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Physics-
Unit 1 Physical Quantities and Measurement
Q.1.2: What is the difference between base quantities & derived quantities? Give
three examples in each case.
Ans: Base Quantities: Base quantities are the quantities on the basis of which other
quantities are expressed. e.g. length, mass, time, etc.
Derived Quantities: The quantities which are expressed in terms of base quantities are
called derived quantities. e.g. speed, force, work, etc.
Q.1.3: Pick out the base unit in the following: joule, newton, Kilogram, hertz,
mole, ampere, meter, kelvin, coulomb and watt.
Ans: Base unit: Kilogram, mole, ampere, meter, and kelvin
Q.1.4: Find the base quantities in each of the following derived quantities: (a)
speed (b) volume (c) force (d) work
Ans: (a) speed = distance covered / time taken
Unit of speed = meter / second = ms-
Base quantities involved in speed are meter and second.
(b) Volume = length x width x height
Volume = m x m x m = m^3
Base quantities involved in volume is meter
(c) force = mass x acceleration = ma
Unit of force 1N = 1Kg x 1ms-2^ = 1kgms-
Base quantities involved in force are kilogram, meter and second
(d) work = force x displacement =FS
Unit of work 1J = 1N X 1m = 1kgms-2^ x 1m = 1kgm^2 s-
Base quantities involved in work are kilogram, meter and second
Q.1.5: Estimate your age in seconds?
Ans: Suppose my age = 15 years.
My age = 15 x 365 days = 5475 days.
My age = 5475 x 24 hours = 131400 hours.
My age = 131400 x 60 minutes = 788400 minutes.
My age = 7884000 x 60 seconds = 473040000 seconds.
Q.1.6: What role SI units have played in the development of science?
Ans: (i) SI system is in use all over the world (ii) Manipulation in this system is quite
easy i.e. the multiple and sub multiple of different units are obtain simply by multiplying
or dividing with ten or power of tens.
Q.1.7: What is meant by vernier constant?
Ans: Least Count/Vernier Constant: The difference between one small division on main
scale & one vernier division is 0.1 mm. It is called least count (LC) of the vernier
callipers. LC of vernier Callipers =
1 mm
10 divisions
, LC = 0.1 mm = 0.01 cm.
Q.1.8: What do you understand by zero error of a measuring instrument?
Ans: Zero Error & Zero Correction: It is a defect in a measuring device (Vernier
Callipers & Screw Gauge). It is caused by an incorrect position of the zero point.
For example: To find the zero error, close the jaws of vernier callipers gently. If zero line
of the main scale coincides with the zero of the main scale then the zero error is zero.
Zero error will exist if zero line of the vernier scale is not coinciding with the zero of the
main scale as shown in the diagram given below:
Q.1.9: Why is the use of zero error necessary in a measuring instrument?
Ans: When making some kind of scientific measurement it is necessary to first check
your measuring instrument for zero error. The zero error is the reading displayed when
you know the true reading should be exactly zero.
For example, using a set of vernier callipers, the zero error is the reading that shows
when the callipers are fully closed. As long as you check for zero error, you can then
use it to correct your readings.
Q.1.10: What is a stop watch? What is the least count of a mechanical stopwatch
you have used in laboratories?
Ans: Stop watch: A stop watch is used to measure the time interval of an event. A
mechanical stopwatch can measure a time interval up to a minimum 0.1 second. Least
count of mechanical stop watch is 0.1 second.
Q.1.11: Why do we need to measure extremely small intervals of times?
Ans: We need to measure small intervals of time as smaller is the time interval better
resolution of measurement is possible.
Q.1.12: What is meant by significant figures of a measurement?
Ans: Significant figures: All the accurately known digits and the first doubtful digit in
an expression are called significant figures.
Rules for determining significant figures
(i) Non-zero digits are always significant. (ii) Zeros between two significant figures are
also significant. (iii) Final or ending zero on the right in decimal fraction is significant.
Q.1.13: How is precision related to the significant figure in a measured quantity?
Q.5: What is the function of balancing screws in a physical balance?
Ans: We use the balancing screws to remove the zero error of the physical balance.
Balancing screws in a physical balance are used to bring the pointer at zero position.
Q.6: On what pan we place the object & why?
Ans: Only for convenience we put the weights on the right pan after keeping a body on
the left pan.
Q.7. How do the prefixes micro, Nano and Pico relate to each other?
Ans. Micro = 10 -
Nano = 10 -6x10-3^ = 10 -3^ micro
Pico = 10 -6x10-6^ = 10 -6^ micro
Q.8. You hair grow at the rate of 1mm per day. Find their growth rate in nms-1.
Ans. Growth rate of hair in one day = 1 x 10-3m x 1/24x60x60 s
= 1x10-3x0.00001157 ms-
=1x10-3x1157x10-8^ ms-
= 11.57 x 10-9^ ms-
1 mm per day = 11.57 nms-
Q.9: A chocolate wrapper is 6.7 cm long and 5.4 cm wide. Calculate its area up to
reasonable number of significant figures.
Ans. Length of chocolate wrapper= l=6.7 cm
Width of chocolate wrapper= w = 5.4 cm
Area A =?
Area A = Length x Width
= 6.7 cm x 5.4 cm
= 36.18 cm^2 = 36 cm^2
Answer should be in two significant figures because in data the least significant figures
are two therefore answer is 36cm^2.
Q.10: Why do we use zero correction?
Ans: Zero correction is used to get correct and exact measurement.
Q. 11: Define prefixes, also name five prefixes commonly used.
Ans: Prefixes are the words or letters added before SI units such as centi, kilo, mega,
giga, and milli. SI units have the advantages that their multiples and sub-multiples can
be expressed in terms of prefixes. Example 2000g = 2kg
Q. 12: What do you understand by scientific notation?
Ans: In scientific notation a number is expressed as some power of ten multiplied by a
number between 1 and 10. For example: A number 678 can be expressed as 6.78 x10^2
Q.13: Which safety equipment’s a school laboratory must have?
Ans: A school laboratory must have safety equipment’s such as
(i) Waste-disposal basket. (ii) Fire extinguisher. (iii) Fire alarm. (iv) First aid box.
(v) Sand and water buckets. (vi) Fire blankets to put off fire.
Q.14: What is a meter rule? What is the least count of a meter rule used in the
laboratories?
Ans: A meter rule is a length measuring instrument. It is commonly used in the
laboratories to measure small length of an object. Least count of meter rule is 1mm
Q.15: What is the least count of a screw gauge?
Ans: Least count of screw gauge = pitch of the screw gauge
no. of divisions on circular scale
least count of screw gauge = 1mm / 100 = 0.01 mm = 0,001cm
Q.16: What is the pitch of your laboratory screw gauge?
Ans: The pitch of our laboratory screw gauge is 1mm.
Q.17: What is the range of your laboratory screw gauge?
Ans: The range of our laboratory screw gauge is 100mm
Q.18: Which one of the two instruments is more precise and why?
(a) Vernier Callipers (b) Screw gauge
OR
A micrometer screw gauge is more precise than Vernier Callipers Why?
Ans: The least of vernier calipers is 0.01cm while the least count of screw gauge is
0.001cm. Smaller is the least count of an instrument more precise will be the
instrument. A micrometer screw gauge is more precise because its least count is small
as compared to the least count of verneir callipers.
Q.19: What do you know about rounding the number?
Ans: (i) If the last digit is less than 5 then it is simply dropped. Example 1.943 is
rounded to 1.
(ii) If the last digit is greater than 5 then it is digit on its left is increased by one. Example
1.47 is rounded to 1.
(iii) If the last digit is 5, then it is rounded to get nearest even number. For example 1.
is rounded to 1.4 and 1.45 is also rounded to 1.
Questions based on understanding:
Q.1: Why do we study physics?
Ans: We study physics because most of the technologies throughout the world are
related to physics. In our daily life, we hardly find a device where Physics is not involved. Consider pulleys that make it easy to lift heavy loads. Electricity is used not only to get light and heat but also mechanical energy that drives fans and electric motors etc. Q.2: Why do we use zero correction? Ans: Zero correction is used to get correct and exact measurement.
Q.2.2: Explain translatory motion and also give examples of various types of
translatory motion.
Ans: (1)Translatory motion: In translational motion, a body moves along a line without
any rotation. The line may be straight or curved.
Types of translatory motion
(i) Linear motion : Straight line motion of a body is known as its linear motion.
Example: Car moving on a straight road
(ii) Circular motion: The motion of an object in a circular path is known as circular
motion.
Example: A car moving along a circular path.
(iii) Random motion: The disordered and irregular motion of an object is called random
motion.
Example: The motion of birds.
(2) Rotatory motion: The motion of a body along a fix axis is called rotatory motion.
Example: motion of the wheel of a bicycle
(3) Vibratory motion: The back and forth motion of a body is called vibratory motion.
Example: the motion of a swing
Q.2.3: Differentiate between the following:
(i) Rest and motion:
Ans: Rest: A body is said to be at rest, if it does not change its position with respect
to its surroundings. Example, a car is standing on the road.
Motion: A body is said to be in motion, if it changes its position with respect to its
surroundings. Example, a car is moving on the road.
The state of rest or motion of a body is relative.
(ii) Difference between circular motion and rotatory motion:
Ans: Any rotatory motion where the radius length and axis of rotation are fixed is called
circular motion. Circular motion is just a special case of rotatory motion. There is no
fixed axis and radius restriction for rotatory motion but there is restriction of fixed axis
and radius for circular motion. For example, all planets have rotatory motion around
their suns. But most of the orbits are elliptical, so the rotation axes and radii vary as they
move around so most planets do not have circular motion.
(iii) Difference between distance and displacement:
Ans. Distance: Length of a path between two points which has magnitude only is called
the distance between those points. It is a scalar quantity. It is represented by “s”
Displacement: Displacement is the shortest distance between two pints which has
magnitude and direction. It is a vector quantity. It is represented by “d”
(iv) Difference between speed and velocity:
Ans. Speed: The distance covered by an object in unit time is called speed. It is a
scalar quantity. Its SI unit is meter per second. Its formula is
V =
s
t
Velocity: The rate of displacement of a body is called velocity. It is a vector quantity.
Its SI unit is meter per second. Its formula is
V =
d
t
(v) Difference between linear and random motion:
Ans: Linear motion: Straight line motion of a body is known as its linear motion.
Example: Car moving on a straight road
Random motion: The disordered and irregular motion of an object is called random
motion. Example: The motion of birds.
(vi) Difference between Scalars and Vectors?
Ans. Scalars: A scalar quantity is described completely by its magnitude only. For
example: mass, length etc.
Vectors: A vector quantity is described completely by its magnitude and direction. For
example: displacement, velocity etc.
Q.2.4: Define the terms speed, velocity and acceleration.
Ans. Speed: The distance covered by an object in unit time is called speed. It is a
scalar quantity. Its SI unit is meter per second. Its formula is
V =
s
t
Velocity: The rate of displacement of a body is called velocity. It is a vector quantity.
Its SI unit is meter per second. Its formula is
V =
d
t
Acceleration: Acceleration is defined as the rate of change of velocity of a body. When
velocity is increasing then the acceleration is positive but when velocity is
decreasing then the acceleration is negative.
Acceleration = change in velocity / time taken
Acceleration = final velocity –initial velocity /time taken
a =
vf − vi
t
Units of Acceleration: SI unit of acceleration is meter per second per second (ms-2)
Q.2.5: Can a body moving at a constant speed have acceleration?
Ans. If the body is moving in a straight line with constant speed then there is no
acceleration but if the body is moving along a circular path with a constant speed, then it
will have acceleration due to the change of direction at every instant.
Q.2.6: How do riders in Ferris wheel possesses translator motion but not rotator
motion?
Ans. Riders in Ferris wheel possess translatory motion because their motion is in a
circle without rotation.
Q.2.7: Sketch a distance-time graph for a body starting from rest. How will you
determine the speed of a body from this graph?
Q.2.11: Why vector quantities cannot be added or subtracted like scalar
quantities?
Ans. The scalar quantities obey the rules of arithmetic & ordinary algebra because
scalar quantities have no direction. Since vectors have magnitude as well as direction,
therefore vectors are added by head to tail rule,
Q.2.12: How are vector quantities important to us in daily life?
Ans. Vectors are very important as they describe physical processes in the real world,
& without understanding them we cannot understand how the real world works. e.g. our
breathing, walking etc.
Q.2.13: Derive equations of motion for uniformly accelerated rectilinear motion.
Ans: From book
Q.2.14:- Sketch a velocity-time graph for the motion of the body. From the graph
explaining each step, calculate total distance covered by body.
Ans: Velocity-time Graph:-
(a) if an object moves at constant velocity v for time t. The distance covered by the
object is v x t. This distance can also be found by calculating the area under the
velocity-time graph. This area is shaded and is equal to v x t. If velocity of the object
increases uniformly from 0 to v in time “t” the magnitude of its average velocity is given
by V =
(0 + v) =
v. Distance covered = average velocity x time =
v x t
Quick quiz + Mini Exercise:
Q.1: When a body is said to be at rest?
Ans: A body is said to be at rest if it does not change its position with respect to its
surroundings. No net force is acting on it.
Q.2: Give an example of a body that is at rest and is in motion at the same time
Ans: Motion and rest are relative concepts. There is no absolute rest. We can define
the state of rest or motion only with respect to another object or a point in space taken
as reference. Example: A passenger sitting in a moving bus is at rest with respect to
other passengers inside the bus. But to an observer outside the bus are in motion. Q.3: Mention the type of motion in each of the following: (i) A ball moving vertically upward. Ans: Linear motion
(ii) A child moving down a slide Ans: Linear motion (iii) Movement of a player in a football ground Ans: Random motion (iv) The flight of a butterfly Ans: Random motion (v) An athlete running in a circular track Ans: Circular motion (vi) The motion of a wheel Ans: Circular motion (vii) The motion of a cradle Ans: Vibratory motion Q.4: Which is the fastest animal on the earth? Ans: Falcon is the fastest animal on the earth. Falcon can fly at a speed of 200km/h. While a Cheetah can run at a speed of 70km/h. Q.5: How a paratrooper comes to ground? Ans: A paratrooper attains a uniform velocity called terminal velocity with which it comes to ground. Terminal velocity is represented by vt which is equal to 1.1 m/s. Q.6: Define uniform speed and variable speed. Ans: Uniform speed: A body has uniform speed if it covers equal distances in equal intervals of time however short the interval may be. Variable speed: If a body covers unequal distances in equal intervals of time however short the interval may be, the speed of the body is said to be variable. Q.7: Define uniform velocity, variable velocity and average velocity. Ans: Uniform velocity: A body has uniform velocity if it covers equal displacement in equal intervals of time however short the interval may be Variable velocity: If speed or direction changes with time then the velocity of such a body is said to be variable. Average velocity: The ratio between displacement and time is known as average velocity. Average velocity =
Distance
Time
, vav =
d
t
Q.8: Write equations of motion for bodies moving under gravity? Ans: (1) vf = vi + gt (2) h = vi t +
gt (3) 2gh = vf^2 – vi^2 Q.9: Write three equations of motion for a body starting from rest. Ans: As three equations of motion are (1) vf = vi + at (2) s = vi t +
at (3) 2as = vf^2 – vi^2 Now putting vi=0 in the above equations we get (1) vf = at (2) s =
at (3) 2as = vf^2 Q.10: Explain the types of motion with examples and also give examples of various types of motion.
Speed of the object =
=2 ms- Speed found from the graph is 2 ms- Q.6: What would be the shape of a speed-time graph of a body moving with variable speed? Ans: When an object does not cover equal distances in equal intervals of time then its speed is not constant. In this case the distance-time graph is not a straight line as shown in figure. The slope of the curve at any point can be found from the slope of the tangent at that point. For example, Slope of the tangent at P =
RS
QS
= 3 ms- Thus, speed of the object at P is 3 ms-1. The speed is higher at instants when slope is greater; speed is zero at instants when slope is horizontal. Q.7: Draw and discuss the Speed-time graph showing constant speed. Ans: When the speed of an object is constant (4 ms-1) with time, then the speed-time graph will be a horizontal line parallel to time-axis along x-axis as shown in figure. In other words, a straight line parallel to time axis represents constant speed of the object. Q.8: Draw and discuss graph of an object moving with uniform acceleration. Ans: When the speed of an object is changing uniformly then in such a case speed is changing at constant rate. Thus its speed-time graph would be a straight line such as shown in figure. A straight line means that the object is moving with uniform acceleration. Slope of the line gives the magnitude of its acceleration Q.9: Draw and discuss graph of an object moving with uniform deceleration. Ans: The graph in figure shows that the speed of the object is decreasing with time. The speed after 5s is 4 ms-1^ and it becomes 2 ms-1^ after 10 s. As acceleration = slope of CD =
2 m s
− 1
− 4 m s
− 1
10 s – 5 s
2 m s
− 1
5 s
= - 0.4 ms- Speed-time graph in figure gives negative slope. Thus, the object has deceleration of 0.4 ms-2. Q.10: Convert ms-1^ to kmh-1^ and convert kmh-1to ms- Ans: 1 ms-1^ = 0.001km x 3600 h- 1 ms-1^ = 3.6 kmh- Now 1kmh-1^ =
1000 m
60 s x 60 s
ms- Q.11: Discuss the use of a LIDAR gun by motor way police.
Ans: A motorway speed camera A LIDAR gun is light detection and ranging speed gun. It uses the time taken by laser pulse to make a series of measurements of a vehicle's distance from the gun. The data is then used to calculate the vehicle's speed. Q.12: How a paratrooper comes to ground? Ans: A paratrooper attains a uniform velocity called terminal velocity with which it comes to ground.
Unit 3 Dynamics
Q.3.2: Define the following terms: (i) Inertia (ii) Momentum (iii) Force (iv) Force of friction (v) Centripetal force Ans. (i) Inertia: Inertia of a body is its property due to which it resists any change in its state of rest or motion. Greater is the mass of a body greater is its inertia.
Inertia∝ themass of a body (ii) Momentum: Momentum of a body is the quantity of motion it
possesses due to its mass and velocity. It is denoted by P. Its formula is P= mv and Its SI unit is kgms-1. (iii) Force: A force moves or tends to move, stops or tends to stop the motion of a body. The force can also change the direction of motion of a body. Its formula is F = ma. Its SI unit is Newton. (iv) Force of friction: The force that opposes the motion of moving objects is called friction. In solids, the force of friction between two bodies depends upon many factors such as nature of the two surfaces in contact and the pressing force between them. (v) Centripetal Force: Centripetal force is a force that keeps a body to move in a circle. Centripetal force always acts perpendicular to the motion of the body. Fc = mv^2 r Q.3.3: What is the difference between: (i) mass and weight (ii) Action and reaction (iii) Sliding friction and rolling friction Ans: (i) Mass and weight: The quantity of matter contained in a body is called its mass. It is a scalar quantity. The S.I. unit of mass is kilogram. Weight is a force with which the earth attracts a body towards its centre. Weight is a vector quantity and is always directed towards the centre of the earth. The S.I. unit of weight is Newton. (ii) Action and reaction: Let a body A exerts a force on another body B. The force exerted by
Ans: First of all when the horse pulls on the cart, the cart exerts an equal but opposite reaction on the hours, the action and reaction. However there is another force between the hours and the ground. The horse’s hooves press down on the ground and the ground push back on the horse. Q.3.11: What is the law of conservation of momentum? Ans: Law Of Conservation Of Momentum: The momentum of an isolated system of two or more than two interacting bodies remains constant. Explanation: Consider an isolated system of two spheres of masses m 1 and m 2. They are moving in a straight line with initial velocities u 1 and u 2 respectively, such that u 1 is greater than u 2. Total initial momentum of the system before collision = m 1 u 1 + m 2 u 2 ………….. (i) After some time mass m 1 hits mass m 2 with some force. Total final momentum of the system after collision = m 1 v 1 + m 2 v 2 ……………… (ii) According to the law of conservation of momentum [Total initial momentum of the system before collision] = [Total final momentum of the system after collision] m 1 u 1 + m 2 u 2 = m 1 v 1 + m 1 v 2 ……………………. (iii) Equation (iii) shows that the momentum remains same which is the law of conservation of momentum. Q.3.12: Why is the law of conservation of momentum important? Ans: Law of conservation of momentum is applicable on all objects in the universe. A rocket and jet engine taking off the recoil of gun are examples of law of conservation of momentum. Q.3.13: When a gun is fired it recoils, why? Ans: Recoil Of Gun: Before firing the gun, both the gun and the bullet are at rest, so that the total momentum of the system is zero. As the gun is fired, bullet shoots out of the gun and acquires momentum. To conserve momentum of the system, the gun recoils. Q.3.14: Describe two situations in which force of friction is needed? Ans: (i) Friction is needed to walk on the ground (ii) To stop a bicycle we apply brakes. Q.3.16: Describe ways to reduce the friction? Ans: (i) The friction can be reduced by making the sliding surfaces smooth. (ii) The friction can be reduced by making the fast moving objects a streamline shape. (iii) The friction can be reduced by lubricating the sliding surfaces. (iv) The friction can reduced by using ball bearings. Q.3.17: Why rolling friction is less than sliding friction? Ans: When a certain body rolls over the surface of another body, it has to contact with the surface only at a single point. But when a body moves over the surface of another body, there is relative motion between two surfaces, thus, friction has some maximum value. That is why rolling friction is less than the sliding friction. Q.3.18: What you know about the following: (i) Tension in a string (ii) Limiting force of friction (iii) Braking force (iv) Skidding of vehicle’s (v) Seatbelts (vi) Banking of roads (vii) Cream separator Ans: (i) Tension in the string: The force exerted by a string when it is subjected to pull is called tension in the string. If a person is holding a block of weight w attached to the end of a string, a
force is experience by him. This force is known as tension. When the body is at rest, the magnitude of tension is equal to the weight of the body suspended by the string. Tension and the weight acts in the opposite direction. (ii) Limiting force of friction:- The maximum value of friction is known as the force of limiting friction (Fs). It depends on the normal reaction between the two surfaces in contact. Fs=uR (iii) Braking force:- Friction between a rotating component and a stationary component causes the drum or disc to slow down such a force is called braking force. The greater the diameter of the disc, the further from the centre of wheel the braking force can be applied. This in turn will generate a greater braking force or torque, on the disc. (iv) Skidding of vehicles:- Skids usually occur while driving when the clutch is suddenly engaged or disengaged, the brakes are applied too hard the vehicle accelerates too quickly or the steering wheel is turned too sharply. These can create a situation where power either too much or too little, causes a loss of traction. If the brakes are applied too strongly, the wheels of the car will lock up (stop turning) and the car will skid due to its large momentum. (v) Seatbelts: They provide an external force to a person wearing seatbelt. The additional time is required for stretching seat belts. This prolongs the stopping time for momentum to change is and reduces the effect of collision. (vi) Banking of roads: Banking of road means to make the road to slide towards the center of curvature with an angle. It is helpful because if the velocity is more or there is less friction between the tyres and the road which reduce the danger of car to move out of circular track. (vii) Cream separator: A separator is a high-speed sinner. It acts on the same principle of centrifuge machines. The bowl spins at very high speed causing the heavier contents of milk to move outward in the bowl pushing the lighter contents inward towards the spinning axis. Cream is lighter than other components in milk. Therefore, milk which is denser than cream is collected at the outer wall of the bowl. Q.3.19: What would happen if all friction suddenly disappears? Ans: Nothing would be steady on the ground; many things would be just sliding and sliding. Q.3.20: Why the spinner of a washing machine is made to spin at a very high speed? Ans: They have a perforated wall having large numbers of fine holes in the cylindrical rotor. When it spins at high speed the water from wet clothes is forced out these holes due to lack of centripetal force. Quick quiz + Mini Exercise: Q.1: How much force you need to prevent the book from falling? Ans: The force which is needed to prevent the book from falling is equal to the weight of the book i.e. R = W. Q.2: Why is it easy to roll a cylindrical eraser on a paper sheet than to slide it? Ans: Cylindrical eraser is a rolling body and can easily roll on a paper sheet than to slide it. Because a rolling friction is the force of friction between a rolling body and a surface over which it rolls. Q.3: Do we roll or slide eraser to remove pencil work from our notebook? Ans: The fact that sliding friction is greater than rolling friction. We slide the eraser to remove the pencil work from our note book. Q.4: In which case do you need smaller force and why? (i) Rolling (ii) sliding Ans: In case of rolling friction we need smaller force because rolling friction is smaller than the
a =
m 1 − m 2
m 1 + m 1
g
Divide Eq. (2) by Eq.(1) to find tension T in the string.
T =
2 m 1 m 2
m 1 + m 2
g
The above arrangement is also known as Atwood machine. Q.10: Derive the relation for tension and acceleration for motion of two bodies attached to the ends of a string that passes over a frictionless pulley such that one body moves vertically and the other moves on a smooth horizontal surface Ans: Tension Case –ii Consider two bodies A and B of masses m 1 and m 2 respectively attached to the ends of an inextensible string as shown in figure. Let the body A moves downwards with an acceleration a. Since body A moves downwards, therefore, its weight m 1 g is greater than the tension T in the string. Net force acting on body A = m 1 g- T According to Newton's second law of motion; m 1 g – T = m 1 a ……………………(1) The forces acting on body B are: i. Weight m 2 g of the body B acting downward. ii. Reaction R of the horizontal surface acting on body B in the upwards direction. iii. Tension T in the string pulling the body B horizontally over the smooth surface. As body B has no vertical motion, hence resultant of vertical forces (m g and R) must be zero. Thus, the net force acting on body B is T. According to Newton's second law of motion; T = m 2 a …………………………..(2) Adding Eqs (1) and Eqs (2) we get acceleration a as
a =
m 1
m 1 + m 2
g
Put this value in Eqs (2)
T =
m 1 m 2
m 1 + m 2
g
Questions based on understanding:
Q.1: Generally, mass and weight are considered similar quantities, but it is not
correct. Explain your answer. Or Distinguish between mass and weight?
Ans: Mass of a body is the quantity of matter possessed by the body. It is a scalar
quantity and does not change with change of place. It is measured by a beam balance.
On the other hand, weight of a body is the force equal to the force with which Earth
attracts it. It varies depending upon the value of g. Weight is a vector quantity. Its SI
unit is newton. Weight is measured by a spring balance.
Q.2: State what will happen to you while you are sitting inside a bus when the bus
starts moving suddenly?
Ans: We have observed that the passengers sitting in a bus fall in backward direction
when the bus starts moving suddenly. It is because the lower parts of their bodies tend
to start motion with the bus as they are in contacted with the bus, while upper parts of
their bodies tend to continue their state of rest due to inertia. Hence, they fall backward.
The same will happen to me as well.
Q.3: State what will happen to you while you are sitting inside a bus when the bus
stops moving suddenly?
Ans: We have observed that the passengers sitting in a bus fall forward when its driver
applies brakes suddenly. It is because the upper parts of their bodies tend to continue
their motion due to inertia, while lower parts of their bodies in contact with the bus stop
with it. Hence, they fall forward. The same will happen to me as well.
Q.4: State what will happen to you while you are sitting inside a bus when the bus
turns a corner to the left suddenly?
Ans: We have observed that when a bus takes a sharp turn to left, passengers fall in
the outward direction to the right side. It is due to inertia that they want to continue their
motion in a straight line and thus fall outwards. The same will happen to me as well.
Q.5: A bullet has a very small inertia due to its small mass. But why is its impact
so strong when it is fired from the gun?
Ans: To explain such situation, we define a physical quantity called momentum.
Momentum of a body is the quantity of motion it possesses due to its mass and
velocity.The momentum P of a body is given by the product of its mass m and velocity
v. Thus
P = mv, Momentum is a vector quantity. Its SI unit is kgms-
since the mass of a bullet is small but its velocity is very large so its momentum is very
large that is why its impact is very strong.
Q.6: Why the impact of a loaded truck on a body coming its way is very large
even if the truck is moving slowly?
Ans: To explain such situation, we define a physical quantity called momentum.
Momentum of a body is the quantity of motion it possesses due to its mass and velocity.
The momentum P of a body is given by the product of its mass m and velocity v. Thus
P = mv, Momentum is a vector quantity. Its SI unit is kgms-
since the truck is moving slowly but the mass of a loaded truck is very large so its
momentum is very large that is why its impact is very strong.
Q.7: What happens when an air-filled balloon is set free?
Ans: When the balloon is set free, the air inside it rushes out and the balloon moves
forward. The action is by the balloon that pushes the air out of it when set free. The
reaction of the air which escapes out from the balloon acts on the balloon. It is due to
this reaction of the escaping air that moves the balloon forward.
Q.8: On what principle a rocket moves in the air?
Ans: A rocket moves on the principle of Newton’s 3rd^ law. When its fuel burns, hot
gases escape out from its tail with a very high speed. The reaction of these gases on
