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Assignments
NCERT Solutions
Very Short Questions Type
Question 1.
During the game of table tennis, if the ball hits a player it does not hurt him. On the other hand when a fast moving cricket ball hits a spectator it may hurt him. State reason.
Answer
It is because of the Momentum. Table tennis ball is light than cricket ball and hence less momentum. So it hurts less to the player
Question 2.
Define the first law of motion.
Question 3.
Why do a back seater moves forward when a fast moving bike is stopped suddenly?
Answer
It is because of the inertia of motion. The body is in motion so when the bike stops , back seater move forward.
Question 4.
When a carpet is beaten with a stick it releases dust. Explain why.
Answer
It is because of the inertia of rest of the dust particles .Dust particle tend to remain at rest whereas the carpet comes into motion,this causes the dust particles to fall down
Question 5.
Name the physical quantity that measures inertia. State its SI unit.
Answer
Mass is the physical quantity that measures inertia. Its unit is Kg
Question 6.
Name the property of bodies by virtue of which they resist a change in their state of rest or of uniform motion.
Answer
Mass
Question 7.
What is the momentum of a body of mass 5 kg moving with a velocity of 0.20 m/s.
Answer
$p=mv = 5 \times .20 = 1 kg m/s$
Question 8.
Write the net force acting on a bus, of mass 2000 kg, moving with a uniform velocity of 60 km/h.
Answer
As acceleration is zero, Force is zero
Question 9.
State the relation between the momentum of a body and the force acting on it.
Answer
Force is equal to rate of change of momentum
$F = \frac {\Delta p}{t}$
Question 10.
A body of mass 25 kg has a momentum of 125 kg m/s. calculate the velocity of the body.
Answer
Given m=25 kg ,p=125 kg m/s
Momentum is given by
$p=mv$
or
$v = \frac {p}{m} = 5 m/s$
Question 11.
Name the physical quantity which is measured/ determined by the rate of change of momentum.
Answer
Force
Question 12.
What is the mathematical formula and SI unit of momentum?
Answer
$p=mv$
SI unit of Momentum is kg m/s
Question 13.
What force would be needed to produce an acceleration of 4 m/s
2 on a ball of mass 6 kg?
Answer
Given m= 6 kg , a=4 m/s2
$F= ma = 6 \times 4 = 24 N$
Question 14.
State Newton’s third law of motion.
Question 15.
In the collision between a heavier body and a lighter body, if the force experienced by the heavier body is $F_1$ and that by the lighter body is $F_2$, write the relation between $F_1$ and $F_2$.
Answer
$F_1$ = $F_2$
Question 16.
Explain why same of the leaves may get detached from a tree if we vigorously shake its branch?
Answer
It is because of the inertia of rest of the leaves .Leaves tend to remain at rest whereas the branch comes into motion,this causes the leaves to fall down
Question 17.
An object of mass 100 kg is accelerated uniformly from a velocity of 5 m/s to 8 m/s in 6 s. Calculates the initial and final momentum of the object. Also find the magnitude of force exerted on the object.
Answer
Given u =5 m/s , v=8 m/s ,t=6 s ,m=100 kg
Momentum is given by
$p=mv$
Initial Momentum ($p_1$)
$p_1 =mu = 100 \times 5 = 500 kgm/s$
Final Momentum ($p_2$)
$p_2 =mv = 100 \times 8 = 800 kgm/s$
Now Force can be calculated as
$f= \frac {\Delta p}{t} = \frac {800-500}{6} = 50N$
Question 18.
Out of the four physical quantities associated with the motion of an object viz force, velocity, acceleration and momentum which one remains constant for all bodies large or small, undergoing a free fall?
Answer
We know that formula for these quantities in Free fall
Acceleration $g$. Acceleration in free fall is constant and it is given by g= 9.8 m/s2
Force $F=ma$ .So it will be different for large or small bodies
Velocity $ v= u + at$ . This will keep changing with time
Momentum $p= mv$. This will also keep on changing
We can easily see that Force,velocity ,momentum will be different. The only thing which is constant is acceleration
Question 19.
A runner presses the ground with his feet before he starts his run. Identify action and reaction in this situation.
Answer
Action- when the runner presses the ground with his feet before starting to run
Reaction- when the runner moves forward due to the forward force applied by the ground on him
Question 20.
An athlete always runs some distance before taking a jump. Why?
Answer
An athlete runs before jumping to gain momentum. Because it helps in jumping higher and longer because of inertia of motion gained due to the motion.
When the athletes Jump they already have a forward motion that would be greater than that of a jump made from standing in one spot.
Question 21.
How are action – reaction forces related in magnitude and direction?
Answer
Action and reaction forces are equal in magnitude and opposite in direction
Question 22.
State the relation between the momentum of a body and the force acting on it.
Answer
$F= \frac {\Delta}{t}$
Question 23.
(i) A heavy and a light object have same momentum. Which of these is travelling faster?
(ii) State the law of conservation of momentum. Give examples.
Answer
$p=mv$
$v=\frac {p}{m}$
We can easily see that with same momentum, light object will be travelling faster
Short Answer Type questions
Question 1.
There are three solids made up of aluminium, steel and wood, of the same shape and same volume. Which of them would have highest inertia?
Answer
Since steel has greatest density and greatest mass, therefore, it has highest inertia.
Question 2.
Two friends on roller – skates are standing 5 m apart facing each other. One of them throws a ball of 2 kg towards the other, who catches it. How will this activity affect the position of the two? Explain your answer.
Answer
Distance between them will increase. Initially the momentum of both the friends are zero as they are at rest. In order to conserve the momentum the one who throws the ball would move backward. The second will experience a net force after catching the ball and therefore will move backwards that is in the direction of the force
Question 3.
Give reason and give the law related to these statements:
(a) It is easier to push an empty box than to push the box full of books.
(b) It is difficult for a fireman to hold a hose which ejects large amount of water with high velocity.
Answer
a.Friction forces depends on the roughness of the surface and mass of the box.So it is easier to push an empty box than to push the box full of books
b. When large amount of water is ejected from a hose at a high velocity, according to Newton's Third Law of Motion, water pushes the hose in backward direction with the same force. Hence, it is difficult for a fireman to hold a hose in which ejects large amount of water at a high velocity
Question 4.
(a) Name the force that keeps the object moving in a circular path with constant acceleration.
(b) What would happen if this force is absent?
Answer
a. Centripetal Force
b. The object will move at tangent to the circular path
Question 5.
It is necessary to run along with the moving bus in the same direction of the bus, while alighting from the bus. Give reasons.
Answer
This is because of the inertia of the body. If we just get it down, our feet will be at rest while the body is still in motion and we will fall down and get injuries.
If we run while getting down, we give the body the time to slowly come to rest
Question 6.
Which of the following has more inertia? Give reason for your answer:
(i) A bicycle and a train
(ii) A rubber ball and a stone of the size.
(iii) A five rupee coin and a one rupee coin.
(iv) A bicycle or a truck.
Answer
i. Train
ii. Stone
iii. Five rupee coin
iv. Truck
Question 7.
Find the change in momentum of a car weighing 1500 kg when its speed increases from 36 km/h to 72 km/h.
Answer
$v_1= 36 km/hr = 10 m/s$, $v_2= 72 km/hr= 20 m/s$, m=1500 kg
$\delta p = p_2 - p_1 = 1500 (20-10) = 15000 kgm/s$
Question 8.
Two objects A and B of same masses and velocity V and 3 V respectively are in motion.
(a) Which object will have larger momentum?
(b) Give reason to support your answer ‘a’.
Answer
Momentum is given by
$p=mv$
Here $m_A=m_B =m$ , $v_A =V$ ,$v_B =3V$
$p_A=mV$
$p_B=3mV$
Clearly $p_B > p_A$
Question 9.
While playing football, the goalkeeper did not get sufficient time to stop a fast ball shot towards him. Why did he hurt his hand while doing so?
Question 10.
The hand gets hurt while punching a wall. Why?
Question 11.
(i) Explain why is it difficult for a fireman to hold a hose, which ejects a large amount of water at a high velocity.
(ii) Why action and reaction do not cancel each other?
Answer
i.When large amount of water is ejected from a hose at a high velocity, according to Newton's Third Law of Motion, water pushes the hose in backward direction with the same force. Hence, it is difficult for a fireman to hold a hose in which ejects large amount of water at a high velocity
ii. Because they act on different bodies
Question 12.From a rifle of mass 4 kg, a bullet of mass 50 g is fired with an initial velocity of 35 m/s. Calculate the initial recoil velocity of the rifle.
Answer
From Law of Conservation of Momentum
$0= 4v + .05 \times 35 $
or
v= -.4375 m/s
Question 13.
An object of mass 1 kg travelling in a straight line with a velocity of 10 m/s collides with, and sticks to a stationary wooden block of mass 5 kg. Then they both move off together in the same straight line. Calculate the total momentum just before the impact and just after the impact. Also calculate the velocity of the combined object.
Answer
$m_1 = 1kg$ ,$m_2 =5 kg$ ,$u_1=10 m/s$, $u_2=0$
Initial Momentum
$p_1 = m_1 v_1 + m_2 v_2$
$p_1 = 10 kg m/s$
Now after the collision ,they stick together move with velocity V
Final Momentum
$p_2 = (m_1 + m_2) V$
$p_2 = 6V$
Now As per law of conservation of Momentum as External force is absent
Initial Momentum=Final Momentum
$p_1=p_2$
$10 = 6V$
V=1.67 m/s
Now
$p_2 = 6V= 10 kg m/s$
Question 14.
What is the effect force, when it is applied in the following cases?
a.Balloon is pressed from opposite sides.
b.A ball is hit with a bat.
c.Spring is compressed.
Answer
a. Change in shape
b. change in velocity and direction
c. change in shape
Question 15.
If you are trying to push a heavy box on a horizontal surface, list various forces acting on the box. State the condition under which this box will start sliding on the surface. How will the magnitude of applied force required to move the box change if:
a)Weight of the box is increased?
b)The surface on which the box is placed is made more rough?
Answer
i. Forces acting on the Horizontal box
a. Reaction of the surface on the box in Upward direction
b. Weight of the box in the vertical direction
c. Push force in the horizontal forward direction
d. Friction force in the horizontal backward direction and it is dependent on the roughness of the horizontal surface and weight of the box
The box will start moving when the horizontal applied force is greater then friction force
ii.
a.Friction force will increase in case the weight of the box is increase, so horizontal applied force will need to be increased
b. Friction force will increase in case the surface is made more rough, so horizontal applied force will need to be increased
Question 16.
A pile of carom coins is hit with a fast sliding strike. What happens to the carom coins and why?
Question 17.
A stone released from the top of a tower of height 19.6 m. calculate its final velocity just before touching the ground.
Answer
s=19.6 m , a= g=9.8 m/s2, u=0, v=?
$v^2=u^2 + 2as$
$v^2 = 2 \times 9.8 \times 19.6= 19.6 \times 19.6$
v=19.6 m/s
Question 18.
A gun of mass 3 kg fires a bullet of mass 30 g. the bullet takes 0.003 s to move through the barrel of the gun and acquires a velocity of 100 m/s. calculate:
a)The velocity with which the gun recoils.
b)The force exerted on gunman due to recoil of the gun.
Answer
a. As per law of conservation of momentum
$0 = 3 v + .03 \times 100$
or v = -1 m/s
b. Force exerted on gunman due to recoil of the gun will be equal force exerted on the bullet.
Now for bullet, u=0 , v=100 m/s , t=.003 sec
$a= \frac {v-u}{t} = 3333.33 m/s^2$
Total force on bullet = .03 \times 3333.33 = 100 N
This same force will b exerted on the gunman due to recoil of the gun
Question 19.
Define SI unit of force. A force of 2 N 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.
Answer
a.The SI unit of force is Newton and it is the force that will give a mass of 1 kg an acceleration of 1 meter per second per second.
b. u= 2 m/s,v=5 m/s ,t=10 sec ,F=2 N
$v=u + at$
$5=2 + 2t$
or a=1.5 m/s2
Now $F=ma$
or $m = \frac {F}{m} = 1.33 N$
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Class 9 Maths
Class 9 Science
