Force and Laws of Motion Class 9th Notes - Free NCERT Class 9 Science Chapter 9 Notes - Download PDF

Force And Laws Of Motion Class 9 CBSE Notes: Chapter 9- Download Free PDF

Force and Laws of Motion is a critical chapter in the Class 9 Physics curriculum, providing the fundamental principles required to understand motion. Careers360's Force and Laws of Motion class 9 notes are written by highly qualified teachers and provide in-depth explanations of key concepts. All students can freely access these ch 9 physics class 9 notes, which are conveniently available in PDF format on the careers360 platform. Force and laws of motion are recognised as fundamental principles in Physics and play an important role in examinations.

Students can strengthen their understanding of these concepts by diligently reviewing the class 9 physics chapter 9 notes. careers360 also offers free NCERT Solutions to help students prepare for exams thoroughly. Force is a measurable physical quantity that includes both magnitude and direction. It manifests as a push or pull on an object. Its application can cause changes in the state of motion or rest, alter velocity and direction, and even change the shape of an object. This chapter is a cornerstone for understanding the complex dynamics of motion and lays a solid foundation for future research in Physics.

Also, students can refer,

• NCERT notes Class 9 Science
• NCERT Solutions for Class 9 Science Chapter 9 Laws of Motion
• NCERT Exemplar Class 9 Science Chapter 9 Laws of Motion

NCERT class 9 science chapter 9 notes

Force is a push or pull on an object, it can change the velocity of an object, change its direction of motion or change the size or shape of the object.

Balanced and unbalanced forces

When the magnitude of the net force applied to an object is zero, there will be no change in the object’s state of motion, such forces on the object are termed balanced forces. When multiple forces are applied to an object, it starts to move in the direction of the force which is greater in magnitude, these are unbalanced forces, and unbalanced forces cause a change in the state of motion of the object.

To bring a change in the speed or direction of an object, the action of the unbalanced force is required.

The first law of motion

Galileo observed the motion of objects on an inclined plane and concluded that no net force is needed to sustain an object’s uniform motion.

The first law of motion states that until driven to change by an applied force, an object will stay in a condition of rest or uniform motion in a straight path.

The first law of motion states that when an object is subjected to an unbalanced external force, its velocity changes, or the object accelerates. Newton's first law of motion, often known as the law of inertia, emphasises objects' ability to resist change in velocity.

Inertia and mass

Inertia is the property of an object to resist the change in its state of motion, if it’s in motion, it tends to be in motion, if it’s in rest, it’ll tend to remain at rest. An object with greater mass will have more inertia, in other words; mass is a measure of inertia.

The second law of motion

Momentum (p) is the product of an object’s mass (m) and its velocity (v)

p=mv

The direction of momentum is the same as that of velocity, and its SI unit is kg m s^-1

Force applied on an object is directly proportional to the change in momentum it caused and, it is inversely proportional to the time taken for this change to take place.

The second law of motion states that an object's rate of change of momentum is proportional to the applied unbalanced force in the force's direction.

The mathematical formulation of the second law of motion

If a force of 'F' is applied to a mass of ‘m' with an initial velocity of 'u'. The force produces a change in its velocity in a time 't,' thus the final velocity is 'v.'

Initial momentum = p₁ = mu

Final momentum = p₂ = mv

Change in momentum ∝ p₂ – p₁

mv- mu

∝ m(v-u)

Rate of change of momentum m(v-u)/t

Therefore, F = m(v-u)/t

F = km(v-u)/t

F = ma

The value of k is chosen to be 1, a is the acceleration of the object.

SI unit of force is N (Newton) or kg m s^-2

When no force is applied to the object, F=0

F= ma

F= m(v-u)/t

Ft = m(v-u)

Since, F=0, therefore,

v=u

Initial velocity equals final velocity, this is in accordance with Newton’s first law of motion.

Third law of motion

When one item puts a force on another, the second object reacts by exerting a force on the first, according to the third law of motion. These two forces have the same magnitude however operate in opposing directions and on separate objects.

Conservation of momentum

Law of conservation of momentum: The total momentum of two objects is conserved even after the collision.

If there are two objects ‘A’ and ‘B’, let their respective masses and velocities be mA , uA and mB , uB.

Let u_A > u_B , so these objects collide with each other.

The collision will last for a time ‘t’, the force exerted by A on B will be FAB , and B exerts a force F_BA on A.

Let their final velocities be v_A and v_B.

Momentum of A before collision = m_Au_A

Momentum of A before collision = m_Av_A

Momentum of B before collision = m_Bu_B

Momentum of B before collision = m_Bv_B

FAB = rate of change of momentum of A= m_A(v_A-u_A)/t

FBA = rate of change of momentum of B= m_B(v_B-u_B)/t

According to Newton’s third law, F_AB = -F_BA

m_A(v_A-u_A)/t = - m_B(v_B-u_B)/t

Simplifying the above equation,

m_Au_A + m_Bu_B= m_Av_A +m_Bv_B

Initial sum of momentum = Final sum of momentum

Significance of NCERT notes for class 9 science chapter 9

• Revision Assistance: Force And Laws Of Motion notes class 9 help students revise the chapter's content and ensure a thorough understanding of the key concepts covered.
• Simplicity: These CBSE class 9 physics ch 9 notes are intended to be easy to understand, with step-by-step explanations to help students grasp the concepts without difficulty.
• CBSE Syllabus Coverage: These ch 9 physics class 9 notes are consistent with the CBSE Science syllabus for class 9, ensuring that students cover all relevant topics and concepts.
• Accessibility: The availability of physics class 9 chapter 9 notes PDF format allows students to access and use them for offline preparation, allowing for greater flexibility in study schedules.
• Clarity of Concepts: The Force And Laws Of Motion class 9 notes provide clear explanations, allowing students to effectively grasp the principles of laws of motion.
• Preparation Tool: Using these class 9 physics chapter 9 notes helps students prepare for exams, allowing them to consolidate their knowledge and perform well in assessments.

NCERT Solutions Of Class 9 Subject-Wise

• NCERT solutions for class 9 Maths
• NCERT solutions for class 9 Science

NCERT Class 9 Exemplar Solutions for Other Subjects:

• NCERT Exemplar Class 9 Maths solutions
• NCERT Exemplar Class 9 Science solutions

Class 10 Chapter Wise Notes