Which ones (if any) of these sentences are true?

[RogerS]

a) Absolute motion is a redundant concept

b) There is no value in the concept of absolute space

c) No matter how fast we run, a light beam will still recede away from us at the speed of light

d) Imagine a car 4m long trying to fit into a garage 3.9m long. If the car travels faster than 22% of the speed of light then it will shrink enough to fit

e) Friction between the seas and the earths' surface causes the moon to drift slightly farther away from earth

 

[StevieB]

a) Absolute motion is a redundant concept

b) There is no value in the concept of absolute space

c) No matter how fast we run, a light beam will still recede away from us at the speed of light

d) Imagine a car 4m long trying to fit into a garage 3.9m long. If the car travels faster than 22% of the speed of light then it will shrink enough to fit

e) Friction between the seas and the earths' surface causes the moon to drift slightly farther away from earth

a) Not redundant if the fixed point you are referencing to is effectively motionless (think celestial body) but in absolute terms, since the universe is still expanding then nothing is motionless, so in a pedantic kind of way absolute motion is impossible to compute.

b) see a) hypothetical but useful

c) tricky, my gut reaction would be light speed minus speed you run, but as this is the minutest % then to all intents and purposes this is correct. Caesium clocks and time dilation shows how speed effects the passage of time, but you run alot slower than a plane flies.

d) Impossible to say - no car will ever travel that fast (even with the Stig driving) and stopping it in the garage will be impossible as once it stops it will expand again and not fit.

e) No idea - the moons gravitational pull causes the tides, so effectively if friction causes us to pull further away gravitational effects become less, which reduce friction, which stops the drift.... will it not reach equilibrium?

how did I do?! 2/5 I suspect

Steve

 

[Anonymous]

i am guesing these are theories which can never be proven, regardless of how good the hadron collider is those particle collisions are not at the speed of light.

with regards to D, the car is a fixed size object it wouldn't expand (lets forget heat causing the materials of the car to expand when travelling at that kind of speed) nor would it shrink.

E) i do believe the moon is creaping further away from us, but not sure why?

 

[PeterBassett]

a) Don't know
b) Don't know
c) True
d) I make that the car will be 2mm too long to fit at 22%C. If you round down it will fit, but only for a very short amount of time. ;-)
e) True

 

[Lons]

D: Sounds like my missus trying to get into a garage - would demolish it so pointless argument :lol: :lol:

 

[RogerS]

In fact they are all correct according to Professor Brian Cox and Albert Einstein.

I can highly recommend 'Why does e=mc2' by Brian Cox and Jeff Forshaw.

The one statement that I can't really get my head round is (c). Common sense suggests that if you had a mate holding a flashlight and you then ran fast enough then you could get to the front of the lightbeam and run alongside it. But if we are to follow Maxwell's equations to the letter, then no matter how fast we run, the beam still recedes away from us at the speed of light. If it were not so then it would mean that the speed of light is different for your mate with the flashlight than it is for you. But since the speed of light is a constant, this can't be so.

Must go and lie down now as my brain is starting to hurt again.

 

[woodbloke]

Must go and lie down now as my brain is starting to hurt again.

Mine started to hurt after I read the first question :lol: - Rob

 

[PeterBassett]

e is because the moon pulls the seas via gravity. This causes the seas to bulge towards the moon and away from the moon, on the front and back of the earth. Front and back here with respect to the moon.

Now, if the earth were tidally locked to the moon, this bulge alone would actually cause the moon to eventually crash into the earth.

However, since the earth rotates once in 24 hours and the moon orbits once in a month or so the face that the earth presents to the moon is always moving forward. It rotates in the same direction as the moons orbit.

Due to friction with the sea this movment drags the bulge of tidal water slightly ahead of where it would be if the earth were tidally locked.

Since the bulge is now ahead of the moon in its orbit the extra mass there pulls at the moon. Pulls it forward along it's orbit, causing its orbital velocity to increase.

Since the velocity has increased it must orbit higher.

Pete

 

[Benchwayze]

I do know that the Earth's gravitational pull causes 'Moon-quakes' that are more severe and longer lasting than quakes on Earth.

Now ... Brain burning out I'm afraid. Absolutely...

John

 

[kasandrich]

Heres another puzzler for you,

If I stand facing an oncoming train, and when it gets into range I fire a pea shooter at it, will the train stop?

My case is that the pea will bounce off, doing a complete change of direction 180degrees, as it changes direction, there must be a point in time where it is stationary, and not going in either direction, you must therefore deduce that the pea stops, when the pea is stationary it will be in contact with the front of the train, therefore it follows that if the pea is stationary and in contact with the train, the train must also be stationary. :roll:

 

[bugbear]

Heres another puzzler for you,

If I stand facing an oncoming train, and when it gets into range I fire a pea shooter at it, will the train stop?

My case is that the pea will bounce off, doing a complete change of direction 180degrees, as it changes direction, there must be a point in time where it is stationary, and not going in either direction, you must therefore deduce that the pea stops, when the pea is stationary it will be in contact with the front of the train, therefore it follows that if the pea is stationary and in contact with the train, the train must also be stationary. :roll:

This reasoning assumes that both peas and trains are perfectly inelastic, which is (obviously) not the case.

BugBear

 

[RogerS]

I like that one, Richard.

Here's another. Imagine a treadmill large enough to stick a plane on. Now fire up the planes engines to take off speed and set the treadmill going until it also reaches take-off speed.

Question - will the plane take-off?

 

[kasandrich]

I like that one, Richard.

Here's another. Imagine a treadmill large enough to stick a plane on. Now fire up the planes engines to take off speed and set the treadmill going until it also reaches take-off speed.

Question - will the plane take-off?

Yes because a planes engines work on the air not the ground.

 

[dannykaye]

a) True

b) True

c) two light beams travelling towards each other an observer travelling with one can measure the passing speed of the other and it will be c not 2c

d) difficult because it is shorter dur to dilation, you could never stop it tho

e) True

here's one that I can never satisfy myself about, an astronaut is floating in space at the centre of a perfect spherical mirror. he turns on a flashlight, what does he see

 

[Jacob]

Heres another puzzler for you,

If I stand facing an oncoming train, and when it gets into range I fire a pea shooter at it, will the train stop?

My case is that the pea will bounce off, doing a complete change of direction 180degrees, as it changes direction, there must be a point in time where it is stationary, and not going in either direction, you must therefore deduce that the pea stops, when the pea is stationary it will be in contact with the front of the train, therefore it follows that if the pea is stationary and in contact with the train, the train must also be stationary. :roll:

This reasoning assumes that both peas and trains are perfectly inelastic, which is (obviously) not the case.

BugBear

Makes no (difference). Except that if elastic, not all particles of the train and/or pea are traveling at the same time with the same speed and direction.

 

[barkwindjammer]

Eat your peas up and get back to work :?

 

[RogerS]

I like that one, Richard.

Here's another. Imagine a treadmill large enough to stick a plane on. Now fire up the planes engines to take off speed and set the treadmill going until it also reaches take-off speed.

Question - will the plane take-off?

Yes because a planes engines work on the air not the ground.

I don't think so. The plane needs airflow to generate lift. It's wheels might be going round and the engines at full-blast but there's no airflow.

 

[RogerS]

a) True

b) True

c) two light beams travelling towards each other an observer travelling with one can measure the passing speed of the other and it will be c not 2c

d) difficult because it is shorter dur to dilation, you could never stop it tho

e) True

here's one that I can never satisfy myself about, an astronaut is floating in space at the centre of a perfect spherical mirror. he turns on a flashlight, what does he see

I'm not sure that he will see anything if it is a perfect mirror. By perfect, any light will be reflected back down it's own path and to the torch. By perfect there will be no internal light scatter inside the mirror so there would be no stray photons coming out at other angles to then bounce off other parts of the mirror. Am I right?

 

[barkwindjammer]

a jet or prop would be thrusting the aircraft forward, its going to move forward and colide with the giant treadmill, the treadmill is in a static position

 

[RogerS]

Heres another puzzler for you,

If I stand facing an oncoming train, and when it gets into range I fire a pea shooter at it, will the train stop?

My case is that the pea will bounce off, doing a complete change of direction 180degrees, as it changes direction, there must be a point in time where it is stationary, and not going in either direction, you must therefore deduce that the pea stops, when the pea is stationary it will be in contact with the front of the train, therefore it follows that if the pea is stationary and in contact with the train, the train must also be stationary. :roll:

This reasoning assumes that both peas and trains are perfectly inelastic, which is (obviously) not the case.

BugBear

Makes no (difference). Except that if inelastic, not all particles of the train and/or pea are traveling at the same time with the same speed and direction.

That is so wrong. If the objects are elastic (which all objects on this planet are to some extent) then the energy from the collision gets absorbed by each object by deforming and then into heat, I believe.

I'm still trying to work out, theoretically, what would happen if they were both inelastic....common sense says that the initial premise should hold and yet if both are stationary then there would have to be a time - (a noticeable time due to the inertia in the train) while the train got back up to speed.