## Physics IX Notes Chapter 3 Dynamic

**Q.1 Define and explain force?**

** Ans. a force:-**an agent which moves or tends to move a stationary body stops or tends to stop a moving body or tends to change the speed or direction of moving body is known as force.

Force is a vector quantity and its unit is Newton. One Newton is that force that moves a body of one kg with the acceleration of 1m/sec^{2}.

**Q.2 state and explain or Newton 1**^{st} law of motion?

^{st}law of motion?

** Ans. Newton’s 1st law:-**this law states that in the absence of any external force a body in rest will always remain in rest and a body in motion will always continue its motion in a straight line with uniform speed.

** Explanation:-**this law has two parts which are explained below.

In the 1^{st} part, it is stated that in the absence of any external force a stationary body will always be at rest, for example, a book is laying on the table so it will remain stationary and will not change it’s by itself unless someone moves it by applying a force.

In 2^{nd} parts it is stated that moving will always move along a straight line unless someone stops it by applying a force but when a ball is kicked it comes to the state of rest after covering some distance it is because the force friction of air and the earth acts upon the ball each moment which compels it to stop.

**Q.3 state and explain law of inertia?**

**Ans.1 inertia:-**Newton’s 1

^{st}law of motion is also known as inertia “inertia” is the property of a body which opposes any change in its state of rest or motion.

** Explanation:-**inertia depends upon the mass of a body in fact mass is the measure of inertia. Greater the mass of a body greater will be inertia greater force will be required to stop it e.g the inertia of hard cricket ball is greater than the inertia of soft ball and greater force will require to stop hard ball.

** Examples**:-(1) when a jerk is given to the branch of a tree all fruits falls down from it because when branch comes into motion at on at that time the fruits in rest and tends remain in rest as a result + they separated and fall down.

** (2)**when a carpet is beaten with a stick. The carpet set into motion suddenly while the dust particles are at rest and tends to remain at rest and tends to remain at rest as a result they fly off from carpet.

** (3) **when bus suddenly stops the lower past of passenger comes into rest while the upper past is still in motion and tends to remain in motion as a result the passenger falls forward.

** (4) **when a person jumps from a moving vehicle, his feet come in contact with ground and comes to state of rest while the upper part is still in motion and tends to remain in motion, As a result person gets hurt.

** (5) **A parcel laying on the seat of a car move with same as that of the car whe breaks are suddenly applied the comes into rest but parcel is still in motion and tends to remain in motion due to inertia as a result the parcel falls into the floor.

**Q.4 state and explain Newton’s 2**^{nd} law in detail?

^{nd}law in detail?

**Ans. statement:-**this law states that when a force acts on a body on acceleration is produced in direction of force which is directly proportional to the applied force but inversely proportional to the mass of the body.

** Explanation:-** if we apply a force “f” on a body of mass “m” and it produces acceleration “a” in the direction of applied force, the it mathematical from is given as

a f……… (i) a 1/m……….(ii)

Comparingeq (i) and eq(ii) we get,

a f. q/m => a f/m

=>a = (k)f/m ………….(iii)

If f = 1 N mass = 1kg then k = 1 so by putting

k = 1 we get a = f/m =>f/,m => = ma ……(iv)

Equation (iv) represents the mathematical form of 2^{nd}eq: of motion.

If we apply equal force on two bodies of different masses, the acceleration of the lighter body will be greater than the acceleration of the heavy body. Similarly greater the applied force greater will be the acceleration produced.

** Example**:-when a cricketer catches the ball he moves his hands backwards. By doing so the acceleration of ball decreases and the force of ball caching in hand is also decreases and by catching the ball the cricketer feels less fain.

**Q.5 state and explain Newton’s 3**^{rd} law with the help of examples?

^{rd}law with the help of examples?

**Ans. Newton s third law:-** states that for every action there is a reaction which is equal in its magnitude but opposite in direction.