# Roller coaster

## Homework Statement

The normal force equals the magnitude of the gravitational force as a roller coaster car crosses the top of a 60m diameter loop-the-loop.

## Homework Equations

What is the car's speed at the top?

## The Attempt at a Solution

hello, this problem doesn't give much information
would normal force be pointing down?

so Fnet= ma=mv^2/r but its doesnt give those variables

can anyone help?
thanks :)

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LowlyPion
Homework Helper
Don't you have all you need?

Draw a diagram.

m*g down and mv2/r up

But it says the Normal force is still m*g

If the net force must be m*g and you have m*g acting down against the centripetal force, what must the centripetal force be?

Is the net force in the y direction zero?

does the centripetal force = 2mg since normal and gravity both point downward?

im lost since this coaster is upside down

LowlyPion
Homework Helper
Is the net force in the y direction zero?

does the centripetal force = 2mg since normal and gravity both point downward?

im lost since this coaster is upside down
Not lost at all.

The centripetal force is 2*m*g.

so would net force in the y direction = zero?

if it is zero, i dont think it would be solvable because we dont know the mass

wouldnt it be 0=mv2/r

ok, i got the answer but i used the net force=2mg
i still dont understand why this is so

LowlyPion
Homework Helper
ok, i got the answer but i used the net force=2mg
i still dont understand why this is so
FNormal = ∑ F = Fcentrip - Fgravity

m*g = m*v2/r - m*g

2*m*g = m*v2/r