AS-level SUVAT / Newtonian physics-based question

In summary, the problem involves calculating the average total upward force acting on a diver to bring his vertical velocity to zero. By equating the diver's vertical kinetic energy on entering the water with the work done by the water in decelerating him to a stop, the average decelerating force is found to be approximately 1411N. However, the model answer states that the total average upward force is 2117N, implying an additional upward force of 706N. This can be attributed to the downward force due to gravity (705.6N) which must be countered by an equivalent upward force for the diver to come to a stop. This may not be immediately obvious to a 16-year-old student without any hints or
  • #1
DJsTeLF
47
0

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.

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?


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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  • #2
DJsTeLF said:
...
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.
 
  • #3
gneill said:
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 realize 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??

Comments welcome
 
  • #4
You don't need the details about buoyancy to solve the problem. All you need to know is that the work done to bring him to a halt has to take into account the change in potential energy due to gravity as well as the kinetic energy he needs to lose. Hence the hint about gravity (okay, it was a joke, too). The result is that there's an extra force acting upward (you don't care about the source) to counteract the downward force due to his weight.
 
  • #5


The additional 706N is most likely the weight of the diver (72kg x 9.8m/s^2 = 705.6N) which is acting in the opposite direction of the upward force. This weight force must be taken into account when calculating the total average upward force on the diver. Therefore, the total average upward force would be 1411N + 706N = 2117N.

In order to fully understand the concept of forces acting on the diver, it is important to consider the forces acting on the diver before and after he enters the water. Before entering the water, the only force acting on the diver is his weight, which is equal to his mass multiplied by the acceleration due to gravity. As he enters the water, an additional force is applied to him by the water, causing him to decelerate and eventually come to rest at a depth of 1.6m.

To calculate the average total upward force acting on the diver, we must consider both the force applied by the water and the force of gravity. The force applied by the water can be calculated using the equation F = ma, where F is the force, m is the mass of the diver, and a is the deceleration caused by the water. This force will be equal to the decelerating force calculated in the attempt at a solution.

However, we must also consider the force of gravity acting on the diver. This force will be equal to his weight, which is acting in the opposite direction of the upward force. Therefore, the total average upward force will be the sum of the decelerating force and the weight force.

In summary, the additional 706N is the weight force acting on the diver, which must be taken into account when calculating the total average upward force. This is because the weight force and the upward force are in opposite directions and must be balanced in order for the diver to come to rest at a depth of 1.6m.
 

1. What is AS-level SUVAT / Newtonian physics-based question?

AS-level SUVAT / Newtonian physics-based question is a type of question that involves applying the principles of SUVAT equations and Newtonian physics to solve a problem or scenario. It is commonly found in AS-level physics exams.

2. What are SUVAT equations?

SUVAT equations are a set of equations used to describe the motion of an object in a straight line. They stand for: S (displacement), U (initial velocity), V (final velocity), A (acceleration), and T (time). These equations are derived from Newton's second law of motion and can be used to solve various kinematic problems.

3. How do I approach an AS-level SUVAT / Newtonian physics-based question?

The best way to approach an AS-level SUVAT / Newtonian physics-based question is to first identify the given variables and the unknown variable. Then, use the appropriate SUVAT equation to solve for the unknown variable. It is important to pay attention to the units and to use consistent units throughout the calculation.

4. What are some common mistakes to avoid when solving AS-level SUVAT / Newtonian physics-based questions?

Some common mistakes to avoid when solving AS-level SUVAT / Newtonian physics-based questions include using the wrong equation, not converting units, and not showing the correct units in the final answer. It is also important to double check calculations and to use significant figures when appropriate.

5. How can I improve my skills in solving AS-level SUVAT / Newtonian physics-based questions?

To improve your skills in solving AS-level SUVAT / Newtonian physics-based questions, practice is key. Make sure to understand the underlying concepts and principles, and then attempt a variety of practice questions. It can also be helpful to work with a study group or seek help from a tutor if needed.

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