1 A small ball of mass 0.8 kg is moving with speed \(10.5 \mathrm {~m} \mathrm {~s} ^ { - 1 }\) when it receives an impulse of magnitude 4 Ns . The speed of the ball immediately afterwards is \(8.5 \mathrm {~m} \mathrm {~s} ^ { - 1 }\). The angle between the directions of motion before and after the impulse acts is \(\alpha\). Using an impulse-momentum triangle, or otherwise, find \(\alpha\).