Gareth56

Does the force one experiences when holding a ball underwater come then from the fact that the water is trying to return to its original position, because it (the water) obviously prefers to be back where is was and I suppose gravity is trying to return it to its original position, but is being prevented by you holding the ball?

Thanks. The problem I'm having is visualizing how water can "inflict" an upward force on an object, in other words what (within the water) is providing the upward force. Is the buoyancy force a result of the downward force when the object placed in the water i.e. the buoyancy force is an example of a reaction force as described by Newton's 3rd Law?

Is it because of Newton's 3rd Law that you find it difficult to push a beachball under water?

Thanks

That's what I thought but someone I know is claiming that he won the NP for both discoveries but only received one prize! Personally I think he's confused but just wanted confirmation from more learned people.

Thanks. Did Einstein do work on both applications or was it just the Photoelectric effect that he worked on?

Is the Photoelectric effect the same thing as the Photovoltaic effect? If not what are the differences?

Thanks

G56

I'm lead to understand that green is the easiest colour to replace when working in a digital format hence green screens etc in films, why is this?

The reason I asked was I read that you can kill yourself falling from a high bridge into a river (or similar scenario) because when you hit the water you're moving too fast for the water to "get out of your way" hence it's like hitting a solid sufrace.

Is it possible to calculate the maximum height that you can dive or jump into a body of water without sustaining serious injury (or worse!!) ?

It's said that Rotation is when an [rigid] object turns about an internal axis e.g. a merry-go-round or gramophone record whereas Revolution is when an object turns about an external axis e.g. the Earth around the Sun.

So why are gramophone record speeds given in terms of Revolutions Per Minute (RPM) and not Rotations Per Minute because the whole of the record is rotating about an internal axis?

Rotating vectors and frames of reference, oh dear now it's getting really confusing for a layman whose interested in physics. I'll just have to accept that Work is not a vector.

Thanks to all.

Couldn't that argument/reasoning be applied to a quantity of Force?

Work doesn't have a direction. Force and displacement do, but the dot product gets rid of direction information.

If I push a car with a certain Force in a certain Direction for a certain Distance then wouldn't I have performed Work on said car? And as the Force has been applied in a certain direction (say easterly) isn't that a vector?

Sorry to appear dim here but I still cannot see why something such as Work which has both a magnitude and a direction can be a scalar!

Suppose there is a small oil spill on the road. A wider tire would have a greater chance of at least some of it being NOT on the oil. Therefore the friction between the tire and the road would be approximately that of the Non-oil covered part of the road. In this case, the tires will not slip and the car remains under control.

 

With a small tire, maybe the entire tire surface is then on the oil. Then the friction is that of only the oil surface; which could cause the car to crash.

But on the other hand a wider tyre may have a greater chance of hitting the patch of oil because it's covers a greater surface area unlike a narrower tyre which would have a lower chance of hitting it.

The website more or less covers my query.

Thanks.

Sorry but I still don't understand why F1 cars need such wide tyres.

If area doesn't enter into the equation for the amount of friction, why do racing cars have such wide tyres?

Thanks.

Displacement and force are both vectors, you've actually written the wrong equation there

 

force x displacement

 

Is different to:

 

[math]Work = force \cdot displacement[/math]

 

They are two different ways of multiplying vectors the second (the dot product) results in a scalar value. Whereas the first (the cross product) results in a vector orthogonal to both of the starting vectors.

What I meant when I wrote WD = Force x Dispacement ....was Force multiplied by Dispalcement.

It seems I need to read upon Dot products and Cross products.

Thanks as always.

I understand that Work is not a vector quantity but a scalar. The definition of Work = Force x displacement. So if I apply a force of magnitude X to an object causing a displacement in a certain direction doesn't that constitute a vector?

Thanks DH, I'm clinging on by my fingernails but it's interesting stuff. One thing I came across over here was a gravity wave detector being built. Are these waves that are caused by the force of gravity?

Gravity Waves

Thank you. So a "real force" is a force that you can measure if you're in freefall or moving uniformly?

What are examples of "real forces"?

Sorry swansont you've lost me again

We do not and cannot feel the effects. That is one of the distinguishing features that differentiates pseudo forces from "real" forces. There is no way to measure pseudo forces (e.g., inertial force, centrifugal force, Coriolis force, gravity). You can observe the effects of these pseudo forces in terms of observed acceleration. That leads to the other distinguishing features that differentiate pseudo forces from "real" forces: (1) The magnitude of a pseudo force is proportional to the mass of the object subject to the force, and (2) Pseudo forces vanish in some reference frames (inertial frames).

I thought the gravitational force was a "real" force? Or is gravity not the same as Fgrav ?

Thanks

Thanks D H, I think I followed that however I did think that the car was in intimate contact with the rails.

Would the same happen i.e. a feeling on heaviness at the bottom of a loop if the car was in magnetic contact with the rails like a maglev train?

So if the normal force balances the gravitational force when I'm standing still why does the magnitude of the normal force increase and where does the "extra" normal force come from when I'm travelling through the bottom of roller coaster loop.