Help! Simple Problem I Think!

1. Oct 8, 2004

Agahnim

hi i need help with a homework problem for physics: it goes like this:

A roller coaster car of mass 310 kg (including passengers) travels around a horizontal curve of radius 40 m. Its speed is 18 m/s. What is the magnitude and direction of the total force exerted on the car by the track?

_______N at _______° above the horizontal.

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The first answer I tried was calculating the normal force exerted by the track (which was simply mass * gravity, and 90 degrees above the horizontal). That was wrong. Then I tried giving the answer as the centripetal acceleration ((m * V^2)/ r), and the angle as 0 degrees above the horizontal. That was wrong too.

2. Oct 8, 2004

Gokul43201

Staff Emeritus
There are two forces acting on the car : (i) its weight acting downwards (ii) the normal reaction, acting at some angle (not vertically upwards) to be determined.

As a result of these forces, the acceleration of the car is v^2/r inwards.

Draw the free body diagram. Resolve all forces along the x, y directions. Apply Newton's second law to the forces and accelerations along these directions.

PS : Next time, post such questions in "Homework Help"

3. Oct 8, 2004

Agahnim

Hi, thanks for the probleb advice! and next time i'll post it in homework help.

But one thing im not understanding: Wont the normal force be directly upwards and the weight be directly down, since the curve is not banked? Therefore the sum of forces in the y-direction will be ZERO right?

Now for the x-direction: The only thing acting on the car is centripetal acceleration since there's no friction. Therefore that acceleration will be in the x-direction. But since centri. accel. is not really a 'force' it can't be counted. so im still confused.

P.S. If someone experienced can calculate the answer to this problem and let me know how he got it, i would like that!!!

4. Oct 8, 2004

Staff: Mentor

As Gokul43201 stated, there are two forces acting on the car: its weight, acting downwards, and the force that the track exerts on the car, which acts in a direction that you must find. I would not call that latter force a "normal" force, since that assumes it acts in a particular direction: it's safe to assume that the car is somewhat "attached" to the track, not merely riding on top of it.

Yes, vertical forces must add to zero; horizontal forces must provide the needed centripetal force.

(I will move this to the homework help section.)