Short Notes - Force and Laws of Motion

Force and Laws of Motion Class 9 Notes Science Chapter 8

• Force: It is a push or pull on an object that produces acceleration in the body on which it acts. S.I. unit of force is Newton.
• A force can do three things on a body.
(a)It can change the speed of a body.
(b)It can change the direction of motion of a body.
(c)It can change the shape and size of a body.
• Balanced forces: Forces are said to be balanced forces if they nullify one another and their resultant force is zero.
• Unbalanced forces: When two opposite forces acting on a body, move a body in the direction of the greater force or forces which brings motion in a body are called as unbalanced forces.
•  First law of motion: An object remains in a state of rest or of uniform motion in a straight line unless acted upon by an external unbalanced force.

Question for Short Notes - Force and Laws of Motion
Try yourself:
Which of the following is NOT a possible effect of a force acting on an object?

•  Inertia: The natural tendency of an object to resist a change in its state of rest or of uniform motion is called inertia.
The mass of an object is a measure of its inertia.
Its S.I. unit is kg.
A body with greater mass has greater inertia.
• Frictional force: The force that always opposes the motion of objects is called force of friction.
• Second law of motion: The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of the force.
Mathematically,
• Momentum: The momentum of an object is the product of its mass and velocity and has the same direction as that of the velocity. Its S.I. unit is kg m/s.
(p = mv )
1 Newton: A force of one Newton produces an acceleration of 1 m/s2 on an object of mass 1kg.
1N = 1 kg m/s2
(F = ma)
• Third law of motion: To every action, there is an equal and opposite reaction and they act on two different bodies.
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FAQs on Force and Laws of Motion Class 9 Notes Science Chapter 8

 1. What are the three laws of motion proposed by Sir Isaac Newton?
Ans. Sir Isaac Newton proposed the three laws of motion: 1) Law of Inertia, 2) Law of Acceleration, and 3) Law of Action-Reaction.
 2. How does Newton's first law of motion relate to the concept of inertia?
Ans. Newton's first law of motion states that an object at rest will stay at rest, and an object in motion will stay in motion unless acted upon by an external force. This concept is related to inertia, which is the tendency of an object to resist changes in its motion.
 3. Can you explain the concept of momentum in the context of Newton's second law of motion?
Ans. Momentum is the product of an object's mass and velocity. According to Newton's second law of motion, the acceleration of an object is directly proportional to the force acting on it and inversely proportional to its mass. Therefore, the concept of momentum helps us understand how an object's motion changes when a force is applied to it.
 4. How does Newton's third law of motion explain the interaction between two objects?
Ans. Newton's third law of motion states that for every action, there is an equal and opposite reaction. This means that when one object exerts a force on another object, the second object exerts an equal and opposite force back on the first object. This law helps explain how forces are exchanged between interacting objects.
 5. Can you provide an example of each of Newton's three laws of motion in everyday life?
Ans. An example of Newton's first law of motion is a ball rolling on a flat surface slowing down due to friction. An example of the second law is a moving car accelerating when the gas pedal is pressed. An example of the third law is a person jumping off a boat causing the boat to move in the opposite direction.

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