physics help, about the centripetal force.

ok, why does the centripetal force actually act outwards.

for example in the january 2004 phy 4 paper.
a car is stationary such that R(reaction force) = WEIGHT.
it then goes past the same point with velocity v.

if there is a CENTRIPETAL FORCE which should act towards the centre of motion.

shouldnt the equation be W + C.F = R
as the weight and the centripetal force both act downwards?

don't have the paper and am not entirely sure what you mean in:

"if there is a CENTRIPETAL FORCE which should act towards the centre of motion.

shouldnt the equation be W + C.F = R
as the weight and the centripetal force both act downwards?"

Does it travel around a circle???

There must be an unbalanced force downwards (the centripetal force) so when the car is moving, the reaction force pushing upwards will be less, the weight will be the same so there will be a nett force downwards of magnitude mv^2/r

When the car's at rest, R1 = mg
When it's moving, taking downwards as positive F = ma, mg - R2 = mv^2/r so R2 = m(g - v^2/r)

If the car is going round on a banked (inclined) road, then the reaction increases in order to provide the horizontal force needed to keep it goin round in circles:


. Reaction
. /\
. \
. \
. Car
. |
. |
. \/
. weight

You get:

R.sin(alpha)=mv^2/r

where r is the radius of the circle and alpha is the angle at which it is banked. Does that help?

When the car's at rest, R1 = mg
When it's moving, taking downwards as positive F = ma, mg - R2 = mv^2/r so R2 = m(g - v^2/r)

so your saying the reaction force will decrease, the force mg will always be the same acting downwards.

Yep, because when it's travelling in the circle, it's accelerating so there is an unbalanced force downwards. The weight is constant because its only affected by the mass of the car and the gravitational field strength.

OH shit i get it, cheers m8.
wot examing board r u on

I got 81/90

really
i got 69/90
i need to resit
wot did u get for phy 1 2 3 and how did your phy 5 practical go

I got 300/300 at AS so dropping 9 in phy4 doesn't really bother me! I thought the practical was ok but my tray was on a slope and I found it hard to count the oscillations with the 100g mass on the ruler cos they were so quick

wot uni u wanna go 2, wot course and wot module marks have u got for maths

Magdalen college, Oxford, Physics, P1 100, P2 94, P3 100, M1 98, M2 92 M3 100, S1 86 , S2 92