9th class physics new book chapter 3 question answer
9th class physics new book chapter 3 question answer.
Q1:- Explain the concept of force by practical examples.
Force is an agency which changes or tries to change the state of an object. Force transfers energy to the object.
Pushing a door, kicking a ball, pulling a door and squeezing a ball are examples of force. Friction, gravity, compression and tension are also forces.
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Q2:- Describe Newton’s laws of motion.
Newton’s first law of motion:-
A object does not change its state of rest or motion in a straight line unless an external force is applied on it.
Newton’s first law is also called law of inertia. Inertia is that property of matter due to which it resists to change its state.
Examples of Newton’s first law of motion:-
A book on a table at rest will be at rest unless an external force is applied on it.
A car moving on a straight road will continue it unless an external force is applied on it
Newton’s second law of motion:-
When we apply a force on an object, it produces acceleration in it and this acceleration is directly proportional to the applied force and inversely proportional to the mass of that object.
$$a\;\propto F\\a\;\propto\frac1m\\After\;combining,\;we\;get\\a\propto\frac Fm\\a\;=\;k\;\frac Fm\\If\;\\a=\;1ms^{-2}\\F\;=\;1\;N\\m\;=\;1\;kg\\then\\K\;=\;1\\So\\a=\frac Fm\\of\\F\;=\;ma$$
Which is mathematical form of Newton’s second law of motion
Examples of Newton’s second law of motion:-
If we apply the same force on two balls, small ball will accelerate faster than a heavy ball.
Hitting a ball harder makes it go faster.
Newton’s Third law of motion:-
For every action, there is always an equal and opposite reaction.
It can also be expressed as of one body exerts a force on the second body. the second body also exerts an equal and opposite force on the first body.
Examples of Newton’s third law of motion:-
Lets the check the force on a book which is lying on the table. The force acting downward on the book is the weight. The book exerts a downward force on the table equal to its weight. The table also exerts a reaction force on the book.
When a bullet is fired from a gun, the bullet moves in the forward direction as an action and the gun recoils back as a reaction.
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Q3:- Define momentum and express Newton’s 2nd law of motion in terms of change in momentum.
The momentum of a moving body is the product of its mass and velocity.
P = m v
According to Newton’s Second law of motion.
$$F\;=\;m\;a\\F\;=\;m\;(\frac{\triangle v}{\triangle t})\\F\;=\;\frac{m\;(\;v_f\;-\;v_i)}{\triangle t}\\F\;=\frac{\;m\;v_f\;-\;m\;v_i}{\triangle t}\\F\;=\;\frac{p_f\;-\;p_i}{\triangle t}\\F\;=\;\frac{\triangle p}{\triangle t}$$
Equation shows that rate of change of momentum of a body is equal to the applied force.
Momentum is a vector quantity and its unit is Ns.
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Q4:- State and explain the principle of conservation of momentum.
Law of conservation of momentum states that for an isolated system momentum remains conserved.
Its mean that if no external force acts on an isolated system, the final momentum of the system is equal to the initial momentum of the system.
Let consider a system of two hard interacting balls.
$$Mass\;of\;1st\;ball\;=\;m_1\\Mass\;of\;2nd\;ball\;=\;m_2\\Velcoity\;of\;1st\;ball\;before\;collision=\;v_1\\Velcoity\;of\;2nd\;ball\;before\;collision\;=\;v_2\\Velcoity\;of\;1st\;ball\;after\;collision=\;v_1^\boldsymbol’\\Velcoity\;of\;2nd\;ball\;after\;collision=\;v_2^\boldsymbol’\\$$
When there is a collision of the two balls, there is a transfer of momentum from one ball to the other.
According to law of conservation of momentum
Momentum before the collision = momentum after the collision
$$\;m_1\;v_1+\;m_2\;v_2=\;\;m_1\;v_1^\boldsymbol’+\;m_2v_2^\boldsymbol’$$
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Q5:- Describe the motion of a block on a table taking into account the friction between the two surfaces. What is the static friction and kinetic friction ?
When block is placed on a table, two main forces act on it, weight in the downward direction and normal reaction in the upward direction. if we push it slightly, block will not move due to static friction between the block and the table. As we increase the applied force, the static friction also increases and attains its maximum limit.
When we apply a greater force then the maximum static friction, the block starts moving. When block moves, friction still exists. It is known as kinetic friction.
The force between two solid surfaces is called sliding friction. Sliding friction has two main categories.
Static Friction:-
The force that opposes the start of motion of a body at rest is called static friction. It has maximum value, after which the object will start moving.
$$static\;friction\;is\;denotes\;by\;F_s\;or\;f_s$$
Kinetic Friction:-
The friction which acts when object is moving is called kinetic friction. Its magnitude is generally less than maximum static friction.
$$kinetic\;friction\;is\;denotes\;by\;F_k\;or\;f_k$$
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Q6:- Explain the effect of friction on the motion of vehicles in context of tyre surface and braking force.
Role of tyre Surface:-
The tyres of vehicles are made with rough surfaces to increase friction between road and tyres. This friction prevents slipping, especially when vehicle turns. If tyres are smooth, vehicle can easily skid and lose control.
Role of braking force:-
Friction is produced between tyres and brake pads, when brakes are applied. At that instant friction between tyres and road increases. The Stopping distance becomes larger on slippery roads due to less friction.
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