ch-4 Impulse and Momentum

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Momentum:
By Momentum, we mean “Inertia in Motion” or more
specifically, the mass of an object multiplied by its
velocity.
Momentum = mass × velocity
or in shorthand notation,
Momentum = m × v
P=m×v
When direction is not an important factor, we can say
Momentum = mass × speed
 S.I Unit of Momentum :
 Kg-m/sec or N-sec
Impulse Changes Momentum:
The greater the impulse exerted on some thing,
the greater the change in momentum. The exact
relationship is,
Impulse = Change in Momentum
or in shorthand notation,
Ft = Δ(mv)
Where Δ is the symbol for “change in”
Law of Conservation of Momentum
If no net external force acts on a system, the total linear
momentum of the system cannot change.
Or It can be stated that
The sum of a system's initial momentum is equal to the sum
of a system's final momentum.
The law of conservation of momentum can be mathematically
expressed as
Total momentum before collision = Total momentum after collision
P1(initial) + P2(initial) = P1(final) +P2(final)
Impulse Changes Momentum:
The greater the impulse exerted on some thing,
the greater the change in momentum. The exact
relationship is,
Impulse = Change in Momentum
or in shorthand notation,
Ft = Δ(mv)
Where Δ is the symbol for “change in”
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Example
• An average force of 300 N acts for a time of 0.05 s on a
golf ball. What is the magnitude of the impulse acting
on the ball?
Solution
Given data:
F  300N
t  0.05 s,
J?
 J  Ft
 J  (300 N )  (0.05 s)  15 N .s
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• A bowling ball of mass 5kg travels at 2m/s and a tennis ball with
mass of 150g. Can both balls have the same momentum? If yes at
what speed must the tennis ball travel to have same momentum?
solution
V =2m/s
Given data:
m  150g  0.150kg, M  5kg , V  2m / s
m = 150g
M = 5kg
p BB  pTB , v ?
 MV  m v
MV (5kg )(2m / s)
v 

 66m / s
m
0.150kg
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• A baseball of mass 0.15 kg has an initial velocity 0f -20 m/s (moving to
the left) as it approaches a bat. It is hit straight back to the right and
leaves the bat with a final velocity of +40 m/s. (a) Determine the
impulse applied to the ball by the bat. (b) Assume that the time of
contact is 1.6 10 sec , find the average force exerted on the ball by the bat.
(c) How much is the impulse exerted by the ball on the bat?
3
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Solution
(1)
Apply the impulse-momentum theorem
J  mvf  mvi  (0.15kg)(40m / s)  (0.15kg)(20m / s)  9 kg m / s
(2) Apply the equation that defines impulse
 J  Ft
F 
9 kg m / s
J

 1500N
t
0.0060s
(3) Apply Newton’s third law of action and reaction and get
Impulse exerted on the bat by the ball equals -9 kg m/s. The negative sign
indicates a direction to the left of the origin of coordinate system.
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Law of Conservation of Momentum
If no net external force acts on a system, the total linear
momentum of the system cannot change.
Or It can be stated that
The sum of a system's initial momentum is equal to the sum
of a system's final momentum.
The law of conservation of momentum can be mathematically
expressed as
Total momentum before collision = Total momentum after collision
P1(initial) + P2(initial) = P1(final) +P2(final)
Applications
Recoil of a gun: why a rifle recoils when a bullet is fired?
A 10 g bullet is fired from a 3 kg rifle with speed of 500 m/s. What is (a) the initial
momentum of the system (bullet and rifle)? And (b) the recoil speed of the rifle?
p b ) i  mb  v b i  (0.010g )  0  0
p r ) i  m r  v ri  (3kg )  0  0
 p bi  p ri  0  0  0
p bf  p rf  p bi  p ri  0,
b=bullet
r=rifle
i=initial
f=final
 p bf  p rf  0, or
p rf   p bf  (0.010kg )  (500m / s )  5kg  m / s
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A 10 g bullet is fired from a 3 kg rifle with speed of 500 m/s. What is (a) the initial
momentum of the system (bullet and rifle)? And (b) the recoil speed of the rifle?
m1 = 3 kg
m2 = 10 gm = 0.01 kg
v1i = 0 m/sec (before firing)
v2i = 0 m/sec (before firing)
v2f = 500 m/sec (after firing)
P=?
v1f = ? (after firing)
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• Elastic Collision

Is a collision in which the total kinetic energy of the collided objects
after collision equals the total kinetic energy before collision.
P1i + P2i = P1f + P2f
K1i + K2i = K1f + K2f

The collided object bounce a part and return to their original shape
without a permanent deformation
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• Inelastic Collision
 - Is one in which the total kinetic energy of the collided objects after
collision is not equal to the total kinetic energy before collision. The two
object experience a permanent deformation in their original shape
P1i + P2i = P1f + P2f
K1i + K2i ≠ K1f + K2f
 - In completely inelastic collision, the two objects coupled and move as a one
object after collision
In both collisions, conservation of momentum is applied
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Example on Elastic collision
•
A ball of mass 0.6 kg traveling at 9 m/s to the right collides head
on collision with a second ball of mass 0.3 kg traveling at 8 m/s to
the left. After the collision, the heavier ball is traveling at 2.33
m/s to the left. What is the velocity of the lighter ball after the
collision?
solution
Given Data
m1  0.6kg, v1  9m / s, m2  0.3kg, v2  8m / s, v1  2.3m / s, v2 ?
 m1 v 1  m 2 v 2  m1 v1  m 2 v 2
 v 2 
m1 v1  m 2 v 2  m1 v1 (0.6kg )(9m / s)  (0,3kg )(8m / s)  (0.6kg )(2.3m / s )

m2
0.3kg
 v 2  14,6m / s
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•
A kg railroad car traveling at 8 m/s to the east as shown in the drawing
below is collided with another car of the same mass and initially at rest
and couple with it. What is the velocity of the coupled system of cars after
the collision?
m1  m2  1.75104 kg, v1  8m / s, v 2  0, V ?
 m1 v1  m 2 v 2  (m1  m 2 )V
V 
m1 v1  m 2 v 2
m v 0
 1 1
 3.5m / s
m1  m 2
m1  m 2
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