# Confused about kinetic energy

#### Someperson

The equation for kinetic energy is:

k = 1/2*m*v2

This confuses me because this indicates it take more and more energy to accelerate...
Lets imagine a rocket in space (assume the mass of the rocket stays the same when accelerating) - the formula suggests it would take more energy (therefore fuel) to go from 50 to 100 km/s than it would from 0 to 50 km/s.

Is it true that it would take more fuel to accelerate from 50 to 100 km/s than it would from 0 to 50 km/s ?????

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#### GiuseppeR7

Kf-Ki=work done (energy inserted)=(1/2)m(Vf-Vi)^2
the energy required to change the speed of the rocket depends on Vf-Vi not on none of the speeds alone.

#### Someperson

I still don't get it:

Imagine 2 rockets, 1 accelerates to 100 m/s and the other accelerated to 50 m/s then to 100 m/s (so both rockets end with the same end velocity)
both have a mass of 1 Kg

Kinetic energy of the first rocket:
1/2*1Kg*(100m/s)2 = 5000 J

Kinetic energy of the second rocket:
1/2*1Kg*(50m/s - 0m/s)2 + 1/2*1Kg*(100m/s - 50m/s)2 = 1Kg*(50m/s)2 = 2500 J

They should end with the same kinetic energy...

#### GiuseppeR7

lol we have both done a SERIOUS mathematical mistake

#### Nugatory

Mentor
Kf-Ki=work done (energy inserted)=(1/2)m(Vf-Vi)^2
the energy required to change the speed of the rocket depends on Vf-Vi not on none of the speeds alone.
The part in bold is wrong (although by coincidence it gives the right answer if one of Vf or Vi happen to be zero).

The correct formula is $K_f-K_i = \frac{1}{2}mV_f^2-\frac{1}{2}mV_i^2$.

yeah XD

#### Nugatory

Mentor
Imagine 2 rockets, 1 accelerates to 100 m/s and the other accelerated to 50 m/s then to 100 m/s (so both rockets end with the same end velocity)
both have a mass of 1 Kg

Kinetic energy of the first rocket:
1/2*1Kg*(100m/s)2 = 5000 J

Kinetic energy of the second rocket:
1/2*1Kg*(50m/s - 0m/s)2 + 1/2*1Kg*(100m/s - 50m/s)2 = 1Kg*(50m/s)2 = 2500 J

They should end with the same kinetic energy...

#### GiuseppeR7

if you have a rocket, in general the final speed is given by this: Vf=g ISP Ln(Final Mass/Initial Mass).
Where the ISP is defined as the Thrust/Wheight of fuel flow.

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#### Someperson

So I see that it does take more fuel to accelerate at a constant rate the higher the velocity? this seems very un-intuitive to me. In every physics simulator I've played (e.g. KSP) fuel always seems to be consumed at the same rate - is this just an illusion or are the physics wrong in these games?

This seems like it leads to a paradox:
Imagine 2 reference frames one moving at 0 m/s and the other at 1 m/s
A rocket it also moving at 1 m/s - it then accelerates to 2 m/s.
The rocket weighs 1 Kg.

The 2 reference frames must agree on the amount of fuel being used (right?)
The first reference frame (the one moving at 0 m/s) thinks the rocket gained 1.5 J of kinetic energy
however the second reference frame thinks the rocket gained 0.5 J of kinetic energy

If burning x amount of fuel release kx amount of energy and both reference frames agree that x amount of fuel was burned then both must also agree on k as well however as shown above they don't.

what is the mistake I am making?

#### jbriggs444

Science Advisor
Homework Helper
are you sure? the "energetic efficency" of a rocket depends almost only on the chemistry of it, not on the speed.
Yes, it is a fact -- if you compute "energetic efficiency" as the energy added to the payload by a fixed quantity of fuel. The fuel delivers a fixed delta-v. For a rocket moving at greater speed, a fixed delta-v corresponds to a greater increase in payload energy. This is the Oberth effect.

It seems like a violation of conservation of energy until you consider the energy in the exhaust stream.

• mic*

#### GiuseppeR7

Intuitively: the rocket is creating a thrust, the rocket is moving so work is done, energy from the chemistry is released. But at higher speed (a being constant because T is) in a unit time a change in speed is a*t but in that time much more space is done by the rocket and so the work is bigger since it is W=T*space and so the energy transfered.
The Kinetic Energy is of course dependent on the frame of reference!
jbriggs444: yeah, now i get to what you where referring...

#### PeroK

Science Advisor
Homework Helper
Gold Member
2018 Award
So I see that it does take more fuel to accelerate at a constant rate the higher the velocity? this seems very un-intuitive to me. In every physics simulator I've played (e.g. KSP) fuel always seems to be consumed at the same rate - is this just an illusion or are the physics wrong in these games?

This seems like it leads to a paradox:
Imagine 2 reference frames one moving at 0 m/s and the other at 1 m/s
A rocket it also moving at 1 m/s - it then accelerates to 2 m/s.
The rocket weighs 1 Kg.

The 2 reference frames must agree on the amount of fuel being used (right?)
The first reference frame (the one moving at 0 m/s) thinks the rocket gained 1.5 J of kinetic energy
however the second reference frame thinks the rocket gained 0.5 J of kinetic energy

If burning x amount of fuel release kx amount of energy and both reference frames agree that x amount of fuel was burned then both must also agree on k as well however as shown above they don't.

what is the mistake I am making?
I don't know if you ride a bicycle. But, it's easy to get up to about 30 kmph. Especially if the wind is behind you. But, at about this speed it gets very difficult to accelerate further. You can tell it's not just the air or the mechnical inefficiency. At that speed, all your power is used to generate a small force that overcomes the various small retarding forces.

You may be getting confused with a constant force creating constant accleration. But, the faster you go, the more Power is required to maintain a constant force. That's what you feel when you reach your maximum cycling speed. You have no more power to generate an accelerating force.

To deal with your paradox. Suppose two cars are moving alongside each other. The cars are not moving relative to each other, but both cars know they are moving relative to the road. And, as that is where the accelerating force must act, they will understand fuel consumption etc.

Another example would be if you are on a moving walkway, going at, say, 10kpm. And someone is running alongside it. It's much easier for you to accelerate relative to the walkway than it is for the runner to accelerate.

If you move to space where there is no medium to get a grip on, then you need other means of propulsion and this complicates the issue. But, the same principle applies: the fuel consumption would depend on the mechanism and that mechanism would be observed and the amount of fuel consumption understood by another observer regardless of the relative motion.

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#### Dale

Mentor
are you sure? the "energetic efficency" of a rocket depends almost only on the chemistry of it, not on the speed.
Dr Stupid is definitely correct on this. The efficiency of a rocket engine most definitely does depend on the speed.

It is best to analyze a rocket using conservation of momentum first, since that forces you to consider the exhaust. Then, when you apply conservation of energy it is easier to remember that the exhaust has kinetic energy too. Most of the fuel's energy goes into accelerating the exhaust, not the rocket.

• mic*

#### Dale

Mentor
If burning x amount of fuel release kx amount of energy and both reference frames agree that x amount of fuel was burned then both must also agree on k as well however as shown above they don't.

what is the mistake I am making?
The mistake is neglecting the KE that goes into the exhaust. That is why I always recommend starting with conservation of momentum for rockets.

#### Drakkith

Staff Emeritus
Science Advisor
2018 Award
The mistake is neglecting the KE that goes into the exhaust. That is why I always recommend starting with conservation of momentum for rockets.
Does the fact that you have to accelerate the fuel as well as the rocket have anything to do with this?

#### GiuseppeR7

Dr Stupid is definitely correct on this. The efficiency of a rocket engine most definitely does depend on the speed.

It is best to analyze a rocket using conservation of momentum first, since that forces you to consider the exhaust. Then, when you apply conservation of energy it is easier to remember that the exhaust has kinetic energy too. Most of the fuel's energy goes into accelerating the exhaust, not the rocket.
yeah, i was thinking about another "efficiency" definition.

#### jbriggs444

Science Advisor
Homework Helper
Does the fact that you have to accelerate the fuel as well as the rocket have anything to do with this?
The Oberth effect (increasing efficiency with increasing speed) and the Tsiolkovsky rocket equation (increasing speed as the log of the fuel to payload ratio rather than in direct proportion to fuel consumption) are independent effects in the sense I think you mean.

#### Dale

Mentor
Does the fact that you have to accelerate the fuel as well as the rocket have anything to do with this?
Both facts are certainly important in a real rocket, but I think that the thing which causes the confusion of the OP is mostly forgetting the KE of the exhaust rather than the fuel-as-payload effect. You can contrive scenarios to neglect the fuel-as-payload effect, but the KE of the exhaust effect is always there if you look at different frames.

E

#### ellis1954

are you sure? the "energetic efficency" of a rocket depends almost only on the chemistry of it, not on the speed.
I did not want to delve into the efficiency of the rocket. I was just giving the general physical reason that a rocket works I.e why it has a time rate of change in acceleration. Since there is a time rate of change in acceleration there must be a time rate of change in velocity. Since there is jerk for rockets the time rate of change in velocity is non linear. There are even terms for the time rate of change of jerk but generally these are stated as the number if derivatives from position. The following link will take you to an explanation of the preceding.

https://en.m.wikipedia.org/wiki/Jerk_(physics)

#### GiuseppeR7

In physics there are no explanations to "why" stuff work. There are only descriptions about it. One of the explanations for the rocket can be: the rocket create a thrust because of the pressure distribuition inside the nozzle, the gas accelereted trough the nozzle is an effect of the pressure distribuition. Honestly i can not understand anything of what you are saying.

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