Gravity and friction
As usual, home education leads me to realise that I don’t know anything about anything (and to think I trained as a ‘professional’ teacher, supposedly so much better than a parent at educating children – er I don’t think so!).
Joe chose a science kit to do this morning, and it was all about forces. Gravity, friction etc. demonstrated by the standard ‘toy car down a ramp’ experiment, etc., which he really enjoyed. Then we got onto centrifugal force, spinning things around his head and in bottles, etc. At which point I had some vague recollection of there being centripetal force as well, and went to look up the difference between the two, thinking that centrifugal was an outwards circular force and centripetal being the opposite but now I’m completely confused, because this article says centrifugal force is not really a force at all!
Anyway it must be time for coffee! Hopefully by the time I get back someone might have educated me …
April 27th, 2007 at 10:05
I’ve now re-educated myself that centrifugal force is a pseudo force as it is a reaction against the centripetal force.
I think.
Time for coffee at this end too!
I’m sure I have things I need to be doing around here . . .
April 27th, 2007 at 12:22
I seem to remember that although not a force in itself a reaction against it is a real force, or something, but then I only got a D in GCSE science.
April 27th, 2007 at 13:21
Imagine you have a little electric car.
You switch it on and put it on the floor and set it going. It will run in a straight line until something stops it.
Now you put a little fan pointing sideways on top of the cart. You switch the fan and the car on. The fan blows air at right angles to the line of travel of the cart.
Now the car runs not in a straight line, but curves gently as the fan pushes against it. Eventually, it runs back round to where it started from.
The force exerted by the fan acts as a centripetal force, and pushes towards the centre of the circle.
Now put the cart and the fan in a heavy metal box. Put the box on the back parcel shelf of your car and drive to a straight stretch of road. Accelerate up to 100mph and do an emergency stop.
The pain in the back of your head is caused by the momentum of the metal box which made it want to continue travelling in a straight line until something stopped it.
This same effect is what causes the sensation of a force pushing out from a centre as in a fairground ride.
April 27th, 2007 at 13:26
In that case I think I have had centrifugal and centripetal muddled up in my mind for about the last 20 years …
April 27th, 2007 at 14:02
Probably caused by the impact to the back of your head…
April 27th, 2007 at 14:36
Your little car doesn’t need to be powered. It will run along the floor in a straight line after you let go until something stops it – friction with the floor will do nicely thanks.
Centrifugal force is the ‘pseudoforce’ you feel if you swing something around your head – you have to pull it in on its string to keep it going in a circle (centripetal) so it is sensible to think of the centrifugal force as a reaction (opposite) force. Except its not there.
The thing on the end of the string won’t fly off from the centre (radially) when you let go; it will fly off (in a straight line) tangentially (like skimming off the edge of the circle).
Gravity is centripetal. The Earth is kept in orbit because it is pulled toward the Sun. If the Sun’s gravity were switched off we’d fly off in a straight line forever, a la Space 1999.
I’ll be testing Joe on this next time I see him.
April 27th, 2007 at 15:17
I just KNEW Rog would get involved! So let me think what we’ll be talking about over dinner….
November 6th, 2007 at 19:29
I THINK IT MIGHT BE BECAESE THE PERSON IN THE CAR MIGHT HAVE A BIG HEAD