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CBSE Class 9 Answered

Hi, my name is Aprameya i had a doubt we know K.E.(Kinetic Energy) = (1/2)mv^2 the v over here is velocity but at what time. Velocity will be different at differnt points of time i mean at 1second it might be something and and at another second it might be something else. so while calculating which velocity do we use
Asked by aprameyachaubeycr7 | 23 Mar, 2020, 02:01: AM
Expert Answer
Consider an example,
An object of mass 5 kg is at rest starts and then start moving with a velocity of 2 m/s and reaches point B from A. 
 
A(u=0).................................B (v = 2m/s)
 
So here in this exampe if you are asked to find the kinetic energy at point A then, 
K.E.A = 1/2 ×mu 
Here, 
u - initial velocity 
u =0 at point A
Thus 
K.E at point A will be zero. 
Now if you want to find kinetic energy at point B, 
K.E. = 1/2 mv2 
v = 2m/s at point B 
Thus, 
K.EB = 1/2 × 5 × 22 
Thus, 
K.E.B = 10 Joule 
Now conisder another example if an object starts moving from point B and reaches point C and at point C the velocity is 4 m/s 
Then the K.E. = 1/2 (5)(4)2 = 40 J
Also the change in the kinetic energy can be calculated using formula, 
K.E. = 1/2 m (v2 - u2)
So in case of motion from B to C the change in kinetic energy is, 
K.E. = K.E.f - K.E.i 
K.E. = 1/2 (5) (42 - 22 ) = 30 J. 
Hope this help you to understand. 
 
Answered by Shiwani Sawant | 23 Mar, 2020, 11:49: AM
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