Hey there! Sign in to join this conversationNew here? Join for free
 You are Here: Home >< Maths

# M3 Newton's law of gravitation question Watch

1. In the following questionthe partucle is moving towards the centre of the earth which means the same direction as the acceleration. I took it that this means that F = (GmM)/(x^2) instead of F = (-GmM)/(x^2) is my assumption wrong? The correct answer as given in the book only comes if F = (-GmM)/(x^2). I attached my working.
Attached Images

2. (Original post by thebrahmabull)
In the following questionthe partucle is moving towards the centre of the earth which means the same direction as the acceleration. I took it that this means that F = (GmM)/(x^2) instead of F = (-GmM)/(x^2) is my assumption wrong? The correct answer as given in the book only comes if F = (-GmM)/(x^2). I attached my working.
I had this exact problem about a month ago. After asking Zacken , I think he said something on the lines of the reason why the force/acceleration is negative is because the displacement x from the earth is decreasing as the particle is in motion. I still struggle to get my head around this but I guess you could always use common sense, e.g if the speed at the surface isn't greater than the original speed then you know you have ****ed up somewhere
3. (Original post by TheFarmerLad)
I had this exact problem about a month ago. After asking Zacken , I think he said something on the lines of the reason why the force/acceleration is negative is because the displacement x from the earth is decreasing as the particle is in motion. I still struggle to get my head around this but I guess you could always use common sense, e.g if the speed at the surface isn't greater than the original speed then you know you have ****ed up somewhere
I see. Thanks!
4. You need to resolve in the direction of increasing, basically.

Reply
Submit reply
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: May 14, 2016
Today on TSR

### Falling in love with him

But we haven't even met!

### Top study tips for over the Christmas holidays

Discussions on TSR

• Latest
• ## See more of what you like on The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

• Poll
Useful resources

## Make your revision easier

### Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams

Can you help? Study help unanswered threads

## Groups associated with this forum:

View associated groups
Discussions on TSR

• Latest
• ## See more of what you like on The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

• The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.