Force and Laws of Motion Class 9
NCERT Class 9 Science – Force and Laws of Motion: Practice Questions
Practice Questions | Fill in the Blanks | Multiple Choice Questions (MCQs) | Board Examination Questions
Practice Questions
1. What is Newton’s First Law of Motion?
Newton’s First Law of Motion states that an object will remain at rest or in uniform motion in a straight line unless acted upon by an external force.
2. Define inertia and give examples.
Inertia is the resistance of any physical object to changes in its velocity. Examples include a passenger moving forward when a car stops suddenly, or dust falling off a carpet when it’s beaten.
3. What is Newton’s Second Law of Motion?
Newton’s Second Law of Motion states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.
4. Explain Newton’s Third Law of Motion with examples.
Newton’s Third Law states that for every action, there is an equal and opposite reaction. Examples include a boat moving forward when a person jumps off it, or a rocket propulsion in space.
5. What is the relationship between force, mass, and acceleration?
The relationship is expressed as F = ma, where F is the force, m is the mass, and a is the acceleration. This is derived from Newton’s Second Law of Motion.
6. What is meant by the term ‘net force’?
Net force refers to the total force acting on an object, taking into account all the individual forces acting on it, and is the vector sum of those forces.
7. How do balanced and unbalanced forces affect motion?
Balanced forces do not cause a change in motion, while unbalanced forces result in acceleration, changing the velocity of the object.
8. Describe how friction affects motion.
Friction opposes the relative motion of two surfaces in contact. It can cause acceleration to decrease or even bring a moving object to a stop.
9. What is conservation of momentum?
Conservation of momentum states that in a closed system, the total momentum before and after an interaction remains constant if no external forces act upon it.
10. How is the concept of action and reaction applied in everyday life?
Action and reaction can be seen when small animals like catapult themselves off the ground or when swimmers push the water backwards to move forward.
Fill in the Blanks
1. The SI unit of force is ______.
Answer: Newton
2. The tendency of an object to resist any change in its state of motion is called ______.
Answer: inertia
3. ______ is the state of motion when an object is not accelerating.
Answer: equilibrium
4. The force exerted by a surface as an object moves across it is called ______.
Answer: friction
5. An object at rest stays at rest unless acted upon by a ______ force.
Answer: net
6. An object ____________ has a net force of zero.
Answer: at rest
7. The ____________ law states that the acceleration of an object is directly proportional to the net force.
Answer: second
8. ____________ is defined as mass times acceleration.
Answer: force
9. ________ action will always have an opposite reaction.
Answer: every
10. An increase in speed is called ______.
Answer: acceleration
Multiple Choice Questions (MCQs)
1. Which of Newton’s laws is also known as the law of inertia?
a) First law
b) Second law
c) Third law
d) Fourth law
Answer: a) First law
2. What does Newton’s Second Law relate to?
a) Speed
b) Force
c) Momentum
d) All of the above
Answer: b) Force
3. What is the formula for calculating weight?
a) W = mg
b) W = m/v
c) W = F/m
d) W = F/v
Answer: a) W = mg
4. What type of friction occurs between stationary objects?
a) Kinetic friction
b) Static friction
c) Sliding friction
d) Rolling friction
Answer: b) Static friction
5. Which law can be demonstrated using a rocket?
a) First law
b) Second law
c) Third law
d) All of the above
Answer: c) Third law
6. Which force opposes the motion of an object?
a) Gravitational force
b) Frictional force
c) Magnetic force
d) All of the above
Answer: b) Frictional force
7. When an object falls freely under gravity, it experiences ______.
a) constant acceleration
b) constant velocity
c) variable acceleration
d) no acceleration
Answer: a) constant acceleration
8. What is the net force acting on an object in equilibrium?
a) Zero
b) Greater than zero
c) Less than zero
d) Depends on the object
Answer: a) Zero
9. Which term describes the quantity of motion an object has?
a) Force
b) Mass
c) Momentum
d) Inertia
Answer: c) Momentum
10. Which law explains why a passenger lunges forward during sudden braking?
a) First law
b) Second law
c) Third law
d) The law of inertia
Answer: a) First law
Board Examination Questions (Recent Years)
1. State and explain Newton’s Second Law of Motion. How is it used to define the unit of force?
Answer: Newton’s Second Law states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. It’s expressed as F = ma. This relationship is used to define the unit of force: one Newton is the force required to accelerate a mass of 1 kg at 1 m/s².
2. Describe an experiment to demonstrate Newton’s First Law of Motion.
Answer: Place a small ball on a smooth surface and give it a slight push. It will continue to move until friction or another force stops it, demonstrating that an object in motion stays in motion unless acted upon by an external force.
3. Explain how seat belts are related to Newton’s First Law of Motion.
Answer: Seat belts are essential for safety as they prevent passengers from continuing in motion when a car suddenly stops, thereby demonstrating the concept of inertia.
4. Discuss the role of friction in everyday life.
Answer: Friction is vital in everyday life as it allows us to walk without slipping, enables vehicles to grip the road, and helps in tasks like writing. However, too much friction can cause wear and tear.
5. How does understanding forces help in designing safer vehicles?
Answer: Understanding forces helps engineers design vehicles with better safety features, such as crumple zones and airbags, that absorb impact and reduce injuries during collisions, applying the principles of motion.
6. What is the importance of an airbag in a vehicle from the perspective of Newton’s laws?
Answer: An airbag reduces the velocity of the occupant’s body in a crash, thereby reducing the force upon impact, explaining Newton’s First and Second Laws regarding motion and force.
7. Explain the concept of action-reaction forces using the example of a swimming swimmer.
Answer: When a swimmer pushes water backward using their hands, the reaction is that the swimmer is propelled forward, demonstrating Newton’s Third Law of Motion.
8. Describe how the concept of momentum is applied in sports.
Answer: Momentum is applied in sports to increase performance; for example, a football player gaining speed maximizes momentum making it easier to tackle opponents or score goals.
9. Discuss the impact of mass on acceleration according to Newton’s Second Law.
Answer: According to Newton’s Second Law, the greater the mass of an object, the smaller its acceleration for a given force, illustrating the need for more force to achieve higher acceleration with heavier objects.
10. Explain the terms ‘static friction’ and ‘kinetic friction’.
Answer: Static friction occurs between stationary objects and prevents them from moving, while kinetic friction occurs between moving objects and slows down their motion.
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