1 A small ball \(B\) is projected with speed \(30 \mathrm {~m} \mathrm {~s} ^ { - 1 }\) at an angle of \(60 ^ { \circ }\) to the horizontal from a point on horizontal ground. Find the time after projection when the speed of \(B\) is \(25 \mathrm {~m} \mathrm {~s} ^ { - 1 }\) for the second time.
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A uniform object is made by attaching the base of a solid hemisphere to the base of a solid cone so that the object has an axis of symmetry. The base of the cone has radius 0.3 m , and the hemisphere has radius 0.2 m . The object is placed on a horizontal plane with a point \(A\) on the curved surface of the hemisphere and a point \(B\) on the circumference of the cone in contact with the plane (see diagram).
- Given that the object is on the point of toppling about \(B\), find the distance of the centre of mass of the object from the base of the cone.
- Given instead that the object is on the point of toppling about \(A\), calculate the height of the cone.
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[The volume of a cone is \(\frac { 1 } { 3 } \pi r ^ { 2 } h\). The volume of a hemisphere is \(\frac { 2 } { 3 } \pi r ^ { 3 }\).]