You are Here: Home >< Physics

# Help with circular motion question! watch

1. The question is the following:

A "car" of mass 1800kg in an amusement park moves through the lowest part of a loop of radius 8.5m at 22ms^-1.

Draw a free body force diagram for the car at this moment and calculate the upward force of the rails on the car.

I've attached what the diagram should look like according to the answers.

The answer given is as follows:

From Newton’s second law:
(mv^2)/r = P - mg
? P= m (v^2/r + g)
P =120 kN

What I don't understand is:

- what P is... I think its supposed to be the upward force of the rails on the car.
- Why the first bit of the answer isn't (mv^2)/r + P = mg because the centripetal force surely acts radially inwards, therefore upwards according to the diagram, and P would also act upwards...

Would really really appreciate any help that you could give me!!!!!!!!!!

Attached Images

2. (Original post by Ninachuuu)
The question is the following:

A "car" of mass 1800kg in an amusement park moves through the lowest part of a loop of radius 8.5m at 22ms^-1.

Draw a free body force diagram for the car at this moment and calculate the upward force of the rails on the car.

I've attached what the diagram should look like according to the answers.

The answer given is as follows:

From Newton’s second law:
(mv^2)/r = P - mg
? P= m (v^2/r + g)
P =120 kN

What I don't understand is:

- what P is... I think its supposed to be the upward force of the rails on the car.
- Why the first bit of the answer isn't (mv^2)/r + P = mg because the centripetal force surely acts radially inwards, therefore upwards according to the diagram, and P would also act upwards...

Would really really appreciate any help that you could give me!!!!!!!!!!

mate, P is the tension in the rail that keeps the car in the circular motion..hence it is the tension that provides the centripetal force....the answer to the question is

(MV^2)/r + mg..........basically this requires you to find the resultant force on the car......solve the equation I am pointed out..that should give you the answer..keep me updated..

### Related university courses

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: March 26, 2011
The home of Results and Clearing

### 1,890

people online now

### 1,567,000

students helped last year
Today on TSR

### GCSE results day guidance

All you need to know is here

### University open days

1. Bournemouth University
Wed, 22 Aug '18
2. University of Buckingham
Thu, 23 Aug '18
3. University of Glasgow