2. Sir Isaac Newton gave us 3 Laws Of Motion, They are :
1st Law-
Everybody continues in its state of rest or uniform motion in straight line unless compelled by
some external force.
2nd Law-
The rate of change of momentum of a body is directly proportional to the applied unbalanced force in
the direction of force.
3rd Law-
To every action there is an equal and opposite reaction.
3.
4. NEWTON’S SECOND LAW OF MOTION STATAES THAT :
i.e.
Where, dp = change in momentum and dt = time taken for this change.
7. Consider a body of mass ‘m’ moving with initial velocity ‘u’.
Let a Force ‘F’ act on the body for time ‘t’ so that the velocity after time ‘t’ is ‘v’.
Initial momentum of body, p1 = mu
Final momentum of body, p2 = mv
M
F
u
t = 0
M
F
v
t = t
8. Now, change in momentum of body
= p2 – p1
= mv – mu
= m(v-u)
Time taken to change this momentum
= (t – 0) = t
Rate of change of momentum =
𝑪𝒉𝒂𝒏𝒈𝒆 𝒊𝒏 𝒎𝒐𝒎𝒆𝒏𝒕𝒖𝒎
𝑻𝒊𝒎𝒆 𝒕𝒂𝒌𝒆𝒏
=
𝒎(𝒗−𝒖)
𝒕
9. According to Newton’s 2nd Law of Motion –
Force applied rate of change of momentum
Or F
𝒎(𝒗−𝒖)
𝒕
………(i)
Since
v = u + at OR
𝒗−𝒖
𝒕
= a
Therefore, eqn. (i) can be written as –
∴ F ma
Or F = kma ……… (ii)
Where K is constant of proportionality.
10. The value of K can be found as:
F = 1 unit
m = 1 unit
a = 1 unit
Then from eqn. (ii) -
1 = k × 1 × 1
Thus, K = 1
And eqn. (ii) becomes -
F = ma
11. We know, Force (F) and acceleration (a) are vector quantities, whereas mass (m) is scalar. Therefore,
Newton’s 2nd law of motion ca be written in vector form as
F = ma
This relation shows that the direction of force applied on the body is same as the acceleration
produced in the body.
12. We know, F = ma
S.I. unit of mass, m = 1 kg
S.I. unit of acceleration, a = 1 ms-2
∴ S.I. unit of Force, = 1 kg ms-2
1 kg ms-2 is also known as 1 Newton or 1N
Also C.G.S. unit of force is,
1 Newton = 105 Dyne
13.
14. A CRICKET PLAYER LOWERS HIS HANDS WHILE CATCHING THE BALL :
If a player does not lower his hand while catching the ball, the time to stop the ball would be very
small. So a large force has to be applied to reduce the velocity of the ball to zero or to change the
momentum of ball. When a player lowers his hands, the time to stop the ball is increased and less
force has to be applied to cause the same change in momentum of ball. Therefore, the hands of the
player are not injured.
15. HUGE DAMAGE TO THE TRAIN TAKES PLACE WHEN IT SUDDENLY COLLIDES
WITH A STATIONARY TRAIN :
When a fast moving train suddenly collides against a stationary train, there is large change in the
momentum of the moving train. This large change in momentum of train is brought about in short time
interval. Hence, a large retarding force acts on the moving train. Due to this large force, huge damage is
done to the moving train.
16. A BOXER MOVES HIS HEAD BACKWARD WHEN HIS OPPONENT BLOWS A
PUNCH:
When a boxer blows a punch on the face of another boxer, then the change in momentum of punch on
striking the face of another player exerts a large force on his face. On the other hand, when the boxer
moves his head backward, then the change in momentum takes place in large time interval. Thus, force
on the face of boxer becomes less.
17. 1. NAME THE UNIT OF FORCE IN C.G.S. SYSTEM.
2. HOW MANY DINES ARE IN ONE NEWTON ?
3. WHY VEHICLES ARE FITTED WITH SUSPENSION OR SPRINGS ?
4. WHAT IS UNIT OF MOMENTUM ?
5. NAME UNIT OF MOMENTUM IN C.G.S. SYSTEM.
6. DEFINE MOMENTUM.
7. IS FORCE VECTOR OR SCALAR QUANTITY ?
