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Impulse:-

“Impulse of a force is defined as the change in momentum produced by the force and it is equal to the product of force and the time for which it acts”. Therefore, a large force acting for a short time to produce a finite change in momentum which is called impulse of this force and the force acted is called impulsive force or force of impulse.

According to Newton’s second law of motion,

\vec{F}\ =m\vec{a} =m\frac{(\vec{v}-\vec{u})}{t}

or, 

\vec{F}t = m\vec{v}-m\vec{u} = \vec{J}

So, Impulse of a force = change in momentum.

If the force acts for a small duration of time, the force is called impulsive force.

As force is a variable quantity, thus impulse will be,

J = \int_{t_{1}}^{t_{2}}F dt

The area under F - t curve gives the magnitude of impulse.

Impulse is a vector quantity and its direction is same as the direction of  vector-f .

Unit of Impulse:- The unit in S.I. system is kgm/sec or newton -second.

Dimension of Impulse:- MLT-1

Conceptual Questions:-

Question 1:-

Figure below shows a popular carnival device, in which the contestant tries to see how high a weighted marker can be raised by hitting a target with a sledge hammer. What physical quantity does the device measure? Is it the average force, the maximum force, the work done, the impulse, the energy transferred, the momentum transferred, or something else? Discuss your answer.

Answer:-

The device will measure impulse. The impulse of the net force acting on a particle during a given time interval is equal to the change in momentum of the particle during that interval. Since the contestant is hitting the target with a sledge hammer the change in momentum is large and the time of collision is small, therefore it signifies that the average impulsive force will relatively large. Suppose two persons bring the harmer from the same height, but they are hitting with different forces. The person who hits with greater force for the short time interval the impulse will be more and this results the height of the mark will be more. Thus the device will measure impulse.

_____________________________________________________________

Question 2:-

Give a plausible explanation for the breaking of wooden boards or bricks by a karate punch. (See “Karate Strikes.” by Jearl D. Walker, American Journal of Physics, October 1975, p.845.)

Answer:-

In the process, breaking of wooden boards or bricks by a karate punch, the collision between the hand and brick is only for a few milliseconds. Because the applied external force is large and the time of collision is small therefore the average impulsive force is relatively large. Thus when you break a wooden board or bricks by a karate punch you have to apply large force for the minimum time which is impulse. Therefore the impact force on the brick or wooden boards will be high.

 

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