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Friday, 24 February 2017

Law of Motion VIII

Impulse and Impulsive Force
Impulsive Force
The force which acts on a body for very short duration of time but is still capable of changing the position, velocity and direction of motion of the body up to large extent is known as impulsive force.
Example 
1. Force applied by foot on hitting a football.
2. Force applied by boxer on a punching bag.
3. Force applied by bat on a ball in hitting it to the boundary.
4. Force applied by a moving truck on a drum.

Note: Although impulsive force acts on a body for a very short duration of time yet its magnitude varies rapidly during that small duration.

Impulse
Impulse received by the body during an impact is defined as the product of average impulsive force and the short time duration for which it acts.
I = Favg x t

Relation Between Impulse and Linear Momentum
Consider a body being acted upon by an impulsive force, this force changes its magnitude rapidly with the time. At any instant if impulsive force is F then elementary impulse imparted to the body in the elementary time dt is given by
                                                      dI = F x dt
Hence total impulse imparted to the body from time t1 to tis

Graph Between Impulsive Force and Time


With the time on x axis and impulsive force on y axis the graph of the following nature is obtained

Area enclosed under the impulsive force and time graph from t1 to t2 gives the impulse imparted to the body from time t1 to t2


Physical Application
i) While catching a ball a player lowers his hand to save himself from getting hurt.
ii) Vehicles are provided with the shock absorbers to avoid jerks.
iii) Buffers are provided between the bogies of the train to avoid jerks.
iv) A person falling on a cemented floor receive more jerk as compared to that falling on a sandy floor.
v) Glass wares are wrapped in a straw or paper before packing.

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