# Is conservation of linear momentum valid in a non inertial frame? A ball of mass m with velocity v hits a wall and  re bounces after elastic collision. As momentum must be conserved , the momentum of the wall should be 2mv? How is it possible considering the fact that the wall has zero KE but 2mv momentum?

148 Points
14 years ago

Dear jee king

here wall is fixed so mass of the wall is assume as infinite

so moment conservation will not give any important result.

so we use velocity of approach=velocity of seperatiom

you can say that momentum of wall is 2mv

mv +0*∞ =-mv' +v1*∞

this equation does not give any result

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RAJEEV CHOUDHARY
9 Points
14 years ago

2) Momentum is conserved only when there is no external force.But in this case external force is there on the wall.That external force is from the ground.

Nagaraju Kandru
31 Points
14 years ago

2)  here, the system of consideration is the wall.

check, whether there is any external force acting on the system.

certainly, there is some external force(along the normal to the wall) on the wall exerted by the ball.

so, conservation of linear momentum is not applicable to the wall.

If we consider (the wall+the ball) as the system, then there is no external force in horizontal direction.

(assuming, the wall is vertical and the ball is moving horizontally).

so, we can apply conservation of linear momentum.

let 'm' be the mass of the ball and 'M' be the mass of the wall. if we take the wall is rigidly fixed.

then ,

(m)x(v) + (M)x(0) = (m)x(v') + (M)x(0).

=>  v = v'.

=> elastic collision.

Most important point regarding conservation of linear momentum is, THE SYSTEM which we consider.

if there is no external force in a particular direction, then apply conservation of linear momentum in that direction.

if some external force acts, then apply IMPULSE EQUATIONS.

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Regards,

Nagaraju Kandru(MSc)

IIT-Guwahati