Force and Laws of Motion

TOPICS TO BE COVERED

1. Definition of force .
2. Effects of force and types of force.
3. Newton’s first law motion.
4. Inertia.
5. Types of Inertia.
6. Momentum.
7. Newton’s 2nd law of Motion.
8. Newton’s 3rd law Motion.
9. Conservation of momentum .

Force :-
Force is an external agent agent and physical quantity that can change the state of rest or motion and it can it change the shape and size of any objects OR
–Force is a physical quantity or external agent that can change the position, shape and state (rest, motion, velocity, direction) of a body

– Force is a vector quantity ( it has magnitude as well as direction )
– S.I. unit of force is Newton (N) or kg m/s²

Define 1 N of force :-
1 N force is the force when a body having mass of 1 kg accelerate with 1m/sec²

Effects of force :-

A force cannot be seen beat we can feel and observe the effects of force .

A force can produce the following effects :-
I ) force can make a stationary body in motion.
Exp :- football can be set the move by kicking or applied force.

II ) force can stop a moving body.
Exp :- By applying breakers in a moving vehicles live bus and train.

III ) force can change the direction of a moving body.
Exp :- in a cricket match , when a moving cricket ball is hit by a bat , then the direction of cricket ball change and it goes in another direction.

IV ) force can change the speed of a moving body.
Exp :- To accelerate the moving vehicle.

V ) force can change the shape and object.
Exp :- By pushing of pulling or pressing and object like clay.

1. Balance force
2. Unbalance force

1. Balance force :-
If the resultant of all the force is acting on a body is a zero , then the force is called balance force.

Example :-
In the tug of war if both the team applied equal force towards them then rope does not move.

2. Unbalance force :-
If the resultant force acting on a body is not zero. then that force is called unbalanced force.

INERTIA :-
It is the property of a matter or a body by which it resist the change in state of object.
Inertia is tendency of a body due to which it resist are change in its state of rest or uniform motion.
Inertia is directly proportional to the mass.

Types of Inertia
1. Inertia of rest
2. Mirchi of motion
3. Inertia of direction

1. Inertia of Rest :-
It is defined as the tendency of body to remains in its position of rest.
For Example :-
A body at rest remains at rest and can not start moving on its own.

2. Inertia of motion :-
It is defined as the tendency of a body to remains in its state of uniform motion along a straight line.
For Example :-
A body in uniform motion can neither get accelerated nor gat retarded on its own, also it can not stop on its own.

3. Inertia of Direction
It is defined as in ability of body change by itself it’s direction of motion.
OR
The tendency of an object moving along a straight line to continue move along the same direction unless some external force changes its direction on motion.

Newton’s 1st Law of Motion :-
Any object remains in the state of rest or in uniform motion along the straight line, unit it is compressed to change the state by applying external forces.

A body at rest remains at rest and a body in motion will continue in motion in a state line with a uniform speed , until or unless it is compressed by an external force to change it state of rest or uniform motion.

If an object is a rest , it will remains at rest and object is in motion in state line with the same speed until we apply and external force.
Example :-
A person starting in a bus falls backward when bus is starts movie suddenly . This happens because the person and bus both are in rest , while bus is not moving , but as the bus starts moving the legs of the person start moving along with bus , but rest portion of his bodies has tendency to remains in the rest , because of this person falls backward if he is not alert.

Momentum :-
Momentum is the quantity of motion in a body and its depends on its mass and velocity.

Momentum of a body is also defined as the product of its mass and velocity.

Momentum is directly proportional to mass.
Example :-
Is a cricket ball and tennis ball move with the save velocity , momentum of cricket ball is more because its mass is larger than that of the tennis ball.

Momentum is directly proportional to velocity.

Example :-
If two cricket balls move with different velocities , then the momentum of the ball which greater velocity possesses more momentum.

If a body is a rest it’s velocity is zero and hence its momentum is zero.
But every moving body possesses momentum.

Momentum is a vector quantity.
S.I. unit of momentum is Kg m/s.
C.g.s. unit of momentum is gm cm/s.

Newton’s 2nd Law of Motion :-

1. 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.

2. The change in momentum is represented as Δp, thus:

3. The change in momentum is further expressed as the difference in momentum, where mv is final momentum and mu is the initial momentum, and t is the time:

4. Factoring out the mass (m) gives:

5. The term v – u is recognized as acceleration (a):

6. To convert the proportionality to an equation, a constant of proportionality (K) is introduced

7. In SI units, the constant of proportionality (K) is equal to 1, thus:

8. This final equation is Newton’s second law of motion.
The image also includes two rearrangements of this equation, showing that:

9. The final statement summarizes the concept in word

Application of Newton’s 2nd law :-

A Cricket player move hand backward on catching a fast moving cricket ball.

When a player moves his hand back, the time taken to stop the ball increases and hence the rate of change of momentum decreases.

Newton’s 3rd Law of motion :-

According to the Newton’s 3rd law of motion :-
Every action there is equal and opposite reaction. If any interaction between two bodies , the force applied by the 1st body on the 2nd is equal and opposite to the force applied by 2nd body on the 1st.

Application of Newton’s 3rd Law :-
In swimming ,
When a person is swimming he push the water back and move forward.

Law of Conservation of Momentum :-

When two or more body act upon one another their total moment remains constant provided no external forces are acting on them.
The sum of momentum of two object before collision and the sum of momentum of two object after collision are equal.

1. Newton’s Second Law for two bodies:

(The negative sign indicates the force exerted by body 1 on body 2 is opposite to the force exerted by body 2 on body 1, according to Newton’s third law)