3. A particle \(P\) of mass 2.5 kg moves along the positive \(x\)-axis. It moves away from a fixed origin \(O\), under the action of a force directed away from \(O\). When \(O P = x\) metres the magnitude of the force is \(2 \mathrm { e } ^ { - 0.1 x }\) newtons and the speed of \(P\) is \(v \mathrm {~m} \mathrm {~s} ^ { - 1 }\). When \(x = 0 , v = 2\). Find

1. \(v ^ { 2 }\) in terms of \(x\),
2. the value of \(x\) when \(v = 4\).
3. Give a reason why the speed of \(P\) does not exceed \(\sqrt { } 20 \mathrm {~m} \mathrm {~s} ^ { - 1 }\).