# Class 9 Science Chapter 9 Force And Laws Of Motion Notes

## 9 Class Science Chapter 9 Force And Laws Of Motion Notes

Class 9 Science Chapter 9 Force And Laws Of Motion Notes. here we will be learn about force, effect of force, types of force, newton’s law of motion, principle of conservation of momentum etc.

##### Class 9 Science Chapter 9 Force And Laws Of Motion Notes

📚 Chapter = 9 📚
💠 Force And Laws Of Motion 💠

❇️ Force :-

🔹 It is the force that enables us to do any work. To do anything, either we pull or push the object. Therefore, pull or push is called force.

🔹 Example :- to open a door, either we push or pull it. A drawer is pulled to open and pushed to close.

❇️ Units of force :-

🔹 In S.I system, force is measured in Newton’s represented by letter “N” while as in C.G.S system its values are represented in dynes.

❇️ Newton :-

🔹 The force applied is said to be one dyne if it produces acceleration of 1cm/sec²and in a body of mass 1Kg.

❇️ Dyne :-

🔹 The force applied is said to be one dyne if it produces acceleration of 1cm/sec²⁻ in a body of mass 1 gram.

❇️ Effect of Force :-

🔹 Force can move a stationary body or object.

• For example :- a football can be set to move by kicking it, i.e., by applying a force.

🔹 Force can stop a moving body.

• For example :- by applying brakes, a running cycle or a running vehicle can be stopped.

🔹 Force can change the direction of a moving object.

• For example, by applying force, i.e., by moving handle, the direction of a running bicycle can be changed. Similarly by moving steering, the direction of a running vehicle is changed.

🔹 Force can change the speed of a moving body. By accelerating, the speed of a running vehicle can be increased or by applying brakes the speed of a running vehicle can be decreased.

🔹 Force can change the shape and size of an object. For example, by hammering, a block of metal can be turned into a thin sheet. By hammering, a stone can be broken into pieces.

❇️ Types if forces :-

🔹 Forces are mainly of two types :-

• (A) Balanced forces
• (B) Unbalanced forces

❇️ Balanced Forces :-

🔹 If the resultant of applied forces is equal to zero, it is called balanced forces.

🔹 an Example :- In the tug of war if both the team apply similar magnitude of forces in opposite directions, rope does not move in either side. This happens because of balanced forces in which resultant of applied forces become zero.

🔹 Balanced forces do not cause any change of state of an object.

🔹 Balanced forces are equal in magnitude and opposite in direction. Balanced forces can change the shape and size of an object. For example, when forces are applied from both sides over a balloon, the size and shape of balloon is changed.

❇️ Unbalanced Forces :-

🔹 If the resultant of applied forces are greater than zero, the forces are called unbalanced forces. An object in rest can be moved because of applying Unbalanced forces.

🔶 Unbalanced forces can do the following :-

• Move a stationary object
• Increase the speed of a moving object
• Decrease the speed of a moving object
• Stop a moving object
• Change the shape and size of an object

❇️ Laws of Motion :-

🔶 Galileo Galilei :- Galileo first of all said that object move with a constant speed when no foces act on them. This means if an object is moving on a frictionless path and no other force is acting upon it, the object would be moving forever. That is, there is no unbalanced force working on the object.

🔹 But practically it is not possible for any object. Because to attain the condition of zero, unbalanced force is impossible. Force of friction, force of air and many other forces are always acting upon an object.

❇️ Newton’s Laws of Motion :-

🔹 Newton studied the ideas of Galileo and gave the three laws of motion. These laws are known as Newton’s laws of motion.

❇️ Newton’s First Law of Motion (Law of Inertia) :-

🔹 Any object remains in the state of rest or in uniform motion along a straight line, until it is compelled to change the state by applying external force.

🔶 Explanation :- If any object is in the state of rest, then it will remain in rest until a external force is applied to change its state. Similarly, an object will remain in motion until any external force is applied over it to change its state. This means all objects resist to in changing their state. The state of any object can be changed by applying external forces only.

❇️ Mass and Inertia :-

🔹 The property of an object because of which it resists to get disturb its state is called inertia. Inertia of an object is measured by its mass. Inertia is directly proportional to the mass. This means inertia increases with increase in and decreases with decrease in mass. A heavy object will have more inertia than the lighter one.

🔹 In other words, the natural tendency of an object that resists the change in state of motion or rest of the object is called inertia.

🔹 Since a heavy object has more inertia, thus it is difficult to push or pul a heavy box over the ground than the lighter one.

❇️ Momentum :-

🔹 Momentum is the power of motion of an object.

🔹 The product of velocity and mass is called the momentum. Momentum is denoted by ‘p’.

🔹 Therefore,

• Momentum of the object = Mass x Velocity Or, p = m×v
• Where, p= momentum, m = mass of the object and v= velocity of the object.

❇️ Momentum and Mass and Velocity :-

🔹 Since momentum is the product of mass and velocity (p = m × v) of an object. This means momentum is directly proportional to mass and velocity. Momentum increases with increase of either mass or velocity of an object.

🔹 This means if a lighter and a heavier object is moving with same velocity, then heavier object will have more momentum than the lighter one.

🔹 If a small object is moving with great velocity, it has tremendous momentum. And because of momentum, it can harm an object more severely.

🔹 For example :- a small bullet having a little mass even kills a person when it is fired from a gun.

🔹 Usually, road accidents prove more fatal because of high speed than in slower speed. This happens because vehicles running with high speed have greater momentum compared to a vehicle running with slower speed.

❇️ Momentum of an object which is in the state of rest :-

• Let an object with mass ‘m’ is in the rest.
• Since, object is in rest, therefore, its velocity, v= 0
• Now, we know that
• Momentum = mass × velocity Or p = m × 0 = 0

🔹 Thus, the momentum of an object in the rest i.e., non – moving, is equal to zero.

❇️ Newton’s Second Law of Motion :-

🔹 According to Newton’s second law of motion, the rate of change of linear momentum of a body is directly proportional to the external force applied on the body, and this change takes place always in the direction of the applied force.

🔹 Now, the rate of change of linear momentum of a body can be obtained by dividing “change in linear momentum’ of the body by the ‘time taken’ for this change. Thus, according to Newton’s second law of motion,

🔶 Mathematical formulation of Newton’s second law of motion :-

• m = mass of body.
• u = initial velocity of the body along a straight line,
• F = an external force applied on the body, which is constant in magnitude,
• t = time for which the force is applied
• v = Final velocity of the body along the same straight line, after t second.

❇️ Third Law of Motion :-

🔹 To every action there is an equal and opposite reaction.

🔶 Applications :-

• Walking is enabled by IIIrd law.
• A boat moves back when we deboard it.
• A gun recoils.
• Rowing of a boat.

🔶 Law of Conservation of Momentum :-

• When two (or more) bodies act upon one another, their total momentum remains constant (or conserved) provided no external forces are acting.
• Initial momentum = Final momentum
• Suppose, two objects A and B each of mass m₁ and mass m₂ are moving initially with velocities u₁ and u₂ strike each other after time t and start moving with velocities v₁ and v₂ respectively.
• Now,
• Initial momentum of object A = m₁u₂
• Initial momentum of object B = m₁u₂
• Final momentum of object A = m₁v₂
• Final momentum of object B = m₁v₂
• So,
• Rate of change of momentum in A, F₂ = m₁v₁ – m₁u₁/t m₁ (v₁ – u₁) /t
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