# AS-level SUVAT / Newtonian physics-based question

1. Homework Statement
Diver enters the water at 7.92 [ms][/-1] and comes to rest 1.6m below the surface.

Calculate the average total upward force acting on the diver which brings his verticle velocity to zero.

From an earlier part of the questions the divers mass is 72kg.

2. Homework Equations
Thus far I have solved this by equating the verticle kinetic engery of the diver on entering the water with the work done by the water in decelerating him to a stop.

The answer I get is [average decelerating force]\approx[/1411N]

The model answer however states that the total average upward force is 1411 + 706 = 2117N.

My question is therefore from where do they / should I get the other 706N from?

3. The Attempt at a Solution
[tex]\Delta[/\frac{1}{2}m[v][/2]] = decelerating force x 1.6m

Decelerating force = 0.5 x 72 x [7.92][/2] / 1.6 = 1411N

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gneill
Mentor
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From an earlier part of the questions the divers mass is 72kg.
...

My question is therefore from where do they / should I get the other 706N from?
It's weighty problem.

It's weighty problem.
Is this meant to be a joke or a hint at the solution?

It did make me consider whether they expect the student to realise that to come to rest the diver would have to be neutrally bouyant in order to properly come to rest under water.

Downward force due to gravity = mass x gravity = 72 x 9.8 = 705.6N

Therefore equivalent upward force would be necessary for diver to come to a stop.

It appears I've found a route to the answer but this seems like a hell-of-an insight for a 16yr old student to have without any 'nudges' towards considering neutral bouyancy in the question??