# Terminal Velocity and Upthrust

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Can someone please help me?!! I really need help in this!! I'm having a test on Fluids in Physics tomorrow, and I have only these 2 questions and I'm not sure....

a) If the upthrust/ air resistance is smaller than the weight, then will it decrease the terminal velocity of a skydiver?

b) And if it does decrease the terminal velocity of a skydiver, then does that also mean, that the skydiver will reach it's terminal velocity faster?

a) If the upthrust/ air resistance is smaller than the weight, then will it decrease the terminal velocity of a skydiver?

b) And if it does decrease the terminal velocity of a skydiver, then does that also mean, that the skydiver will reach it's terminal velocity faster?

Last edited by Aleksander Krol; 3 months ago

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Can someone please help me?!! I really need help in this!! I'm having a test on Fluids in Physics tomorrow, and I have only these 2 questions and I'm not sure....

a) If the upthrust/ air resistance is smaller than the weight, then will it decrease the terminal velocity of a skydiver?

b) And if it does decrease the terminal velocity of a skydiver, then does that also mean, that the skydiver will reach it's terminal velocity faster?

**Aleksander Krol**)Can someone please help me?!! I really need help in this!! I'm having a test on Fluids in Physics tomorrow, and I have only these 2 questions and I'm not sure....

a) If the upthrust/ air resistance is smaller than the weight, then will it decrease the terminal velocity of a skydiver?

b) And if it does decrease the terminal velocity of a skydiver, then does that also mean, that the skydiver will reach it's terminal velocity faster?

If the upthrust/air resistance is smaller than the weight there is no terminal velocity because there is a resultant downwards force. The skydiver in that situation is accelerating downwards.

2. As his speed increases the air resistance also increases.

If, eventually the air resistance (plus upthrust) is equal to his weight, the upwards and downward force (weight) are balanced.

At this point the resultant force is zero and the skydiver will continue at that constant falling speed while those conditions remain.

That is terminal velocity.

3. The smaller the air resistance the greater the terminal velocity.

4. The smaller the air resistance the longer it takes to reach that terminal velocity. (With no air you never reach it!)

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(Original post by

1. Referring to a)

If the upthrust/air resistance is smaller than the weight there is no terminal velocity because there is a resultant downwards force. The skydiver in that situation is accelerating downwards.

2. As his speed increases the air resistance also increases.

If, eventually the air resistance (plus upthrust) is equal to his weight, the upwards and downward force (weight) are balanced.

At this point the resultant force is zero and the skydiver will continue at that constant falling speed while those conditions remain.

That is terminal velocity.

3. The smaller the air resistance the greater the terminal velocity.

4. The smaller the air resistance the longer it takes to reach that terminal velocity. (With no air you never reach it!)

**Stonebridge**)1. Referring to a)

If the upthrust/air resistance is smaller than the weight there is no terminal velocity because there is a resultant downwards force. The skydiver in that situation is accelerating downwards.

2. As his speed increases the air resistance also increases.

If, eventually the air resistance (plus upthrust) is equal to his weight, the upwards and downward force (weight) are balanced.

At this point the resultant force is zero and the skydiver will continue at that constant falling speed while those conditions remain.

That is terminal velocity.

3. The smaller the air resistance the greater the terminal velocity.

4. The smaller the air resistance the longer it takes to reach that terminal velocity. (With no air you never reach it!)

I've checked the ms, and the answer to it was given as, "Upthrust is smaller than weight and there is a small decrease in terminal velocity" I don't really get it. how is there a "small decrease in terminal velocity"?? :/

P.s. btw thanks for taking your time and explaining it in the best way as possible. it was lucid.

Last edited by Aleksander Krol; 3 months ago

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but there was this question, where they've only given upthrust as 4N and they've asked the following, "Comment on the size of this force and its effect on the skydiver's terminal velocity."

I've checked the ms, and the answer to it was given as, "Upthrust is smaller than weight and there is a small decrease in terminal velocity" I don't really get it. how is there a "small decrease in terminal velocity"?? :/

P.s. btw thanks for taking your time and explaining it in the best way as possible. it was lucid.

**Aleksander Krol**)but there was this question, where they've only given upthrust as 4N and they've asked the following, "Comment on the size of this force and its effect on the skydiver's terminal velocity."

I've checked the ms, and the answer to it was given as, "Upthrust is smaller than weight and there is a small decrease in terminal velocity" I don't really get it. how is there a "small decrease in terminal velocity"?? :/

P.s. btw thanks for taking your time and explaining it in the best way as possible. it was lucid.

Firstly 'upthrust' is not normally the word for air resistance. Upthrust is what is described by Archimedes Principle.

So what is the 4N force? Is it air resistance or Archimedes upthrust?

If it is the upthrust, this force is constant and due only to the weight of the fluid displaced. (The volume of air displaced by the diver). So it remains at 4N.

On the other hand, the air resistance increases as the skydiver accelerates and falls faster.

Normally in these cases the 'upthrust' is ignored, as the air resistance, when it eventually reaches the same value as the skydiver's weight, is around 800N (for a body of mass around 80kg).

The 4N Archimedes upthrust is negligible.

I think what the question is asking is, does this small, constant, 4N upthrust force have any effect on the terminal velocity?

The answer is

**yes**but a very small effect: 4N being only 0.5% of the weight of the diver.

However, this small 4N force is assisting the air resistance, as it is also acting upwards on the body.

Bearing in mind that the larger the upward force the larger the total 'drag', then this small increase in the drag due to this 4N upthrust will slow the diver down a little bit more.

So his terminal velocity will be a little bit slower as he has been slowed down a little bit more.

I hope this helps.

Please ask again if there is any doubt remaining.

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(Original post by

It's difficult to reply with only parts of questions quoted. But....

Firstly 'upthrust' is not normally the word for air resistance. Upthrust is what is described by Archimedes Principle.

So what is the 4N force? Is it air resistance or Archimedes upthrust?

If it is the upthrust, this force is constant and due only to the weight of the fluid displaced. (The volume of air displaced by the diver). So it remains at 4N.

On the other hand, the air resistance increases as the skydiver accelerates and falls faster.

Normally in these cases the 'upthrust' is ignored, as the air resistance, when it eventually reaches the same value as the skydiver's weight, is around 800N (for a body of mass around 80kg).

The 4N Archimedes upthrust is negligible.

I think what the question is asking is, does this small, constant, 4N upthrust force have any effect on the terminal velocity?

The answer is

However, this small 4N force is assisting the air resistance, as it is also acting upwards on the body.

Bearing in mind that the larger the upward force the larger the total 'drag', then this small increase in the drag due to this 4N upthrust will slow the diver down a little bit more.

So his terminal velocity will be a little bit slower as he has been slowed down a little bit more.

I hope this helps.

Please ask again if there is any doubt remaining.

**Stonebridge**)It's difficult to reply with only parts of questions quoted. But....

Firstly 'upthrust' is not normally the word for air resistance. Upthrust is what is described by Archimedes Principle.

So what is the 4N force? Is it air resistance or Archimedes upthrust?

If it is the upthrust, this force is constant and due only to the weight of the fluid displaced. (The volume of air displaced by the diver). So it remains at 4N.

On the other hand, the air resistance increases as the skydiver accelerates and falls faster.

Normally in these cases the 'upthrust' is ignored, as the air resistance, when it eventually reaches the same value as the skydiver's weight, is around 800N (for a body of mass around 80kg).

The 4N Archimedes upthrust is negligible.

I think what the question is asking is, does this small, constant, 4N upthrust force have any effect on the terminal velocity?

The answer is

**yes**but a very small effect: 4N being only 0.5% of the weight of the diver.However, this small 4N force is assisting the air resistance, as it is also acting upwards on the body.

Bearing in mind that the larger the upward force the larger the total 'drag', then this small increase in the drag due to this 4N upthrust will slow the diver down a little bit more.

So his terminal velocity will be a little bit slower as he has been slowed down a little bit more.

I hope this helps.

Please ask again if there is any doubt remaining.

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