CBSE Class 9 Science Force and Laws of Motion Assignment Set B

Read and download free pdf of CBSE Class 9 Science Force and Laws of Motion Assignment Set B. Get printable school Assignments for Class 9 Science. Standard 9 students should practise questions and answers given here for Science in Grade 9 which will help them to strengthen their understanding of all important topics. Students should also download free pdf of Printable Worksheets for Class 9 Science prepared as per the latest books and syllabus issued by NCERT, CBSE, KVS and do problems daily to score better marks in tests and examinations

Force And Laws Of Motion Class 9 Science Assignment Pdf

Class 9 Science students should refer to the following printable assignment in Pdf for Force And Laws Of Motion in standard 9. This test paper with questions and answers for Grade 9 Science will be very useful for exams and help you to score good marks

Class 9 Science Assignment for Force And Laws Of Motion

 Force and Laws of Motion

Q 1 Why does a boxer move his head backwards to minimize the effect of on coming punch?

Q 2 Briefly explain how an expert karate player breaks a slab of ice with a single blow.

Q 3 What is meant by balanced forces?

Q 4 A swimmer swims forward , even though he pushes water backward while swimming. Why?

Q 5 When a force acting on a body has an equal & opposite reaction, then why should the body move at all.

Q 6 What do you mean by impulsive force?

Q 7 Define impulse.

Q 8 Define momentum.

Q 9 What force is needed to produce an acceleration of 2 m/s2 in a body of mass 3kg?

Q 10 Give a simple experiment to illustrate the inertia of rest.

Q 11 Why it is advised to tie the luggage with a rope on the roof of buses?

Q 12 In oil tankers some space is left at the top while filling them. Explain why?

Q 13 A person is prone to more serious injuries when falling from a certain height on a hard concrete floor than on a sandy surface. Explain why.

Q 14 A body is moving on a rough level road with a speed of 15m/s along a given direction. Does any force is needed to maintain this
speed? Why?

Q 1 Two cars weighing 1500 kg are made to collide with a wall. The initial & final velocities of the car are - 15.0 m/s & 2.6 m/s respectively. If the collision lasts for 0.15 s, then find impulsive force exerted on the car.

Q 2 A bullet of mass 20 g and with the velocity of 150ms-1 moving horizontally strikes a wooden material and comes to rest in 0.02 s. Calculate the magnitude of the force exerted by the wooden material on the bullet.

Q 3 Two football players of opposite teams collide while they are trying to hit a football on the ground and after colliding, they move off together. One with a mass of 60 kg was travelling with a velocity of 5.0 m s-1 and the other footballer with a mass of 55 kg was moving faster with a velocity 6.0 m s-1 towards the first player. What is the direction and the velocity with which they move after they become entangled?

Q 4 A large bus and a van, both moving with a velocity of magnitude v, have a head-on-collision and both the vehicles stop after the collision. If the time of the collision is 1 sec then, a) Which vehicle experiences smaller force of impact?

b) Which vehicle experiences the smaller momentum change?

c) Which vehicle experiences the greater acceleration?

d) Why is it that the truck suffers less damage than the car?

Q 5 Define inertia.

Q 6 What force is needed to produce on acceleration of 2 m/sin a body of mass 3kg?

Q 7 What is meant by balanced forces?

Q 8 What is force?

Q 9 A person is prone to more serious injuries when falling from a certain height on a hard concrete floor than on a sandy surface. Explain why.

Q 10 Give a simple experiment to illustrate the inertia of rest.

Exam Questions NCERT Class 9 Science Chapter 9 Force and Laws of Motion

Question. Calculate the force required to impart to a car a velocity of 30 m/s in 10 s. The mass of the car is 1500 kg.
Ans :
u = 0, v = 30 m/s, t = 10 s
v = u + at
30 = 0 + a × 10
a = 3 ms–2
Now m = 1500 kg, a = 3 ms–2
Required Force = F = ma
1500 × 3 = 4500 N

Question. Calculate the force required to impact to a car, a velocity of 30 ms–1 in 10 seconds. The mass of the car is 1,500 kg.
Ans :
Here u = 0 ms–1; v = 30 ms–1; t = 10 s; a = ?
Using                   v = u + at, we have
                         30 = 0 + a (10)
                           a = 3 ms–2
Now                     F = ma = 1,500 × 3
or                        F = 4,500 N

Question. An object undergoes an acceleration of 8 ms–2 starting from rest. Find the distance travelled in 1 second.
Ans :
Acceleration, a = 8 ms–2
Initial velocity, u = 0
Time interval, t = 1 s
Distance travelled, s = ?
Using the equation of motion, s = ut + 2
1 at2, one gets
s = 0 × 1 + 2
1 × 8 × 12 = 4 m
The object travels a distance of 4 m.

Question. Name three non-SI units of force. Define them.
Ans : Some commonly used non-SI units force are :
Dyne : The dyne is the CGS unit of force. One dyne is the force which produces an acceleration of 1 cm/s2in a body mass 1 g, i.e.,
dyne = 1 G * 1 cm/s2
Gram-weight : It is the gravitational unit of force in CGS system. This is denoted as g-wt.
One gram weight is the force which produces an acceleration of 981 cm/s2 in a body of mass 1 g. Thus,
g-wt = 1 g * acceleration due to gravity (g)
Or, 1 g-wt = 1 g * 981 cm/s2 = 981 dynes
[a 1 g * 1 cm/s2 = 1 dynes]
Kilogram-weight : It is the gravitational unit of force in SI system. This unit is denoted as kg-wt and it is the force which produces an acceleration of 9.81 m/s2
in a body of mass 1 kg. Thus,
1 kg-wt = 1 kg * 9.81 m/s2

Question. Explain inertia and momentum.
Ans : Inertia : The natural tendency of an object to resist a change in their state of rest or of uniform motion is called inertia. For example : A book lying on a table will remain there until an external force is applied on it to remove or displace it from that position. Momentum : Momentum of body is the quantity of motion possessed by the body. It is equal to the
product of the mass and velocity of the body and is denoted by p. p = mv
Momentum is a vector quantity and its direction is same as the direction of velocity of the object. Its SI unit is kilogram metre per second (kg ms–1).

Question. Give 3 examples in daily life which shows inertia.
Ans : Three examples of inertia in daily life are :
(i) When we are travelling in a vehicle and sudden brakes are applied we tend to fall forward.
(ii) When we shake the branch of a tree vigorously, leaves fall down.
(iii) If we want to remove the dust from carpet we beat the carpet so that dust fall down.

Question. Name the various types of forces.
Ans : Various types of forces are :
(i) Muscular force,
(ii) Tension,
(iii) Gravitational,
(iv) Reaction,
(v) Frictional forces,
(vi) Electrical forces,
(vii) Magnetic forces.

Question. Use law of conservation to explain why a gun recoils.
Ans : Gun and bullet both are at rest before firing. Hence, their initial velocities are zero. So, the initial momentum of the gun and the bullet is zero before fire. When a bullet is fired from the gun, it moves forward with a large velocity. The bullet imparts an equal and opposite momentum to the gun due to which the gun recoils backwards. Thus, the final momentum of the system is zero.

Question. Why is the movement of a rocket in the upward direction?
Ans : (i) The movement of a rocket in the upward direction can also be explained with the help of the law of conservation of momentum.
(ii) The momentum of a rocket before it is fired is zero. When the rocket is fired, gases are produced n the combustion chamber of the rocket due to the burning of fuel. These gases come out of the rear of the rocket with high speed. The direction of the Momentum of the gases coming out of the rocket is in the downward direction. To conserve the momentum of the system (rocket gases), the rocket moves upward with a momentum equal to the momentum of the gases. The rocket continues to move upward as long as the gases are ejected out of the rocket.

Question. What is momentum?
Ans : The momentum of an object is the product of its mass and velocity and has the same direction as that of the velocity. The SI unit is kg m/s. (p = mv)

Question. State Newton’s third law of motion.
Ans : To every action, there is an equal and opposite reaction and they act on two different bodies.

Question. Which type of force does not change the state of rest or of motion of an object?
Ans : Balanced force.

Question. Why do athletes have a special posture with their right foot resting on a solid supporter for athletic races?
Ans : Athletes have to run the heats and they rest their foot on a solid supports before start so that duringthe start of the race the athlete pushes the support with lot of force and this support gives him equal and opposite push to start the race.

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