a body moves in a circular path its direction of motion changes
continuously. Thus, the
velocity of a body moving in a
circular path at a constant speed changes continuously. The change in
velocity gives rise to acceleration
in the moving body. Therefore, circular motion is an accelerated
motion even when the speed
remains constant.
34. What is meant by angular displacement?
Ans.:
the angle subtended at the centre of a circle by the initial and the final
positions of a body moving along the circumference of a
circle is called angular displacement of the body. Its unit is radian. The unit radian is denoted by rad.
35. What is meant by angular velocity?
Ans.:
The rate of change of angular velocity is called angular acceleration. It is
denoted by the Greek
letter
.
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