First slide
Angular momentum
Question

A projectile of mass m is launched with an initial velocity vi making an angle θ with the horizontal as shown in figure. The projectile moves in the gravitational field of the Earth. The angular momentum of the projectile about the origin when it is at the highest point of its trajectory is

Moderate
Solution

At the highest point of the trajectory,

x = 12R = vi2sin 2θ2g and y = hmax = (vi sin θ)22g

The angular momentum is then

L1 = r1×mv1

      = [vi2sin 2θ2gi^+(visin θ)22gj^]×mvxii^

      = -mvi3sin2θcos θ2gk^

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