Given below are the force and laws of motion class 9 worksheet
(a) Objective Type Questions
(b) Very Short questions
(c) Short questions
(d) Long answer questions
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Multiple Choice questions
Question 1
Which is true from Newton's Third law of motion?
a. For every action force there is a 50% reaction force in the opposite direction.
b. For every action force there is an equivalent reaction force in the opposite direction.
c. Both 1 and 2
d. None of the above
Answer
Answer is b
Question 2
In the equation \( F = ma \), what does m represent?
(a) Mass
(b) Meters
(c) Force
(d) Acceleration
Answer
Answer is Mass
Question 3
Newton's First Law of Motion is also called:
a. Law of Friction
b. Law of Reaction
c. Law of Action
d. Law of Inertia
Answer
Answer is d
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/s^{2}
$F= ma = 6 \times 4 = 24 N$
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
$F_1$ = $F_2$
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.
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
Long Answer type questions
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 difference between balanced force and unbalanced force?
Answer
Question 15
State the various effects of force?
Answer
The various effects produced by a force are as follows:
a.It can change the speed of an object making it move faster or slower.
b.It can change the direction of motion of an object.
c. It can change the shape of an object.
Question 16
Give reason for each of these
Answer
Question 16.
(i)State the law of conservation of momentum.
(ii(Using this law explain why a gun recoils. 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. (Take g = 9.8 m/s^{2})
Answer
s= 19.6 m , u=0, a= 9.8 m/s^{2} ,v=?
$v^2 = u^2 + 2as$
v= 19.6 m/s
Question 18.
A boy of mass 50 kg running 5 m/s jumps on to a 20 kg trolley travelling in the same direction at 1.5 m/s. find their common velocity.
Answer
Here m=50 kg ,$u_1 = 5m/s$ ,M= 20 kg ,$u_2=1.5 m/s$
Now
$mu_1 + M u_2 = (M+m)v$
$ 50 \times 5 + 20 \times 1.5 = 70 v$ v
or v= 4 m/s
Question 19.
Car A of mass 1500 kg travelling at 25 m/s collides with another car B of mass 1000 kg travelling at 15 m/s in the same direction. After collision, the velocity of car A becomes 20 m/s. Calculate the velocity of car B after collision.
This Worksheet on Force , Momentum & Laws of Motion Class 9 physics is prepared keeping in mind the latest syllabus of CBSE . This has been designed in a way to improve the academic performance of the students. If you find mistakes , please do provide the feedback on the mail.