Physics question?!

Hello,

I don't really know this question.
The question is:
6 (a) (i) How does the velocity of the rocket change as the rocket moves upwards?

[2 marks]

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Give a reason for your answer.

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The answer is, because it decreases as air resistance acts oposite to motion but does anyone know how?

Reply 1

username2491333

It will start off slow, it then accelerates thus is goes faster. It will eventually reach a point that it is travelling at a constant velocity. Also referred to as Terminal Velocity.

The rocket starts slow because it is still speeding up, from stationary, it starts to move. The engines will propel the rocket and it will speed it up. The constant velocity (terminal velocity) is achieved because the forces at work on the rocket is balanced. The force from the engine is equal to the opposing force which (in the earth's atmosphere) is air (air resistance).

Reply 2

username2491333

Original post by Oblivion99

It is fair to say that the faster you go, the more air you will encounter. An example is riding a bike, as you go faster, you will encounter more air. Once the air resistance is equal to the opposing force, you reach terminal velocity (where you travel at a constant velocity).

Reply 3

physicsphysics91

Original post by AljadePalaran

Also handy to think of terms of water, i.e its much harder to run in water than it is to walk.

Reply 4

username2385417

Well going from rest to having any velocity is an acceleration; defined as a change in either speed or direction.

When the forces that are the upward thrust of the engine and the force of air resistance that acts in the opposite direction to the thrust become balanced, the rocket will have reached terminal velocity. It's speed will become constant.

I don't think the answer requires any more detail than that at this level.

Reply 5

username2491333

Original post by leavingthecity

The level of detail depends on what your're on. For GCSE level, it is expected as it is taught in the syllabus.

Reply 6

username2385417

Original post by AljadePalaran

The level of detail depends on what your're on. For GCSE level, it is expected as it is taught in the syllabus.

The wording of the question suggests to me that OP only needs the level of detail given in most of the posts here including mine. The wording doesn't suggest an understanding of fluid dynamics or anything is what I mean.

Reply 7

username1560589

Answer: Not enough information.

Reason: You need to know the mass of the rocket, mass of the fuel, ejection velocity of the fuel, mass of the planet and have some model for the air resistance.

Reply 8

username2491333

Original post by morgan8002

Answer: Not enough information.

Reason: You need to know the mass of the rocket, mass of the fuel, ejection velocity of the fuel, mass of the planet and have some model for the air resistance.

Really? I admit I may have missed most of it but I am pretty sure the mass of the planet would not have a single grain of significance. Perhaps the gravitational pull but the mass is completely irrelevant. The mass of the rocket is there because it allows the calculation of another force. Weight (mass x gravitational pull). As of the ejection velocity of the fuel, it would have some impact as they judge the up thrust but it is going into too much detail at GCSE level. However I am not sure because GCSEs are getting considerable harder.

Reply 9

username1560589

Original post by AljadePalaran

I forgot the flow rate of the fuel, but you get my point.

If you know the other factors then the mass of the planet gives the same information as the gravitational pull, ignoring the pull of other bodies.

Reply 10

LotusTK

The rocket will initially be accelerating upwards due to the upwards acting thrust force of the engines. At a certain velocity the air resistance will be equal to the thrust of the engines and hence reduce the resultant force acting on the rocket to zero. Since the force is now zero, acceleration will be zero, and the velocity will remain constant.

Vague question but thats probably along the lines of what its after...