How do I get an intuiutive understanding of the relationship between Power, Energy, Force and Speed?

Question

I try to aid my understanding of physics principles by figuring out appropiate analogies, or qualitatively picturing the 'what is going on' in an intuitive sense.

Obviously this isn't always possible; certain theories being notoriously unintuitive. But classical mechanics usually falls into the intuitive camp.

However, I'm having trouble gaining a 'what is actually happening' picture of the relationship between energy, force and speed. Now I understand the derivations of kinetic energy, the work-energy theorem, and power, etc - it's just the 'picture' I can't get.

As an example, I can't 'see' what is happening to the motive force as an engine revs faster. Assuming a constant power engine, as a car accelerates it's velocity increases and so the motive force decreases. However, I can't 'picture' why. I understand the maths and I can see why this is mathematically true. I just can't visualise why from a physical point of view.

Can anyone give a physical description of the decrease?

Just to clarify, it's not friction, resistance, etc that I can't visualise. It's the following:-

In simplified form, Power=Force x Velocity
Now if something is accelerating at constant power then because velocity is increasing, force must necessarily decrease. This is true even in the absence of all opposing forces, and it is the reason for this decrease that I can see mathematically but not physically.

So forget the car example and think of a spacecraft in outer space. Assume the spaceship has a propulsion mechanism that supplies a constant output power. With the engine on the ship would accelerate. As the ship got faster the propulsion force reduces (because P=Fv=const.). As the force reduces then so does acceleration (F=ma). But the velocity is still increasing, and therefore the force still decreasing. The force will therefore reduce and reduce as the velocity increases. At some velocity the force is almost 0.

But I can't visualise (I can do the math) the cause of this decrease.

Answer 1

To be honest knowing the maths is enough.

Answer 2

Rightly or wrongly, I relate the force/velocity relationship with how a car gets easier to push the faster it moves - as a car gains velocity, its inertia translates into momentum which assists the movement of that car in that particular direction. This also explains why car will coast if you put it into neutral - its momentum is what keeps it going until air resistance, friction, etc. overcomes it and it slows down.

So a car will require less force from its engine to maintain its speed at 100mph than the same car at 30mph (wind resistance and all other factors discounted of course), which is why the car's engine will struggle to maintain 30mph in 5th gear.

Answer 3

Power - Energy - Force and Speed

(Energy "Sun") - (Power "Light") - (Force "Reaction") - (Speed "Movement")

Better Solve This!

Answer 4

what?