NEWTON’S SECOND LAW OF MOTION
“The rate of change of
linear momentum of a body is directly proportional to the external force
applied on the body, and this change takes place in the direction of the
applied force”
Mathematically:
Consequences
of Newton second law of motion
1. No force is required to move a body
uniformly in a straight line
According to Newton’s second
law of motion
F = ma
If F = 0, then a = 0 Thus
velocity of body is constant.
2. Accelerated motion is always due to an
external force
Force is required for
acceleration i.e. for changing speed, direction or both.
Question 1. An
object is accelerating at 2 m/s2. If the net force is tripled and the
mass of the object is halved, what is the new acceleration?
a will
become 6 times i.e. 6 x 2 = 12m/s^{2}
Impulse
The forces which act on a body for a
short time are called impulsive forces. Impulse of a force is a measure
of total effect of a force. It is given by the product of force and time
for which the force acts on the body.
Impulse
= force x time = change in momentum
Applications of the Concept of Impulse
1. A cricket player lowers his hands
while catching a cricket ball to avoid injury. In this way, he increases
the time of catch to reduce the momentum of the ball to zero. As
Impulse = force x time = change in
momentum
Or force = change in momentum/time,
thus player has to apply a smaller force against the ball in order to
stop it.
2.
An athlete is advised to come to stop slowly, after finishing a fast
race, so that time of stop increases and hence force experienced by him
decreases.
Question 1. If the net force of 10 N was continuously applied on 5 kg
object at rest, how long will it take to raise its velocity to 80 m/s?
Ans. u = 0, v = 80 m/s, F = 10 N, m =
5 kg, t =?
F = ma = m (vu)/t
t = m (vu)/F = 5x (800)/10 =
40s
Question
2. A batsman hits back a ball straight in the direction of the bowler
without changing its initial speed of 12 m/s. If the mass of the ball is
0.15 kg, determine the impulse imparted to the ball.
Ans. m = 0.15 kg, u = 12 m/s, v =  12 m/s
Impulse = m (vu) = 0.15(12 – 12)
=  3.6 Ns
*The
negative sign indicates that the direction of the impulse is from
batsman to the bowler.
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