A Puzzler

[Quandram]

Push a sofa on castors on a moving road. Which happens first

a) sofa goes forward
b) you fall over

b) every time

[ONEDUNME]

Welcome back from Thickieland Seen

[Seen]

Originally Posted by Quandram

Push a sofa on castors on a moving road. Which happens first

a) sofa goes forward
b) you fall over

b) every time

But the force comes the engines - and in this example the force comes from the feet which are touching the road.

I think I might change my mind again.....

[kompressaur]

it seems the scientists aren't 100% agreed, in which case i'd be fucking amazed if TDP comes up with the answer

[Quandram]

Originally Posted by Seen

But the force comes the engines - and in this example the force comes from the feet which are touching the road.

I think I might change my mind again.....

If you attach jet engines to a towerblock would it move?

(assuming it wasn't on castors)

[Doc]

ok two examples for you to consider - thought experiments if you like

1) You have a bicycle suspendened on cables from the ceiling. You spin the front wheel - does the bicycle move backwards or forwards? No it won't (assuming friction in the bearings is neglible)

2) you have a plane flying through the air with its under-carriage down. Can you speed up the plane by spinning the wheels? No

[Quandram]

How did the plane get in the air Doc?

[kompressaur]

if this was down to TDP then

komp (me) would say that he hasnt finished the runway yet but it will be ok to take off soon (repeat for another 2 years) slick would say the problem is with the plane and to just bin the plane idea. Mickey would say that he had the very same idea 2 years ago. Winrew would ask why we need a plane anyhow when a coach would get us there just the same. Kiko would say that he thought the departure lounge was the holiday and wtf are we doing on this plane anyhow. Gett1lots would be out on the tarmac throwing stones at the workmen trying to fix the plane. etc etc

[Doc]

I've got another plane you're all welcome to board Komp

[Doc]

How did the plane get in the air Doc?

I put it there

[Doc]

Okay Quandrum

start from first principles

The plane doesn't move unless a (net) force acts on it. There are only 5 forces here that are relevant

1) Gravity acting downards - constant force in the vicinity of the ground
2) Normal reaction acting upwards - equals the gravity-thrust (or zero at the point of takeoff if we reach that)
3) Lift generated by air rushing over and under the wings
4) Friction of the wheels on the runway (moving or not) . Horizantal and directly proportional to the normal reaction
5) Thurst force acting horizontally (lets assume through the plane of the wheel axles - not true, but simplifies it a little so we don't have to consider centre of gravity etc.


The friction force is independent of speed. What it will do is cause the the wheel centres (i.e. the axles) to accelerate backwards (to the left) and the wheels to rotate clockwise (if we imagine the plane taking off from left to right as we look). Once the thrust overcomes this, there is a net forward force, which will cause the plane to accelerate to the right and take off.

[Kiko]

Yes it would take off. Simple as that.

[Computer Guy]

Originally Posted by Quandram

If you attach jet engines to a towerblock would it move?

(assuming it wasn't on castors)

if the dates September 11th, possibly.

[Quandram]

Originally Posted by Doc

Okay Quandrum

start from first principles

The plane doesn't move unless a (net) force acts on it. There are only 5 forces here that are relevant

1) Gravity acting downards - constant force in the vicinity of the ground
2) Normal reaction acting upwards - equals the gravity-thrust (or zero at the point of takeoff if we reach that)
3) Lift generated by air rushing over and under the wings
4) Friction of the wheels on the runway (moving or not) . Horizantal and directly proportional to the normal reaction
5) Thurst force acting horizontally (lets assume through the plane of the wheel axles - not true, but simplifies it a little so we don't have to consider centre of gravity etc.


The friction force is independent of speed. What it will do is cause the the wheel centres (i.e. the axles) to accelerate backwards (to the left) and the wheels to rotate clockwise (if we imagine the plane taking off from left to right as we look). Once the thrust overcomes this, there is a net forward force, which will cause the plane to accelerate to the right and take off.

the only way the plane can move right is if the wheels move it that way. thus the runway moves quicker counter acting it so you get no forward movement and no take off.

[Doc]

you're missing a key point quandram - the wheels aren't driven - the only reason they rotate is the friction from the ground.

Look at it from the perspective of the forces - once the thrust overcomes the friction (which won't be long) - and the friction is independent of speed - the plane will start to move forward, with the wheels slipping on the ground