A $120kg$ dolphin decelerates from $15m/s$ to $7.5m/s$ in $5s$ to join another dolphin in play. What average force was exerted to slow the first dolphin if it was moving horizontally? (Assume that gravitational force is balanced by the buoyant force of the water)

<p>From equation of uniformly accelerated motion</p><p><span contenteditable="false"> <math> <mi>v</mi> <mo>=</mo> <mi>u</mi> <mo>+</mo> <mi>a</mi> <mi>t</mi> </math> </span></p><p><span contenteditable="false"> <math> <mn>7.5</mn> <mo>=</mo> <mn>15</mn> <mo>+</mo> <mn>5</mn> <mi>a</mi> </math> </span></p><p><span contenteditable="false"> <math> <mi>a</mi> <mo>=</mo> <mo>-</mo> <mn>1.5</mn> <mi> </mi> <mi>m</mi> <mo>/</mo> <msup> <mrow> <mrow> <mi>s</mi> </mrow> </mrow> <mrow> <mrow> <mn>2</mn> </mrow> </mrow> </msup> </math> </span></p><p><span contenteditable="false"> <math> <mi>n</mi> <mi>o</mi> <mi>w</mi> <mi> </mi> <mi>F</mi> <mo>=</mo> <mi>m</mi> <mi>a</mi> </math> </span></p><p><span contenteditable="false"> <math> <mfenced open="|" close="|" separators="|"> <mrow> <mrow> <mover accent="true"> <mrow> <mrow> <mi>F</mi> </mrow> </mrow> <mo>? </mo> </mover> </mrow> </mrow> </mfenced> <mo>=</mo> <mn>120</mn> <mo>×</mo> <mn>1.5</mn> </math> </span></p><p><span contenteditable="false"> <math> <mo>=</mo> <mn>180</mn> <mi> </mi> <mi>N</mi> </math> </span></p>

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