# Momentum

A body of mass 5kg is hanged still in air. It then explodes into two fragments X and Y with masses 2kg and 3kg respectively. Find the ratio of the momentum of X to Y just after the explosion.

Ans : 1:1

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讓我提供另一個解題方法吧,你慢慢一步一步看跟一跟我的flow

FIRST PART:

Since the body is originally still in air ( v = 0), the initial linear momentum is zero.

AND there is no external force applied to the body.

The total linear momentum is then conserved.

It follows that the total linear momentum after the explosion is also zero.

However, you know that the two fragments are moving at opposite direction.

(Each fragment has linear momentum)

If each fragment has linear momentum but the sum of the momenta is zero,

then they should have linear momentum of same magnitude but of opposite sign.

Hence the ratio of momentum of X to Y just after the explosion is equal to 1:1(if the direction is not our concern)

If the direction is to be considered as well, the ratio will be -1:1 or 1:(-1),

right?

Second PART:

^^

希望你能明白我的邏輯,要完全明白何謂conservation of linear momentum啊!

Remarks:

1) Never use conservaion of energy in explosion problem. (Mechanical Energy of the system does increase!)

Use conservation of linear momentum ^^

2) The correct term is called linear momentum.

3) the expression KE = p^{2} over 2m is quite useful.

But if you have not heard of it, forget about it!

Source(s): CE&AL Physics knowledge