1:- What is Force?

Ans:- The push or pull of an object is called force. It is a physical quantity that can change the

speed, state, direction and dimension of an object.

A force has both magnitude and direction, so it is a vector quantity.

The SI unit of force is Newton.

The SI base units‎: ‎kg·m/s²

The force is represented by 'F'

Ex:- playing a guitar, scrubbing the floor, rowing the boat, kicking a ball, knocking a nail,

launching a rocket, pushing a trolley, opening the door, pulling a box, tug-of-war,

raising a flag, drawing back a bow

2:- What is push?

Ans:-A force that is used to move an object away from the body is called push.

Ex:-rowing the boat, kicking a ball, knocking a nail, launching a rocket, pushing a trolley

3:- What is pull?

Ans:- A force that is used to move an object towards the body is called pull.

Ex:-opening the door, pulling a box, tug-of-war, raising a flag, drawing back a bow

5:- What is acceleration?

Ans:- The rate of change in velocity in per unit of time is called acceleration. It has both fixed

direction and magnetude. so it is a vector quantity.

acceleration is denoted by the symbol 'a'

The SI unit of acceleration is m/s²

4:- what is the formula of Force?

Ans:- Since mass is the direct proportional of force and acceleration is also direct proportional of force. therefor

F ∝ m

F ∝ a

F ∝ m × a

F = k m × a

k is a proportionaly constant whose value in SI unit is 1

Therefor , F = ma

The SI unit of mass (m) = kg

The SI unit of acceleration (a) = ‎m/s²

^{so the base unit of Force =}kg. m/s²

5:- What force would be needed to produce an acceleration of 4m/s² in a ball of

mass 6 kg?

Solve:- m = 6kg

a = 4m/s²

F = m × a

= 6 × 4 kg m/s²

= 24 kg m/s²

= 24 N

4:- What is mass?

Ans:- Mass is the matter contained in a body. It has magnitude but does not have direction

so it is a scaler quantity.

Mass is shown by the symbol 'm'.

The SI symbol of mass is kg.

5:- What is Weight?

Ans:- Weight is the force of gravity by which earth attracts any object towards its centre.

SI unit of weight is Newton and it is denoted by 'N'

Weight = mass * acceleration due to gravity near the Earth's surface

= mass * g

where g = 9.8 m/s²

example: If a person has a mass of 100kg, what is his weight?

What is a Balanced Force?

Ans:- A force where two forces of equal size, act on a body, in opposing directions is known as a Balanced Force. In Balanced Force, a body continues to be in its position i.e. it may be continuously moving with the same speed and in the same direction or it may be still in its position.

Ex:- in a tug of war, if there is no movement in the rope, the two teams are exerting equal, but opposite forces that are balanced. Again, the resulting force (net force) is zero.

An apple hangs from a tree, the weight of the apple is balanced by the force exerted by the branch on the apple.

A stationary object is an example of balanced force.

What is Unbalanced Force?

Ans:- A force where two forces acting in opposing directions on a body, are not equal in magnitude and in size, is known as an unbalanced force. In an unbalanced force, a body changes its position. Unbalanced force also helps an object to change its speed and direction.

👉 When unbalanced forces are acted in a same direction Then the resultant force is the sum of the forces applied. and the direction of resultant force is the same of the applied force.

👉 When unbalanced forces are acted from different direction Then the resultant force is the difference of the forces applied. and the direction of resultant force is in the direction of greater force.

What is Net Force?

The net force is the force which is the sum of all the forces acting on an object simultaneously. Net force can accelerate a mass.

Practice Problems

1. A 50 kg on at table mass has an extra 20 N acting down on it and a 20 N force acting to the right. What is the net force on the mass?

2. A mass has a 20 N force acting to the right on it and a 12 N force acting to the left on it. What is the net force on the mass.

3. A 100 kg mass is moving at 20 m/s. It is being pushed by a machine from the right with a 74 N force. What does the friction force have to be to keep the 100 kg mass moving with constant velocity?

Newton's law of motion

The father of physics Newton introduce The Newton's law of motion in his book prinspiya in 1687

What is Newton's first law?

Ans:- An object continues to remain at rest or in a state of uniform motion along a straight line unless an external unbalanced force acts on it . This postulate is also known as law of inertia

👉Newton's first law is also known as Galileo's law or law of Inertia.

👉Newton's first law gives the definition of force.

What is law of inertia?

A property of matter by which it continues in its existing state of rest or uniform motion in a straight line, unless that state is changed by an external force is called inertia.

The law of inertia states that if the forces acting upon an object are balanced, then the acceleration of that object will be 0 m/s/s. Objects at equilibrium will not accelerate. According to Newton, an object will only accelerate if there is a net or unbalanced force acting upon it

If msdd

equilibrium = the condition in which all forces balance

Some examples of Newton's first law or Inertia

👉 Wearing a seat belt in a car while driving.

👉 गोली मारने से कांच में छेद हो जाना

👉कम्बल को डंडे से प्रहार करने पर धूल कण का झड़ जाना

👉स्थिर वस्तु के अचानक गतिशील हो जाने पर सवार व्यक्ति का पीछे की ओर झुक जाना

👉चलती हुई मोटर कार के अचानक रुकने पर उसमे बैठे यात्री का आगे की ओर झुक जाना ( A driver of automobile brakes abruptly and, by inertia, shoots forward. )

👉A stone in the ground is in a state of rest.

👉A book lying on the table remains at rest as long as no net force acts on it.

👉A moving object does not stop moving by itself.

What is momentum?

Ans:- Momentum is the quantity of motion of a moving body. It is used to measure the amount of motion in a body. it is the product of mass and velocity of the body. if an object has mass and have motion than it also has momentum.

it is a vector quantity.

It is denoted by p.

The SI unit of momentum is kg.m/s

p ∝ m

p ∝v

p = m × v

here p = momentum

m = mass

v = velocity

Notes:- 1:- When mass is increased than the momentum will also increase

Ex:- tennis ball heats less than the cricket ball because tennis ball has less mass than cricket ball. hens cricket ball has more momentum.

2:- When velocity is increased then the momentum will also increase

Ex:- bullet that is fired from gun has more velocity than the cricket ball thrown by cricketer. hens bullet has more momentum.

Q:- What is the momentum of a man of mass 75 kg when he walks with a uniform velocity of 2m/s

Q:- An object's momentum is 80 kg m/s and its mass is 16000g. find the velocity of the object.

Q:- An elephant of mass 2000 kg moving at 5m/s. calculate the momentum.

Q:- A bullet of mass 0.02kg moving at 400m/s. calculate the momentum.

Change in Momentum=Mass x (v - u)

Newton's second law of motion

The rate of change of momentum of a body is directly proportional to the applied force and takes place in the direction in which the force acts

F ∝ ∆ p

F ∝ 1 / t

F ∝ ∆ p × 1/t

F = k ∆ p × 1/t

k is a proportionaly constant whose value in SI unit is 1

F = ∆ p × 1/t

F = m (v - u ) / t

F = ma

Force = mass × acceleration

Newton's second law provides us the formula of force

Newton's third law of motion

Newton’s Third Law of Motion states that

“When one object exerts a force on a second object, than the second object also exerts a force on the first that is equal in magnitude and opposite in direction.”

This law is sometimes called the “Law of Action and Reaction”.

Force exerted by first body is called action.

Force exerted by second body is called reaction.

Every action has an equal and opposite reaction.

Ex:-

1: A swimmer swimming forward
The swimmer pushes against the water (action force), the water pushes back on the swimmer (reaction force) and pushes her forward.

2: A ball is thrown against a wall

The ball puts a force on the wall (action force), and the wall puts a force on the ball (reaction force) so the ball bounces off.

3: A space shuttle lifts upward

The Space Shuttle engines push out hot gases (action force), and the hot gases put a force on the shuttle engines (reaction force) so the shuttle lifts (there is no sling shot doing it!)

Notes:- In newton's third law action and reaction force are equal in magnitude and opposite in direction but they do not cancel each other

The action - reaction forces do not cancel each other because they are acting on two different objects.

The action and reaction forces are reciprocal on a object.

Law of conservation of momentum

The sum of momenta of the two objects before collision is equal to the sum of momenta after the collision provided there is no external unbalanced force acting on them. this is known as the law of conservation of momentum.

Let two object 'A' and 'B' whose mass is M_A and M_B respectively. They are travel on straight path on same direction at different velocities u_Aand u_B .

Force and Pressure - class 9

1:- What is Force?

Ans:- The push or pull of an object is called force. It is a physical quantity that can change the

speed, state, direction and dimension of an object.

A force has both magnitude and direction, so it is a vector quantity.

The SI unit of force is Newton.

The SI base units‎: ‎kg·m/s2

The force is represented by 'F'

Ex:- playing a guitar, scrubbing the floor, rowing the boat, kicking a ball, knocking a nail,

launching a rocket, pushing a trolley, opening the door, pulling a box, tug-of-war,

raising a flag, drawing back a bow

2:- What is push?

Ans:-A force that is used to move an object away from the body is called push.

Ex:-rowing the boat, kicking a ball, knocking a nail, launching a rocket, pushing a trolley

3:- What is pull?

Ans:- A force that is used to move an object towards the body is called pull.

Ex:-opening the door, pulling a box, tug-of-war, raising a flag, drawing back a bow

5:- What is acceleration?

Ans:- The rate of change in velocity in per unit of time is called acceleration. It has both fixed

direction and magnetude. so it is a vector quantity.

acceleration is denoted by the symbol 'a'

The SI unit of acceleration is m/s2

4:- what is the formula of Force?

Ans:- Since mass is the direct proportional of force and acceleration is also direct proportional of force. therefor

F ∝ m

F ∝ a

F ∝ m × a

F = k m × a

k is a proportionaly constant whose value in SI unit is 1

Therefor , F = ma

The SI unit of mass (m) = kg

The SI unit of acceleration (a) = ‎m/s2

so the base unit of Force = kg. m/s2

5:- What force would be needed to produce an acceleration of 4m/s2 in a ball of

mass 6 kg?

Solve:- m = 6kg

a = 4m/s2

F = m × a

= 6 × 4 kg m/s2

= 24 kg m/s2

= 24 N

4:- What is mass?

Ans:- Mass is the matter contained in a body. It has magnitude but does not have direction

so it is a scaler quantity.

Mass is shown by the symbol 'm'.

The SI symbol of mass is kg.

5:- What is Weight?

Ans:- Weight is the force of gravity by which earth attracts any object towards its centre.

SI unit of weight is Newton and it is denoted by 'N'

Weight = mass * acceleration due to gravity near the Earth's surface

= mass * g

where g = 9.8 m/s2

example: If a person has a mass of 100kg, what is his weight?

What is a Balanced Force?

Ans:- A force where two forces of equal size, act on a body, in opposing directions is known as a Balanced Force. In Balanced Force, a body continues to be in its position i.e. it may be continuously moving with the same speed and in the same direction or it may be still in its position.

Ex:- in a tug of war, if there is no movement in the rope, the two teams are exerting equal, but opposite forces that are balanced. Again, the resulting force (net force) is zero.

An apple hangs from a tree, the weight of the apple is balanced by the force exerted by the branch on the apple.

A stationary object is an example of balanced force.

What is Unbalanced Force?

Ans:- A force where two forces acting in opposing directions on a body, are not equal in magnitude and in size, is known as an unbalanced force. In an unbalanced force, a body changes its position. Unbalanced force also helps an object to change its speed and direction.

👉 When unbalanced forces are acted in a same direction Then the resultant force is the sum of the forces applied. and the direction of resultant force is the same of the applied force.

👉 When unbalanced forces are acted from different direction Then the resultant force is the difference of the forces applied. and the direction of resultant force is in the direction of greater force.

What is Net Force?

The net force is the force which is the sum of all the forces acting on an object simultaneously. Net force can accelerate a mass.

Practice Problems

1. A 50 kg on at table mass has an extra 20 N acting down on it and a 20 N force acting to the right. What is the net force on the mass?

2. A mass has a 20 N force acting to the right on it and a 12 N force acting to the left on it. What is the net force on the mass.

3. A 100 kg mass is moving at 20 m/s. It is being pushed by a machine from the right with a 74 N force. What does the friction force have to be to keep the 100 kg mass moving with constant velocity?

Newton's law of motion

The father of physics Newton introduce The Newton's law of motion in his book prinspiya in 1687

What is Newton's first law?

Ans:- An object continues to remain at rest or in a state of uniform motion along a straight line unless an external unbalanced force acts on it . This postulate is also known as law of inertia

👉Newton's first law is also known as Galileo's law or law of Inertia.

👉Newton's first law gives the definition of force.

What is law of inertia?

A property of matter by which it continues in its existing state of rest or uniform motion in a straight line, unless that state is changed by an external force is called inertia.

The law of inertia states that if the forces acting upon an object are balanced, then the acceleration of that object will be 0 m/s/s. Objects at equilibrium will not accelerate. According to Newton, an object will only accelerate if there is a net or unbalanced force acting upon it

If msdd

equilibrium = the condition in which all forces balance

Some examples of Newton's first law or Inertia

The SI base units‎: ‎kg·m/s²

The SI unit of acceleration is m/s²

The SI unit of acceleration (a) = ‎m/s²

^{so the base unit of Force =}kg. m/s²

5:- What force would be needed to produce an acceleration of 4m/s² in a ball of

a = 4m/s²

= 6 × 4 kg m/s²

= 24 kg m/s²

where g = 9.8 m/s²