Extra questions on Force ,Momentum and Laws of Motion
Given below are the force and laws of motion class 9 extra questions and important questions
(a) Objective Type Questions
(b) Very Short questions
(c) Short questions
(d) Long answer questions
(e)Law of Conservation of Momentum Long answer type questions
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Objective type
Question 1.
A passenger in a moving train tosses a coin which falls behind him. It means that motion of the train is
(a) accelerated
(b) uniform
(c) retarded
(d) along circular tracks
Question 2.
A truck of mass M is moved under a force F. If the truck is then loaded with an object equal to the mass of the truck and the driving force is halved,then how does the acceleration change?.
(a) 1/2
(b) 2
(c)1/4
(d) 1/8
Question 1.
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 2.
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 3.
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 4.
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 5. 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 6.
How are action – reaction forces related in magnitude and direction? Answer
Action and reaction forces are equal in magnitude and opposite in direction
Question 7.
State the relation between the momentum of a body and the force acting on it. Answer
$F= \frac {\Delta}{t}$
Question 8.
(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
Question 9.
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 10.
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 11.
A pile of carom coins is hit with a fast sliding strike. What happens to the carom coins and why? Question 12.
A stone released from the top of a tower of height 19.6 m. calculate its final velocity just before touching the ground. Answer
Question 13.
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 14.
(i)Define SI unit of force.
(ii)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
(i).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.
(ii). 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 $m = \frac {F}{m} = 1.33 N$
Question 15.
Why is it advised to tie the luggage with a rope on the roof buses? Answer
Reason is given as
When a moving bus suddenly stops, the luggage on its roof still tends to continue in the state of motion due to inertia of motion. Thus, to avoid the falling of the luggage, it is tied with a rope on the roof of a bus.
Long Answer type questions
Question 16.
(a) Define momentum. Write its SI unit.
(b) A bullet of mass 0.02 kg is fired from a gun weighing 7.5 kg if the initial velocity of bullet is 200 m/s. Calculate the speed with which the gun recoils. Answer
a. Momentum is defined as the product of mass and velocity and it is a vector quantity. SI unit of momentun is kg m/s
b. Applying law of conservation of momentum
$0=7.5v + .02 \times 200$
v=-.53 m/s
Question 17.
State any three changes that a force brings about on a body. Give one example of each. Answer
a. It can change the speed of the object. A ball slows down on the surface due to friction
b. it can change the direction of the object. In cricket,batsman hit the ball to change the direction of ball
c. It can change the shape of the object. Example stretched spring
Question 18.
Tabulate two differences between balanced and unbalanced forces. Write one example of each. Question 19.
A book slides down a table top and comes to rest position after a certain distance. Name the force acting on the book. What types of forces are acting on the book? (Balanced/ unbalanced). Which force brought the book to rest? Answer
When the book slides down the table top, two forces are acting
a. Friction force in upward direction
b. Gravitational force in downward direction.
The two forces are not equal and thus unbalanced force is there which moves the book downward.
Frictional force is responsible for bringing the book to rest position after certain distance after sliding
Question 20..
The mud particles sticking on the rim of a bicycle wheel leave the rim of the wheel tangentially when it starts moving. Explain. Answer
This happens because of inertia of direction. When the bicycle wheel rotates, centripetal force is not exerted on the mud particles, so mud particles because of inertai of direction continue to move in straight line tangential to the wheel
Question 21.
When a motor car makes a sharp turn at a high speed, we tend to get thrown to one side. Why? Answer
This can again be explained on the basis of the law of inertia. We tend to continue in our straight-line motion. When an unbalanced force is applied by the engine to
change the direction of motion of the motorcar, we slip to one side of the seat due to the inertia of our body
Question 22..
What do you understand by inertia? Do all bodies have the same inertia? Illustrate giving an example. Question 23..
State reason for the following:
a.A person is hit harder, when the person falls on a hard floor than when he falls on sand or cotton.
b.A gunman gets jerk in backward direction while firing a gun.
c.A bullet fired on a glass window makes a fine hole while a stone smashes when hits it. Answer
a. In hard surface, the body comes to rest in shorter time the fall on cotton or sand, so force is more
b. The gun exerts force on the bullet.Now by Newton third law, an equal and opposite force acts on the gun and which is turn is exerted on the gunman
c. When the bullet strikes the glass window, the part of the glass window which comes in contact with the bullet immediately shares the large velocity of bullet and makes a hole, while the remaining part of the glass remains at rest and is therefore not smashed due to inertia of rest.
But when a slow moving stone strikes the same glass window, the various parts of the glass window gets enough time to share the velocity of the stone, and the glass is smashed.
Question 11.
A stone of mass 1 kg is thrown with a velocity 20 m/s across the frozen surface of a lake and comes to rest after travelling a distance of 50 m. what is the force of friction between the stone and ice? Answer
m=1 kg,u=20 m/s ,a=? , s=50 m ,v=0
$v^2 = u^2 + 2as$
a= -4 m/s2
Force of friction = $1 \times 4 =4 N$
Question 12.
State Newton’s second law of motion and prove that Newton’s first law of motion is a special case of Newton’s second law of motion. Question 13.
Answer the following:
(i) What is meant by momentum of a body? How can it be measured? Write its SI units.
(ii) Explain how does Newton’s first law give the qualitative definition of force? Question 14.
(i) Define momentum. Write its SI unit.
ii) How much momentum will an object of mass 10 kg transfer to the floor if it falls from a height of 5 m? (g = 10 m/s2)
(iii) Explain how a karate player can break a pile of tiles with a single blow of his hand. Answer
ii. s= 5m , a= g=10 m/s2 ,u=0, v=?
$v^2= u^2+ 2as$
v= 10 m/s
Momentum of object when it touches the floor = $10 \times 10 = 100 kg m/s$
This same momentum will get tranferred to the floor
iii.karate player strikes the pile with his hand very fast. The large momentum of his hand is reduced to zero in a very short time. This exerts a large force on the pile of tiles which is sufficient to break them apart.
Question 15.
Do action and reaction act on the same body or on different bodies? Explain your answer with the help of example. How are they related in magnitude and direction? Write the total momentum of the gun and the bullet before firing. Question 16.
State reason for the following:
a.A runner presses the ground with his feet before he starts his run.
b.To take the boat away from the bank of a river, the boat man pushes the bank with an oar. Answer
a. The runner pushes the ground with his feet and in turns , the ground applies the force on the runner to starts his run
b. When the boat manexerts a force of action on the bank with his ore, the bank exerts an equal and opposite force of reaction on the boat. Hence, the boat moves away from the bank.
Question 17.
Give an example to show that friction is an important factor in satisfying Newton’s third law of motion. Question 18.
(a) State the law of conservation of momentum.
(b) A boy of mass 60 kg running at 3 m/s jumps on to a trolley of mass 140 kg moving with a velocity of 1.5 m/s in the same direction.
Find their common velocity. Answer
By using law of conservation of momentum
$m_1 u_1 + m_2 u_2 = (m_1 + m_2) v$
$60 \times 3 + 140 \times 1.5 = (60 + 140)v$
v=1.95 m/s
Law of Conservation of Momentum Long answer type questions
Question 19.
(a) State law of conservation of momentum. Write SI unit of momentum.
(b) Two cars each of mass 1000 kg are moving in a straight line but in opposite directions. The velocity of each car is 5 m/s before the collision during which they stick together. What will be the velocity of the combined cars after collision? Answer
b. As per law of conservation of momentum
$1000 \times 5 - 1000 \times 5 = 2000v$
or v=0
Question 20.
(a) Define momentum. Write its SI unit.
(b) A car and a truck have same momentum. Whose velocity is more and why?
(c) A bullet of mass 20 g moving with a speed of 500 m/s strikes a wooden block of mass 1 kg and gets embedded in it. Find the speed with which block moves along with the bullet. Answer
a. $p=mv$ and SI unit is kg m/s
b. Since car is having less mass then truck ,velocity of car will be more
c. m= 20 g= .002 kg ,u=500 m/s , M= 1kg
Now
$mu = (M+m) v$
$ .02 \times 500 = (1.02) v$
or v= 9.8 m/s
Question 21.
Prove the law of conservation of momentum with clear explanation, diagram and equation. Question 22.
(a) State law of conservation of momentum. Express it mathematically, when two bodies of masses $m_1$ and $m_2$ moving in same direction along same straight line with velocities $u_1$ and $u_2$ respectively, collide and their respective velocities after collision become $v_1$ and $v_2$.
(b) A hammer of mass 500 g moving with 50 m/s strikes a nail. The nail stops the hammer in a small time interval of 0.01/s. what is the change in momentum suffered by the hammer? Also calculate the force of the hammer on the nail. What will be the force of the nail on the hammer? Give reason for your answer. Answer
a. As per law of conservation of momentum, When two or more bodies acts upon each other their total momentum remains constant provided no external forces are acting
Initial Momentum
$p_1 = m_1 u_1 + m_2 u_2$
Final Momentum
$p_2 = m_1 v_1 + m_2 v_2$
Now
$p_1 =p_2$
$m_1 u_1 + m_2 u_2= m_1 v_1 + m_2 v_2$
b. m= 500g = .5 kg , u=50 m/s ,t=.01 s, v=0 ,a=?
$\Delta p = p_2 - p_1 = mv - mu= -25 kg m/s
$v=u+at$
a=-5000 m/s2
Force of Hammer=$ ma= .5 \times 5000= 2500 N$
Force of Nail= Force on Hammer= 2500N
Question 22. Name and define the various types of inertia. Illustrate each of them with suitable example. Answer
We have basically three types of Inertia-:
(a) Inertia of Rest: This is the resistance offered by body to change its state of rest
Examples:
(i) Person sitting in a car falls backwards, when the car suddenly starts. It is because the lower portion in contact with the car comes in motion where as the upper part tries to remain at rest due to inertia of rest.
(ii)When a carpet is beaten with a stick then the dust particles due to inertia of rest tends to remain in rest while the fibers of carpet becomes in motion.
(b) Inertia of Motion:This is the resistance offered by body to change of uniform motion is known as Inertia of motion.
Examples:
(i) When a moving car suddenly stops the person sitting in the car falls forward because the lower portion of the body in contact with the car comes to rest whereas the upper part tends to remain in motion due to inertia of motion.
(ii) An athlete runs a certain distance before taking a long jump because the velocity acquired by running is added to the time of jump, so that it can cover a long distance.
(c) Inertia of Direction: This is the resistance offered by body to change its direction of motion.
Examples:
(i) When a car moves round a curve the person sitting inside is thrown outwards in order to maintain his direction of motion due to inertia of motion
Question 23.
(i) Define momentum. State its SI unit.
(ii) An object of mass 50 kg is accelerated uniformly from a velocity of 4 m/s to 8 m/s in 8 s. calculate the initial and final momentum of the object. Also find the magnitude of the force exerted on the object. Answer
i. Momentum is defined as the product of mass and velocity and it is a vector quantity. SI unit of momentun is kgm/s
ii. u= 4 m/s ,v=8 m/s ,t=8 sec, a=?
$v=u+at$
a= .5 m/s2
Initial Momentum = $50 \times 4 = 200 kgm/s$
Final Momentum = $50 \times 8 = 400 kgm/s$
Force exerted = $ma = 50 \times .5 = 25 N$
Question 24
State the Law of Conservation of Momentum. Deduce this from Newton's second law of motion?
Answer
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. p1+p2=p'1+p'2 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,
\( F = \frac{{p_2} - {p_1}} {t} \)
If there is no external force, F=0 and
\( \frac {{p_2} - {p_1}} {t} = 0 \)
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.
Summary
This Extra questions on Force ,Momentum and Laws of Motion for 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.
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