Choose which statements are true or false. For those that you choose as being fa

Question

Choose which statements are true or false. For those that you choose as being false, explain why:

a)If momentum is conserved, so is kinetic energy
b) A ball is thrown into the air. Its momentum is the same at the same height on the way up as on the way down.
c) A ball is thrown into the air. Its kinetic energy is the same at the same height on the way up as on the way down.
d) A ball is thrown into the air. Its momentum is conserved while it is in the air.
e) In any explosion, momentum is always conserved

Explanation / Answer

a. If momentum = p and Kinetic energy = KE , then we know , KE = p^2 / 2m Hence if momentum is conserved then so is Kinetic energy . (TRUE ) b . Momentum is a vector and its direction is the same as velocity vector . When the ball is going up the momentum vector is in upward direction but while coming down even though the magnitude of the momentum vector is the same as while going up but its direction is now opposite and in the downward direction . ( FALSE ) c . Kinetic energy is scalar , and and is given by = 0.5 m (|v|^2) , |v| is magnitude of velocity . The magnitude of velocity is at the same height while going up and coming down , so the KE is same for both the cases. (TRUE ) d.The momentum of the ball is not conserved because there is an external force which acts on it , the gravitational pull of the earth . (FALSE ). e. The explosion occurs in a very small period of time and involves large impulse , during this short period we can assume the of gravity to be negligent and hence there are no external forces on the fragments of explosion and the momentum is conserved. (TRUE ) .

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